Traditionally ATM has a long and distinguished service record for voice communications. It is also ideally suited to multiplexing environments and can be readily configured to carry VoIP traffic streams.

In fact today we find that most consumer ADSL2+ implementations do offer a choice of PPPoE or PPPoA as their transport protocols (at least here in Perth). PPPoA stands for Point-to-Point Protocol over Asynchronous Transfer Mode.

The importance of this cannot be overlooked as it means that ATM in some form or other will be with us for some time into the future. In fact the Japanese have just recently deployed a communications satellite with an onboard ATM switch. They obviously think there is life in ATM yet.

Introducing Asynchronous Transfer Mode (ATM)

Originally intended to be a unified networking strategy Asynchronous Transfer Mode (ATM) is a connection-oriented, circuit-switched, cell relay “Jack-of-all-trades” transport protocol that uses small uniform fixed-sized cells to redress Quality of Service (QoS) issues so important to voice/video communications and the multitude of streaming applications upon which we are all so dependant.

ATM Origins and Development

During development of the standards for the Asynchronous Transfer Mode (ATM), in the mid 1980s, the goals were to create a unified networking strategy that could act as an all-round transport system for real-time video and audio as well as for image, text and email. ATM is pretty much a “Jack-of-all-trades” transport system. The two groups primarily responsible for the development of the ATM standards were the International Telecommunications Union [ITU 2004] and the ATM Forum [ATM 2004].

Over time we have seen that the majority of implementations and uses that ATM has fulfilled have been primary concerned with telephony and IP networks. Ethernet and the Internet Protocol (IP) are packet-switched network technologies that use packets of variable size referred to as frames.

ATM Protocol Basics

In marked contrast to packet-switched networking technologies; ATM is a connection-oriented, Data Link Layer (OSI Reference Model Layer 2), circuit-switched, cell relay protocol that runs over Synchronous Optical Network (SONET) Physical Layer links (OSI Reference Model Layer 1) using cells of identical and never varying size. Consistent predictability is the underlying ethos here.

Being a connection-oriented channel-based technology means that ATM must always establish a “logical” connection between the two endpoints prior to commencement of data exchange. Significantly, ATM encodes data traffic into small uniform fixed-sized cells. ATM cells are always 53 bytes in size and are comprised of 48 bytes of data and 5 bytes of header information.

ATM Cell Structure

Regardless of the original size of the packets to be transmitted ATM breaks all packets, data, and voice streams into 48-byte chunks and then adds a 5-byte routing header to each one thereby making a total of 53-bytes for each and every cell. The 5-byte header is essential for later reassembly. During development of ATM it was considered that 10% (5 bytes) of each cell (payload) being dedicated to the header for routing information was more than sufficient.

ATM multiplexes these 53-byte cells instead of the larger packets and in so doing reduces the worst-case queuing jitter by a factor of almost 30, thereby removing the need for echo cancellers. I will discuss queuing jitter along with other types of jitter shortly.

ATM Cell Formats

ATM defines two different cell formats the Network-Network Interface (NNI) and the User-Network Interface (UNI). Most ATM links use the UNI cell format.

ATM Adaption Layers (AAL)

ATM Adaptation Layers (AAL) are the rules for segmenting and reassembling packets and streams into cells. It is the AALs that provide the support for the various services delivered by ATM.

Currently, there are five different AALs and the information concerning which one is being used for each cell on a cell-by-cell basis is not contained within the cell or in the cell header. Rather, this information is negotiated by or configured at the endpoints on a per-virtual-connection basis. Here are the five different AALs and their main uses:

1. AAL1

   Constant Bit Rate (CBR) Services, Circuit Emulation

2. AAL2

   Variable Bit Rate (VBR) Services

3. AAL3

   Variable Bit Rate (VBR) Services

4. AAL4

   Variable Bit Rate (VBR) Services

5. AAL5

   Data Transport

ATM Connectivity

Because ATM is a connection-oriented channel-based technology it must establish a “logical” connection between the two endpoints prior to commencement of data exchange. ATM does this by implementing Virtual Circuits, Channels, Paths and Identifiers as follows:

• Virtual Circuits (VC)

  Virtual Circuits (VC) are admirably suited to multiplexing scenarios. Simply by including an 8-bit or 12-bit Virtual Path Identifier (VPI) and a 16-bit Virtual Channel Identifier (VCI) pair in every ATM frame's header each Virtual Circuit (VC) is uniquely identifiable.

• Virtual Channel

  An ATM Virtual Channel represents the basic means of communication between two end-points. Cells are given a unique identifier called the Virtual Channel Identifier (VCI) which is placed into the ATM cells' header. All ATM cells containing identical VCIs are transported in the same Virtual Channel.

• Virtual Path (VP)

  A Virtual Path (VP) denotes the transport of ATM cells belonging to virtual channels which share a common identifier called a Virtual Path Identifier (VPI). The VPI is included in the header of every ATM frame. In other words a Virtual Path (VP) is a bunch of Virtual Channels (VC) connecting the same end-points. These will also have a common traffic allocation.

• Virtual Path Identifier (VPI)

  The Virtual Path Identifier's (VPI) length varies depending on the interface it is sent on (inside the network or on the edge of the network.

ATM Traffic Contracts

When an ATM circuit is set up each ATM switch is informed of the traffic class of the connection. These ATM contracts constitute part of ATM's Quality of Service (QoS) mechanisms. There are four basic types of contracts:

1. Constant Bit Rate (CBR)

   A constant specified Peak Cell Rate (PCR) is set

2. Variable Bit Rate (VBR)

   An average cell rate is specified. This may peak at a certain predefined maximum level for a certain length of time before becoming problematic

3. Available Bit Rate (ABR)

   A minimum guaranteed rate is specified

4. Unspecified Bit Rate (UBR)

   Traffic is allocated all remaining transmission capacity

ATM Traffic Contract Delivery and Monitoring

Traffic Shaping

The intended objective of traffic shaping is to ensure that cell flow will meet its traffic contract and is usually done at the entry point to an ATM network.

Traffic Policing

To maintain network performance it is possible to “police” virtual circuits against their traffic contracts. Basic policing works on a cell by cell basis, but this is sub-optimal for encapsulated packet traffic. If a circuit is exceeding its traffic contract, the network can either drop the cells or mark the Cell Loss Priority (CLP) bit (to identify a cell as being discardable farther down the line).

Benefits of Using Small Fixed Size Cells

The major benefits derived from using small data cells are a reduction in queue delay and jitter; particularly in multiplexing data streams. By using small, fixed-sized cells ATM is able to transport large data files all the while maintaining minimal queuing delays. Minimal queuing delays are essential to the delivery of both voice/video communications.

Queue Delay

Queue delay related issues include problems associated with end-to-end-round-trip delays and delay variance particularly when carrying voice traffic. High traffic volumes and/or congested networks along with the arrival variance associated with variable route routing are among the main causes of queue delay issues.

Jitter

Although jitter results from queuing delay issues deviations or displacement of various aspects of high frequency pulses such as amplitude, phase timing and signal pulse width as a direct result of electromagnetic interference (EMI) and crosstalk (noise) also cause jitter. Think of jitter as being the production of “jerky” results or in video applications flicker. By using small fixed-size cells ATM is able to overcome the effects of queue delay as well as other types/sources of jitter.

Multi Purpose Transport Protocol

Asynchronous Transfer Mode (ATM) carries many different data types and formats (text, audio, video, graphics, photos etc.) from a multitude of sources and of variable sizes. When combined with standard queuing strategies, maximum queuing delays were common. This is totally unacceptable where voice and real-time video traffic is concerned.

Compression/Decompression Algorithms (Codec)

Because of the way in which many Compression/Decompression Algorithmswork special considerations need to be implemented in order to ensure they work properly as intended including:

Time

The nature of time as we humans perceive it is an analogue continuum (that is to say time is a linear progression). Once past, there is no way as yet to recover the loss.

Jitter and Queue Delay

Jitter and queue delay are of great importance because of the nature and manner of operation of the compression/decompression (codec) algorithms used in the conversion of a digitalized data stream back into an analogue audio signal. This conversion process (digital-to-analogue) is very much a “real-time, on-the-fly” process and is more attuned to” just-in-time” transport protocols.

Real-Time Streaming

In order to produce reliable, consistently “acceptable” output the codec needs the data items (the digitized voice data) to be presented to it in a predictable, regulated and evenly spaced in time data stream, hence the term “real-time streaming”.

Late Arrivals

If the data arrives after its allotted position/reception window in the time sequence (relating to that part of the data-stream) the codec will simply drop it. Not surprisingly this is unacceptable for IP telephony. Remember to keep in mind that time is analogue in nature and once a “time window” elapses, the “lost” time becomes unrecoverable.

Codec Packet Handling Options

If the transport protocol is unable to present the data as and when the codec expects it, the codec, has no choice but to assume either silence, make a “best guess” or simply drop the packet. Any way is unacceptable where voice is concerned as the conversation rapidly becomes untenable and the message does not get through.

ATM Deployment Indicators and Scenarios

ATM WAN Core Implementation

ATM production environment implementations have over time proved to be very successful in the Wide Area Network (WAN) scenarios. Numerous telecommunication providers and Internet Service Providers (ISPs) have implemented ATM in their Wide Area Network (WAN) cores.

Slow Links

For slow links less than 2M-bit/s, ATM still makes sense, which is why many ADSL systems use ATM as an intermediate layer between the physical link layer and a Layer 2 protocol like PPP or Ethernet.

Linear Audio and Video Streams

Interest in using native ATM for carrying live video and audio has increased recently. It is in these environments, where ATM can deliver the low latency and very high Quality of Service (QoS) required for handling linear audio and video streams.

Gigabit Ethernet

Today we are finding that for both new WAN implementations and for existing WAN implementation upgrades, high speed, high performance Ethernet (Gigabit Ethernet, 10Gbit Ethernet, and Metro Ethernet etc.) are rapidly replacing ATM as the technology of choice.

Relative Performance

At the time ATM was designed, 155Mbit/s (135Mbit/s payload) over fiber-optic cable was very fast in comparison to the other carrier/transport technologies available at the time. Since then however; these other technologies have evolved and are now considerably faster than they once were.

Jitter

Today; a 1,500 byte (12,000 bit) full-size Ethernet packet takes only 1.2 µs to transmit across a 10Gbit/s optical network. With this sort of speed, jitter is no longer the issue it once was. By overcoming the potential adverse effects of jitter through this ramping up of network transfer speeds we have at the same time removed the need for using small uniform cells to overcome jitter.

Complexity

Unfortunately, due to ATM's complexity it proved to be unsuitable for deployment in many of the scenarios that its creators had originally intended.

Converged Networks

The speed and traffic shaping requirements of many converged networks are also proving to be very challenging for ATM.

Texas Instruments Inc.

This device extends the battery life in certain types of medical tools, scales, and data acquisition applications.

Rock & Roll Hall of Fame Building- I. M. Pei

The architect tends to design buildings that depict the high-tech movement. He always works on larger scale projects and uses geometric designs to shape his buildings.

The "Butterfly Effect" - Prof. Edward Lorenz

Professor Lorenz realized that small differences in a forceful system such as the atmosphere could trigger unsuspected results. These explanations eventually led him to formulate what became known as the butterfly effect. "This term came from a paper he wrote in 1972 entitled Predictability: Does the Flap of a Butterfly's Wings in Brazil Set off a Tornado in Texas?"

Bose stereo - Professor Amar Bose

These stereo speakers are world-renowned for giving high-end performance despite their reticent size.

Ethernet - Robert Metcalf

Ethernet is a relationship between the unit frame-based computer networking technologies for (LANs). The Ethernet controls our access to certain types of data processing models, i.e. Internet.

The Internet Archive - Brewster Kahle

The Internet Archive (IA) consists of an online library containing the vast information on the Web and other multimedia resources. This information includes certain snapshots from various times from software, WebPages, audio visual and other sources.

Rockman amplifier - Tom Scholz

The Rockman is used in conjunction with headphones and an amplifier used for certain guitars. If any of you are familiar with Boston then you know who invented this.

Spacewar, the first computer game-Steve Russell

Spacewar was the first digital computer game of our time. The idea behind the game involved spaceships attempting to shoot each other while manipulating within the galaxy. I remember this game; I used to play it all the time on my Atari. I wish I still had it.

Hypertext - Prof. Vannevar Bush PhD

Hypertext is defined as words or text that leads the user to information associated with those words.

GPS (Global Positioning System) - Ivan Getting

The GPS uses satellites that transmit microwave signals. These signals enable receivers to determine certain types of pertinent information such as direction, time, speed and location.

The Advantages of Fiber Optical Networking

First of all, we must note that the biggest advantage of using fiber optic networking and hence the use of fiber optic cable as a transmission medium is the high degree of immunity to noise, cross-talk and Electromagnetic Interference (EMI) that this medium provides.

Spanning Large Distances - With the fiber optic technologies currently available today signal degradation and regeneration issues are not what they once were and so the distance factor that so limits copper-based media is of negligible consequence where fiber optic transmission is concerned.

Environmental Damage - Environment factors such as moisture and Radio Frequency Interference (RFI) are also not of the same criticality as they are for copper-based media. The reasons for fiber optic cable as a transmission medium providing a high degree of immunization to noise (EMI) as opposed to other transmission media all stem from the use of light to convey the information (signals) and the construction of the medium (the fiber optic cable).

Security - Due to the degree of difficulty in “tapping” fiber optic transmission lines without being detected, fiber optic transmission media offer a more secure medium than copper-based or wireless technologies.

The result is that fiber optic transmission media are the media of choice when it comes to “long haul” applications such as intercontinental, cross-continental and oceanic (marine) backbone links. It is also the preferred medium for tier one ISP backbone links. This means that new WAN implementations and applications are now predominantly fiber optic cable based. Wireless rollouts being the major exception.

Additional information regarding fiber optic cable construction, signal propagation, signal regeneration, connectors, cable rollout and modes (single-mode and multi-mode fibers) can be found at Fiber Optic Cable.

I will now discuss the major standards and implementations of fiber optic networking starting with the Fiber Distributed Data Interface (FDDI) standard and then the Synchronous Optical Networking (SONET) and the Synchronous Digital Hierarchy (SDH).

Fiber Distributed Data Interface (FDDI)

FDDI which evolved from the IEEE 802.4 token bus timed token protocol is a fault tolerant 100Mbit/sec token passing counter-rotating dual ring LAN standard that permits data transmission between two end-point devices that can be many tens of kilometers apart.

As its name indicates, fiber optic cable is the main form of physical transmission medium used in FDDI. Although a copper-based implementation called, Copper Distributed Data Interface (CDDI) does exist. Although conceived as a LAN standard FDDI has also been used for MAN and WAN implementations.

FDDI Topology - In essence FDDI is a ring network similar to IBM's Token Ring network but with a number of critical differences. The most noticeable of which is that a FDDI uses a dual-attached, counter-rotating token ring topology (see Figure 1: FDDI).

Fault Tolerance - One ring acts as the primary transmission ring and in the original implementations was capable of delivering transmission speeds of up to 100Mbit/sec. The other or secondary ring was originally intended solely to act as a backup.

This meant that the secondary ring was inactive and remained so for as long as the primary ring was functional. In the event of failure of the primary ring the secondary ring would become active. Now all traffic goes to the secondary ring for transmission. It is this built-in redundancy that makes FDDI is a fault tolerant technology.

Higher Effective Sustained Data Throughput - Another factor in FDDI's favor was that it used a much larger frame size than Ethernet which meant that it was capable of much higher effective sustained throughput rates than standard 100Mbit/sec Ethernet. Administrators also had the option of using the secondary ring for data transport rather than having it stand idly by thereby doubling transmission capacity to 200Mbit/sec.

Coverage and Scalability - Not only can FDDI traverse large distances it also scales much better than 100Mbit/sec Ethernet. This means it provides superior support for expanding enterprise networks consisting of hundreds or thousands of users.

Fiber Distributed Data Interface II (FDDI-II) - FDDI-II is a more recent development of FDDI that has added support for circuit-switched services thereby enabling FDDI to carry both voice and video signals as well. For more on FDDI including applicable standards please see About Fiber Distributed Data Interface (FDDI).

Synchronous Optical Networking - SONET

Synchronous Optical Networking (SONET) is an established high-speed WAN alternative for communicating digital information using lasers or Light-Emitting Diodes (LEDs) over optical cable offered by several telecommunications companies.

SONET was originally developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone and data traffic as well as providing the mechanisms that allow for interoperability between equipment from different vendors. The result is that there are multiple, very closely related standards that describe synchronous optical networking including:

Synchronous Digital Hierarchy (SDH) - The SDH standard was developed by the International Telecommunication Union (ITU) and is documented in standard G.707 and its extension G.708. SDH is used throughout the world but not in North America

Synchronous Optical Networking (SONET) - The SONET standard as defined by GR-253-CORE from Telcordia™. Primarily used exclusively in Canada and the USA where SDH has not been implemented, although it can be found in other countries.

Synchronization is Key - Through the use of atomic clocks synchronous networking data transport rates are very tightly regulated which allows for entire inter-country networks to operate synchronously while greatly reducing the amount of buffering required between elements in the network. This reduction in overhead (buffering) translates into greater effective net data throughput rates.

Encapsulation - Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either ATM or so-called Packet over SONET/SDH (POS) networking.

Generic Transport Containers - SDH and SONET are generic all-purpose transport containers for moving voice and data rather than just communications protocols per sec.

SDH and SONET Frame Structures

Standard packet or frame oriented data transmission frames usually consist of a header and a payload with the header of the frame being transmitted first, followed by the payload and a trailer (e.g. CRC). With synchronous optical networking both the header, which is referred to as the overhead and the payload still exist but the big difference is that the overhead is not all transmitted before the payload, rather the transmission is interleaved.

Interleaved Transmission - With interleaved transmissions the transmission of the conversation goes like this:

First of all, a portion of the overhead (header) is transmitted. This is followed by part of the payload. After which the next part of the overhead is transmitted. This is followed by the next part of the payload and so on until the entire frame has been transmitted. Figure 2: Interleaving above shows this.

SONET Frame Size and Transmission Sequence - SONET frames are 810 octets in size, transmitted as 3 octets of overhead, followed by 87 octets of payload, nine times over until 810 octets have been transmitted. The total frame transmission time is 125 microseconds.

SDH Frame Size and Transmission Sequence - SDH frames are 2430 octets in size transmitted as 9 octets of overhead, followed by 261 octets of payload, also nine times over until 2430 octets have been transmitted. Again the total frame transmission time is also 125 microseconds.

It doesn't take much brain power to see that SDH is capable of an effective data throughput rate three times that which the North American implementation of SONET can achieve.

Ethernet over Fiber Optic Cable

Today we see the Gigabit Ethernet over fiber optic cable and 10G Ethernet over fiber optic cable standards being the most common implementations of optical local area networks (LANs) currently being rolled out. They are also used extensively as the network core layer's transport medium of choice particularly Ethernet networks.

The majority of the big players in the networking hardware arena like Cisco, Juniper, and Redback etc all produce numerous products with fiber optic support including Ethernet over Fiber Optic modules. Note see Network Design: Hierarchiesfor more about network design and the functions and features of a network's core layer.

Asynchronous Transfer Mode (ATM) is a connection-oriented Data Link Layer (OSI Reference Model Layer 2), circuit-switched, cell relay protocol that runs over Synchronous Optical Network (SONET) Physical Layer (OSI Reference Model Layer 1) links. ATM encodes data traffic into small uniform (53 bytes; 48 bytes of data and 5 bytes of header information) fixed-sized cells.

Origins of Asynchronous Transfer Mode (ATM)

During development of the standards for the Asynchronous Transfer Mode (ATM), in the mid 1980s, the goals were to create a unified networking strategy that could act as an all-round transport system for real-time video and audio as well as image, text and email. A “Jack-of-all-trades” transport system if you will.

The two groups primarily responsible for the development of the ATM standards were the International Telecommunications Union [ITU 2004] and the ATM Forum [ATM 2004].

Main Implementations of ATM

The majority of implementations and uses that ATM has fulfilled have been primary concerned with telephony and IP networks.

Unlike Ethernet and the Internet Protocol (IP) which are packet-switched based network technologies, that use packets of variable size referred to as frames, ATM is a circuit-switched cell relay protocol that uses cells of identical and never varying size. Consistent predictability is the underlying ethos here.

Benefits of Using Small Fixed Size Cells

The major benefits of using small data cells were to reduce jitter in multiplexing data streams as well as overcoming problems associated with end-to-end-round-trip delays and delay variance particularly when carrying voice traffic.

The reason this is important is inherently due to the nature of operation of the compression/decompression (codec) algorithms used in the conversion of a digitalized data stream back into an analogue audio signal, which is very much a “real-time” process.

To be able to do an “acceptable” job the codec needs the data items (the digitized voice data) presented to it in an evenly spaced (in time) stream hence the term “real-time streaming”. The nature of time as we humans perceive it is an analogue continuum (that is to say time is a linear progression).

If the transport protocol is unable to present the data as and when the codec expects it, the codec, has no choice but to assume silence or make a “best guess”. Either way is unacceptable where voice is concerned as the conversation rapidly becomes untenable and the message does not get through.

If the data arrives late then the time sequence relating to that part of the data-stream will have passed and the codec will simply drop it. Once again, this is unacceptable for IP telephony. Remember that time is analogue by nature and once a “time window” elapses, the “lost” time becomes unrecoverable.

Queue Delay and Jitter

Asynchronous Transfer Mode (ATM) carries data from a multitude of sources and variable sizes including voice, audio and many other variable sized files. When combined with standard queuing strategies, maximum queuing delays were common.

Because ATM was designed to implement a low-jitter network interface this situation is intolerable whenever voice and video communications are to take place. The answer was to use small-fixed size cells (packet) to overcome the effects of queue delay.

With small fixed-sized cells, ATM is able to transport both large datagrams while still maintaining short/minimal queuing delays.

Asynchronous Transfer Mode (ATM) Cell Structure

ATM breaks all packets, data, and voice streams into 48-byte chunks, adding a 5-byte routing header to each one. The 5-byte header is essential for later reassembly.

The reason for the header being 5-bytes in length is that 10% of the payload of every cell is considered to be more than enough to dedicate to routing information.

ATM multiplexed these 53-byte cells instead of packets and in so doing reduced the worst-case queuing jitter by a factor of almost 30, removing the need for echo cancellers.

ATM defines two different cell formats the Network-Network Interface (NNI) and the User-Network Interface (UNI). Most ATM links use the UNI cell format.

Asynchronous Transfer Mode (ATM) Adaption Layers (AAL)

ATM Adaptation Layers (AAL) are the rules for segmenting and reassembling packets and streams into cells. It is the AALs that provide the support for the various services delivered by ATM.

Currently there are five different AALs and which one is in use for each cell is not included in the cell. Instead, it is negotiated by or configured at the endpoints on a per-virtual-connection basis.

• AAL1 - Constant Bit Rate (CBR) Services, Circuit Emulation
• AAL2 - Variable Bit Rate (VBR) Services
• AAL3 - Variable Bit Rate (VBR) Services
• AAL4 - Variable Bit Rate (VBR) Services
• AAL5 - Data Transport

Asynchronous Transfer Mode (ATM) Connectivity

Being a connection-oriented channel-based technology means that ATM needs to establish a “logical” connection between the two endpoints prior to commencement of data exchange.

Virtual Circuits (VC)

By including an 8-bit or 12-bit Virtual Path Identifier (VPI) and a 16-bit Virtual Channel Identifier (VCI) pair in the ATM frame's header each Virtual Circuit (VC) is uniquely identifiable. Virtual Circuits (VC) are admirably suited to multiplexing scenarios.

Virtual Channel

An ATM Virtual Channel represents the basic means of communication between two end-points. Cells are given a unique identifier called the Virtual Channel Identifier (VCI) which is placed into the ATM cells' header. All ATM cells containing identical VCIs are transported in the same Virtual Channel.

Virtual Path (VP)

A Virtual Path (VP) denotes the transport of ATM cells belonging to virtual channels which share a common identifier called a Virtual Path Identifier (VPI) which is included in the header of every ATM frame. In other words a Virtual Path (VP) is a bunch of Virtual Channels (VC) connecting the same end-points, and have a common traffic allocation.

Virtual Path Idetifier (VPI)

The Virtual Path Idetifier's (VPI) length varies depending on the interface it is sent on (inside the nework or on the edge of the network.

Asynchronous Transfer Mode (ATM) Traffic Contracts

When an ATM circuit is set up each switch is informed of the traffic class of the connection. These ATM contracts constitute part of ATM's Quality of Service (QoS) mechanisms. There are four basic types of contracts:

1. Constant Bit Rate (CBR) - A constant specified Peak Cell Rate (PCR) is set
2. Variable Bit Rate (VBR) - An average cell rate is specified. This may peak at a certain predefined maximum level for a certain length of time before becoming problematic
3. Available Bit Rate (ABR) - A minimum guaranteed rate is specified
4. Unspecified Bit Rate (UBR) - Traffic is allocated all remaining transmission capacity

Traffic Shaping

The objective of traffic shaping is to ensure that cell flow will meet its traffic contract and is usually done at the entry point to an ATM network.

Traffic Policing

To maintain network performance it is possible to police virtual circuits against their traffic contracts. If a circuit is exceeding its traffic contract, the network can either drop the cells or mark the Cell Loss Priority (CLP) bit (to identify a cell as discardable farther down the line).

Basic policing works on a cell by cell basis, but this is sub-optimal for encapsulated packet traffic (as discarding a single cell will invalidate the whole packet).

Asynchronous Transfer Mode (ATM) Deployment Scenarios

ATM has proved very successful in the Wide Area Network (WAN) scenario and numerous telecommunication providers have implemented ATM in their Wide Area Network (WAN) cores.

For slow links less than 2Mb/s, ATM still makes sense, which is why many ADSL systems use ATM as an intermediate layer between the physical link layer and a Layer 2 protocol like PPP or Ethernet.

Interest in using native ATM for carrying live video and audio has increased recently. In these environments, low latency and very high quality of service are required to handle linear audio and video streams.

Asynchronous Transfer Mode (ATM) the Future

Currently the future for ATM does not look very bright as it seems that in all likelihood gigabit Ethernet implementations (10Gbit-Ethernet, Metro Ethernet) will replace ATM as a technology of choice in new WAN implementions.

At the time ATM was designed, 155 Mbit/s (135 Mbit/s payload) over fiber-optic cable was fast in comparison to other technologies but since then networks have become much faster. A 1500 byte (12000-bit) full-size Ethernet packet takes only 1.2 µs to transmit on a 10 Gbit/s optical network, removing the need for small cells to reduce jitter.

The complexity of ATM is another factor that makes deployment of ATM unsuitable in many of the scenarios that its creators had originally intended.

The speed and traffic shaping requirements of converged networks also challenges ATM.

Now that we have covered the basics of twisted pair copper cable functionality as a transmission medium we will move right on into discussing the shielded varieties after which we will have a look at the unshielded varieties.

The Shielding Layer

The single biggest difference between shielded and unshielded twisted pair cables is the presence of the shielding layer. The whole idea of the shielding layer is to make the cable less susceptable to Electro-Magnetic Interference (EMI), noise/cross-talk and the signal degradation they cause.

Shield Layer Implementations

The main types of shielded twisted pair cable are defined by the manner of their construction as follows:

• One Pair Shielding - The shielding layer covers both members of a pair and only one pair at a time
• Multiple Shielded Pairs - When a cable is composed of more than a single shielded pair the sets of pairs of wires are contained in a sheath which may or may not be shielded as well
• Screening - When the shielding layer covers bunches of pairs it is referred to as screening. In the case of cables with small numbers of pairs this is usually implemented by manufacturers in the form of having only one shielding layer that is used to cover all pairs within the cable.
• Screened Shielded Twisted Pair (S/STP) - Also known as Screened Fully Shielded Twisted Pair. The shielding layer covers both members of a pair and an additional shielding layer is added to cover the bunches of pairs.
  • Most Resilient - This is the type of shielded pair cable that is most resistant to the effects of external and internal interference
  • Most Expensive - The multiple layers of additional shielding have the predictable effect of making this type of cable the most expensive of all of the twisted pair cables that have been widely implemented into networks

Grounding

• Proper Grounding Essential - The shield layer must be grounded in order for it to work
• Grounding Using the Shield Layer - Since the shielding layer is metalic it can also serve as a ground
• Shield Failure - Engineers; through experience, know that using the shield as the ground has its draw-backs; shield functional failure being but one
• Drain Wire - As a result shielded and/or screened twisted pair cable usually has an additional special purpose grounding wire; called a drain wire built in
• Reliability and Effectiveness - Using a special purpose drain wire is far more effective and reliable
• Drain Wire Becomes Essential - As cables become bigger and contain more and more collections of bundles of shielded twisted pair wires the drain wire becomes essential rather than a nice extra.

Cost Vs. Resilient Reliability

The additional costs that are incurred when installing shielded varieties of twisted pair cable have; on a materials cost basis alone, proven to be the major reason for the smaller business and the home Local Area Network (LAN) alike to forego any potential resilience to interference that the use of shielded cable might endow their networks in order to reduce costs.

Balanced Pair Differential Mode

Don't forget that both STP and UTP use the twisted balanced pair differential mode transmission principles to help reduce the interference or crosstalk from other neighbouring pairs within a cable as well as that from external EMI sources.

Twist Characteristics

The period of the twist (which is usually the same as for UTP - 3 twists/inch) is designed to reduce EMI - Elector-Magnetic Interference from both; the other pairs of the same cable and exterior influences as well, thereby preserving the signal during its travel along the wire.

Advantage of Shielded Twisted Pair (STP)

• EMI - Less susceptable to the negative effects of EMI

Disadvantages of Shielded Twisted Pair (STP)

• Manufacturing Costs - The extra shielding makes the cable considerably more expensive to manufacture
• Production Volumes - Much smaller volumes of STP are produced in comparison to UTP and this disparity really impacts to-market expense and thus makes STP considerably dearer
• Competitive Market-Place Environment - When combined with the greater cost to manufacture STP is considerably dearer than UTP
• Availability - The enormous volumes of UTP produced mean that it is more readily available than STP
• Bulky - It also makes the cable bulkier

Shielded Twisted Pair (STP) Usage Indicators

• Mission critical applications
• Environments with high levels of background EMI
• Environments containing a high potential of electrical interference due to proximity of other wiring and electrical and electronic equipment
• Environments where electro-magnetic surges are known to occur such as the turning on and off of devices that draw considerable voltage and current flow
• Environments containing other sensative electrical and electronics equipment such as hospital intensive care units, coronary care units, surgical theaters and life support systems

STP Loses Niche Market Edge

Today coaxial and fiber-optic cabling are the preferable options in these scenarios.

Standard Shielded Twisted Pair (STP)

• The Shield Layer - STP cabling includes metal shielding over each individual pair of copper wires which provides protection against external EMI
• Token Ring Networks - the IBM Cabling System specifications for Token Ring Networks specified the use of 150 ohm shielded twisted pair cables

When the Network Cabling Guide returns in Part 6 we will move straight into the most common of all copper-based transmission media; Unshielded Twisted Pair - (UTP), and there is a considerable range and diversity of performance capabilities to be found here. In fact Unshielded Twisted Pair (UTP) cabling is without doubt the predominant type of cable to have been used in computer networking applications to date. Until next time enjoy.

LAN Segmentation Using Transparent Bridges

Architecture Matters

Transparent bridges can be used with Ethernet networks. Other architectures require different types of bridges. For example Token Ring networks use a type of bridging technology called source-route bridging.

Reduced Outlay and Greater Return On Investment (ROI)

It's all about more bangs for the buck! Today a compact workgroup switch that supports transparent bridging such as those from D-Link, 3Com, Netgear and others can be obtained for well under $100.

Unmanaged Vs Managed

Switches that support transparent bridging can be of the managed or unmanaged varieties. The big difference; apart from the cost that is, is that unmanaged switches are a lot more user friendly than the managed variety.

Improved Network Performance

LAN segmentation using transparent bridges reduces the size of the network's collision domain(s) because it turns large collision domain(s) into a number of smaller collision domains with fewer machines per collision domain (LAN segment).

Fewer Collisions

Reducing the number and frequency of collisions improves the network's data transfer speed and efficiency

Reduced Competition

Improved data transfer rates and shorter wait states because fewer machines per segment means less competition for the finite available transmission bandwidth

More Effective Bandwidth Allocation

Improved Effective Available Network Bandwidth

Fewer nodes per segment means each node effectively gets a greater “share” of the available bandwidth.

Maximizing Returns

Since the available network bandwidth is a finite resource any measures that improve this aspect will have the greatest noticeable effect on the largest number of devices both individually and as a”collective”.

More Responsive Network from the Users Perspective

Improved Network Responsiveness

Segmentation of large collision domains into a number of smaller collision domains will result in users noticing an increase in the responsiveness of the network in terms of a reduction in the amount of time that their computer takes to gain access to the transmission media in order to transmit the job that the user has requested.

Faster Transmissions

Users will also see an overall improvement in the time taken to complete a data transmission or data reception task.

Shared Internet Services Improved

Internet searches; for instance, will be done in shorter periods of time than was the case prior to the segmentation.

Reduced Network Congestion

Transparent bridging can greatly reduce network congestion & bottlenecks. Only traffic that belongs on a segment will be placed onto that segment by a device implementing transparent bridging. How transparent bridging works its magic is a topic that will be discussed in a future article of the CCNA Summary Series.

Greatly Reduced Administrative Overheads

Reduced Installation Time

Because of the plug "n" play nature of transparent bridges administrators will spend far less time in installing this type of bridge or a switch.

Switches and Transparent Bridging

Most switches today support transparent bridging. The main difference between these switches and the traditional transparent bridge is in port density. Switches have massively higher port densities than traditional transparent bridges. It's all a matter of evolution.

Smaller Footprint

With higher port densities a modern switch that supports transparent bridging will occupy less physical space and so are ideal to use as workgroup out-of-the-closet scenarios.

I am also making mention of them here because due to their very nature transparent bridging devices are all about no administration. Plug "n" Play they most certainly are. But what do/can you do when loops enter the picture? Well; in most instances, you certainly can't interface with the software that is running on most of these local workgroup transparent bridges and switches because there is none.

The answer is not to be found blowing in the wind but rather in the form of built-in protocols and algorithms that take care of all of these issues while the device stays true to its “no user input required” philosophy.

Loop Avoidance

Redundancy, Reliability, Robustness and Fail-Over

For reasons of redundancy and the resultant increase in reliability and robustness that it delivers network segments may by configured to be reachable via more than one port (even if in a round-a-bout manner).

Reasons for Creating Networking Loops

• The implementation of redundant fail-over solutions is a major player in the creation of networking loops
• For reasons such as the “safety-net” aspect of implementing multiple hot-swappable “just-in-case” “spare parts” devices
• The idea that if a mission-critical device were to fail then the already installed and powered-up “spare” device can immediately jump into action thereby reducing the system and network outages cause by device failures would be negated. Unfortunately this is another culprit when it comes to loop creation.

But what's all the fuss about?

• Loops cause great network chaos and all round confusion
• Devices such as transparent bridges that operate at the data link layer (2) are particularly hard hit by the negative effects of network loops
• Something had to be done to prevent them from totally crippling the network

This is why loop avoidance is important. Correct configuration and physical connectivity are essential but the moment we introduce redundant pathways loops will exist even in the most simple of networks.

In fact loops can occur in a network with as few as two switches.

The Spanning Tree Algorithm

The Spanning Tree Algorithm is used by switches in Spanning Tree Protocol (STP) mode to counteract the effect of loops and redundant pathways. STP mode has meant that network administrators can introduce redundant pathways and redundant switches into their networks without creating loop-hell.

• When in Spanning Tree Protocol (STP) mode; switch ports that create redundant links will be selectively shut and for as long as the primary (lowest cost path) port remains available they will stay that way.
• If the lowest cost port fails then the Spanning Tree Algorithm will be used to recalculate the network and the redundant pathway port will become active. The message will still get through.

Multi-Destination MAC Addresses

• Frames intended for multiple destinations (transmit once-receive many) such as; broadcasts and multicasts, are forwarded through multiple ports by the transparent bridge.
• Broadcasts will be flooded
• Multicasts will be sent through the appropriate ports

For Example:

• A multicast may be for two or three or however many different segments that need to be reached via more than one port of the transparent bridge or switch at a time
• Those ports that do not have destination nodes for which the multicast is intended do not receive the transmission

Transparent Bridging Implementations

The most common global implementation of transparent bridging today is in the form of integrated multiservice/multifunction devices such as ADSL broadband Modem Routers with built in switches. It is these devices that have enabled networks of all sizes to share their Internet connection among considerable numbers of hosts, nodes and users.

Benefits of Transparent Bridging Include

• Successful isolation of intra-segment traffic, thereby reducing the traffic seen on each individual segment. This is called filtering and occurs when the source and destination MAC addresses reside on the same bridge interface.
• Filtering usually improves network response times, as seen by the user. The extent to which traffic is reduced and response times are improved depends on the volume of intersegment traffic relative to the total traffic, as well as the volume of broadcast and multicast traffic

Learning, flooding and forwarding are all part of normal operation for a transparent bridge/switch and the decisions involved in carrying out these functions are accomplished through the building, maintenance and referencing of the associations of entries in the device's filter (forwarding) table.

Until next time, enjoy!

Transparent Bridging

This is a “no-brainer” technology that delivers plug "n" play capabilities to users and key networking components like workgroup switches.

Transparent Bridging Benefits In a Nutshell

Routers, Switches, and Bridges

While routers interconnect networks, switches and bridges interconnect network segments and devices. Cisco® refers to this as the access layer and that is exactly what these devices do. They give the user or device access to network resources including the infrastructure such as transmission media.

You have got to get on the cable, before you can get to the gateway/router, before you can “surf” the net. We are going to check out a “no-brainer” way of doing this. It's called transparent bridging.

Transparent Bridges

Transparent bridges (including modern switches) are so named because their presence and operations are transparent to network hosts and users alike.

Learning, Forwarding, and Filtering

Making Decisions

In order to fully understand how a transparent bridge decides whether or not it should forward a frame and selects the appropriate interface on which to place that frame we need to take a look at the primary functions of a transparent bridge: learning, forwarding, filtering and loop avoidance.

Loop Avoidance

Another role played by transparent switches is loop avoidance which is one of the topics that will be discussed in Magic of Transparent Bridging Part Two.

Out-of-the-Box

Because a new out-of-the-box transparent bridge has never been exposed to the network that it is about to be connected to I shall start by describing the learning process using a fresh out-of-the-box device first.

Learning

When transparent bridges are first powered on, they learn the network device locations (including workstations) by analysing the source Media Access Control (MAC) Address of all incoming frames from all attached networks.

Memory

Before anything can learn it needs to have the magic ingredient - MEMORY. Transparent bridging and the devices that use it are just like us in that their capacity to learn depends on:

• Writing to Memory - The ability to lay down memory in preparation for future recall
• Reading from Memory - The ability to recall that which is stored in memory
• Recall on Demand - The capacity to recall at will. It is this ability that allows you to randomly access the contents of memory or in computer lingo - Random Access Memory (RAM)
• Selective Recall - The ability to selectively recall preferred memories at will
• Application - The capacity to apply the contents of memory to the situation at hand
• Being Smart - The being smart part is all about selecting the most appropriate memory item to recall with regards to your current dilemma

The Filter Table

That part of memory that we are interested in here is the memory which the device (switch) uses in performing the process of transparent bridging. It is called a Filter Table. For reasons that will become obvious shortly the Filter Table is also known as:

• Forwarding Table
• Media Access Control (MAC) Address Table
• Hardware Address Table

Nomenclature and Misconceptions

The important thing here is that all of the above listed names are indeed correct alternate names (synonyms) for the Filter Table. Furthermore; they are more or less interchangeable, it is however most definitely wrong to use the term Routing Table in reference to a transparent bridging device's Filter Table.

Filter Table Building Example:

Here is a short example of how a device uses transparent bridging to build its Filter Table:

• The transparent bridging device sees a frame arrive on Port 1
• Examining the source address field of this frame the device learns that the transmission was from Host A
• It concludes that Host A can be reached through the segment connected to Port 1
• The transparent bridging device examines the contents of its Filter Table. Because this is a new device and this is the first network conversation that this transparent bridge has experienced there will be no entries in its Filter Table.
• The transparent bridging device will now create a new entry in its Filter (forwarding) Table listing Host A as reachable via Port 1
• Every time a frame arrives at the transparent bridging device this process is repeated
• It is in this way that a transparent bridge builds its Filter Table
  • This is what is Referred to as the LEARNING Process

Forwarding - Flooding

Transparent bridges/switches examine incoming frames to determine their intended destination Media Access Control (MAC) Address they will then:

• Look-Up Their Filter Table - In transparent bridging mode the bridge/switch will always refer to its Filter Table (forwarding) to check for any entries relating to this MAC Address. Who knows? There may already be an entry indicating through which port this destination can be reached.
• In our case we have a new transparent bridge. So; unless the frame is intended for Host A, the bridge will not have any entries corresponding to the intended destination MAC Address in its Filter Table
• Because the transparent bridge doesn't know anything about this MAC Address it can not make the decision to forward the frame exclusively out of the one correct specific port
• So; by default, whenever a transparent bridge encounters a frame with a destination MAC Address that it hasn't encountered before (the first time) it will “flood” the frame out of every port except the one on which it arrived.
• Our device is no different and that is exactly what it does. It sends copies of the frame out every port except the one which it arrived on.

Forwarding - Filtering

Filter Table Building

As more traffic transpires over time the transparent bridge will continue building its Filter Table.

• New entries are created for each new (learned) source MAC Address along with the corresponding port through which this address can be reached.

Known Source MAC Address and Port Entries

Eventually the transparent bridge will lookup the source MAC Address of an incoming frame in its Filter Table and it will find an existing entry.

• If; the known entry (previously recorded) in the transparent bridge's Filter Table and the current frame's source MAC address and port number match, the transparent bridge will not need to add this source MAC address to its Filter Table.

Known Destination MAC Address and Port Entries

• Now the transparent bridge examines the frame in order to determine the destination MAC address of the newly arrived frame
• It will then check its Filter Table for any entries pertinent to the intended destination MAC Address of the current frame
• This time it finds an entry that lists this particular destination MAC Address as being reachable via one of its (the bridge/switch) ports

Since the destination MAC Address of the current frame is known to be contactable via a specific port on the transparent bridge/switch the switch/bridge will now be able to selectively place the frame onto the correct network segment via the appropriate port.

Filtering

Now instead of 'flooding` this frame out of all ports; bare the one through which it arrived, the transparent bridge/switch will be bale to selectively and exclusively forward the frame only out of the port indicated by its Filter Table as being the port via which this destination MAC can be reached.

This is what is known as filtering.

Only the segment on which the destination MAC Address resides now receives the transmission thereby allowing all other devices located on all other ports to be free to transmit themselves.

Not Party To the Conversation

This means that devices can transmit or receive simultaneously while other network conversations; that they are not party to, are in progress. The importance of this cannot be underestimated as it results in considerable improvements to network performance.

There is still one proviso however; and that is that only one conversation at a time can take place on a segment. The network is a one conversation at a time/segment network.

Collision Domains - The Implications

Suppose a device that uses transparent bridging has six ports and each of these ports is used to connect different devices or network segments. The result would be that:

• The network will now be comprised of six independent segments
• All conversations between nodes on the same segment will remain confined to that segment. This is known as a collision domain.

Now suppose that nodes on two different segments are having a conversation then:

• The transmission will be contained within those two segments
• The other four segments are free to go about their own conversations

By segmenting collision domains transparent bridge greatly improve network efficiency, performance and the potential available effective bandwidth.

Improved Network Bandwidth and Efficiency

Thus a six port transparent bridge can cater for three different two segment conversations simultaneously. Whereas prior to network segmentation only one conversation at a time could take place over the entire network. This is a much greater improvement in both efficiency and bandwidth while reducing the potential for collisions.

Such technologies as loop avoidance, redundancy, the spanning tree algorithm and STP mode will all be discussed in The Magic of Transparent Bridging Part Two. Until then enjoy!!

Let's start with copper-based cables since they are the most common and the easiest to install. This is particularly true when making custom cable runs. Before I discuss the types of copper cable that are readily obtainable at most hardware or computer shops let us look at the categories of cable which you may encounter out there in the wild.

First of all I will start with the oldest types and then progressively discuss the more recent varieties. Please take note that it is highly possible that those just entering into the world of network cabling may not ever actually encounter some of the earlier types of copper cabling other than to remove them.

The reasons as to why this is so will be made clear at the appropriate time. For now though you can safely assume that being made of a considerably thick copper core does give the early coaxial cable scrap metal value.

Coaxial Cable

Coaxial cabling consists of a single central copper conductor core surrounded by a foam filler layer. Both layers are surrounded by a braided metal shield and then by another outer covering sheath usually plastic.

The inner foam layer; which is fire resistant, was also intended to provide insulation between the center conductor and the braided metal shield which acts to reduce (even block) any outside EMI from fluorescent lights, motors, and other computers. It also helps to make coaxial cable tear resistant which as you will soon learn has both an upside and a down-side.

The final layer; usually composed of some waterproof material such as plastic is there for overall protection and to provide resistance to environmental damage such as that which water might cause.

Coaxial cabling is notorious for being difficult to install but it is highly resistant to signal interference and can support greater cable lengths between network devices than twisted pair cable (STP & UTP). One of the reasons for its difficult to install nature is due to the outer braided metal layer and the overall thickness in diameter of both the inner conducting signal carrying layer and the overall thickness of the entire cable as a package.

Types of Coaxial Cable

There are two types of coaxial cabling; with each being named for reasons that will soon become obvious:

1. Thick Coaxial Cable

• Thicknet - Also referred to as thicknet and was the first variant to be used with Ethernetnetworks
• Thick Coaxial Cable has an extra protective plastic cover to assist in resistance (tolerating) to damp/humid environment tolerance by keeping moisture away from the center conductor. It is this extra layer that makes thick coaxial cable thicker than thin coaxial cable and hence the name.
• Ethernet - When carrying Ethernet signals the term 10Base5 is used to describe the specifications to which the cable must adhere in order to be suitable
• Ten - The 10 refers to the cable's maximum data throughput of up to 10 Megabits/sec
• Base - The BASE tells us that this cable is intended for use with baseband signaling as opposed to broadband signaling.
• Baseband signaling means signals and/or systems whose range of frequencies is measured from zero to a maximum bandwidth or highest signal frequency. Generally the entire spectrum range is used with every transmission. This meant that only one signal could be propagate across the medium at a time.
• Five - The 5 refers to the maximum segment length being 461.5 meters (approximately 1600 feet or 500 yards)
• Linear - In addition 10BASE5 cables had to be one linear run
• Terminators - Another requirement was that 50 ohm resistive terminators had to be installed at both ends of a cable run
• Transceivers - For 10BASE5 networks transceivers had to be installed at 2.5 meters. This was so as that reflections from multiple taps were not in phase which helped in reducing the amount of noise and signal degradation. Suitable attacment locations were marked on the cable.
• N Connectors - Transceivers could also be connected by using N connectors at the end of a cable segment
• T-Connectors - T-Connectors are not allowed
• Attachment Unit Interface (AUI) - Transceivers are connected to nodes via an interface that was known as the Attachment Unit Interface (AUI) which is a 15-pin, double row D-Sub type of connectort that used clips rather than screws for cable resistance (to stop the cable from being accidently dislodged)
• 10BASE5 coaxial cable is rather stiff and measured approximately 9.5 mm (0.375 inches) in diameter with an impedance of 50 ohms
• 100 Nodes - A 10BASE5 network segment can support up to 100 nodes
• Thick coaxial cable was a great choice when running longer lengths in a linear bus network compared to 10BASE2 (thin coaxial cable)
• Non-Flexible - Thick Coaxial Cable's greatest disadvantage is that it is very difficult to install because of its thickness and lack of flexibility

1. Thin Coaxial Cable

• Thinnet - Also referred to as thinnet. When carrying Ethernet signals the term 10Base2 is often used
• Ten - The 10 means that the cable was rated at a maximum data throughput rate of 10 Megabits/sec
• BASE - The BASE tells us that this cable is intended for use with baseband signaling as opposed to broadband signaling. Baseband signaling means signals and/or systems whose range of frequencies is measured from zero to a maximum bandwidth or highest signal frequency. Generally the entire spectrum range is used with every transmission. This meant that only one signal could be propagate across the medium at a time.
• Two - The 2 refers to the approximate maximum segment length of 200 yards (185 meters)
• Thin Coaxial Cable - Was very popular in school networks, especially linear bus networks
• Termination - Another factor that needs to be taken into consideration when working with coaxial cabling of all varieties is the need to terminate any cable segments that were not fixed to another device.
• T-Connectors - When wiring a 10BASE2 network, special care has to be taken to ensure that cables are properly connected to all T-connectors and appropriate terminators are installed.
• Installation Unfriendly - Although; easier to work with and install than thick coaxial cable it was still cumbersome and difficult to work with

• Comparative Improved Flexibility - This state of affairs was due in part to the solid copper core and its relatively thick diameter and the outer mesh metal layer. You may have used Bar-B-Que gas bottles that used hoses that had an outer metal mesh layer and will therefore have encounter this problem before. 10BASE2 was more flexible than 10BASE5 but it was still a problem.
• Single Point of Failure - A failure at any point of the network cabling tends to prevent all communications which made 10BASE2 difficult and time consuming to administer, manage, maintain and troubleshoot.
• 10BASE2 Implementation - The characteristics of 10BASE2 and thin coaxial cable meant that it was ideal for small networks of 2-4 machines.
• Competition - The main reason that we never saw wholesale rollout of 10BASE2 was that CAT 3 cable and the forthcoming CAT 5 cable were considerably cheaper and easier to install were being to build up a head of steam in terms of sales and general acceptance. As a result 10BASE2 was never really main-stream consumer first choice for use as a network medium. However; Its academic installation base did mean that it hung around for a long time.

Coaxial Cable Connectors

The BNC connector is the most common type of connector to be found in association with coaxial cabling. BNC - Bayone-Neill-Concelman or bayonet connector.

There are also other types of connectors and adapters that have been used in networks that were using thin coax as their medium including: a T-connector, barrel connector, and terminator.

Crimp

As always it is the connectors on the cable that are the weakest points in any network and coaxial connectors are no exception. It is therefore particularly important to use coaxial BNC connectors that crimp rather than screw onto the cable as they consistently produce stronger and more reliable connectivity.

Vampire Tap

One of the major advantages of 10BASE5 cable was that it allowed for new connections to be made while all existing connections remained live (still in use). The device used to do this is called a vampire tap.

With practice the cabler was able to clamp the vampire tap onto the existing cable by forcing a spike inside the tap to pierce through the outer shielding layers to make contact with the solid copper core that performed the transmission and reception of the signal.

Other spikes; which bit into the outer conductor, were deployed at the same time as the copper core was being spiked when a special tool clamping/crimping tool was used to perform the clamping procedure. These clamps/crimps were readily available from hardware stores, electronics stores and electrical supplies stores.

It was not long before we began to see vampire taps built into the transciever because this allowed the use of more flexible multi-wire cables

Repeaters - Repeaters are used in scenarios where there is a greater distance between machines than the manufacturer's cable specifications cite as being the maximum distance that this particular copper cable could be run if you hope to get any worthwhile service. In order to increase the maximum distance between nodes that the manufacturer's specifications stipulated as being so repeaters can be used.

In the next article we will be continuing our investigation of network cabling using copper-based cables by having a look at twisted pair cabling starting with shielded Twisted Pair (STP) and Unshielded Twisted Pair (UTP)

The Need for a Transmission Medium

The first recognised computer network medium was known as sneaker-net. This was because way back when in the early days of computing those users who wanted to transfer or share data with other users would make a copy onto floppy disk and get up and go to the intended recipient and hand it to them.

The sneaker-net came into being as observers; usually other university types noted that the majority of these young computer types wore sneakers and since they used their sneakers as the primary means of transporting the data the term sneaker-net came into use. It was probably not meant in a complimentary way. The term “geek” hadn't even been thought of at the time or else the term may have been “geek-net” but we will never know.

Oh! And by the way the floppy disks that I am referring to here are the original floppy disks which were large, flat, round, non-rigid, discs of magnetic medium and not the small 3 1/2 “ types that were and are still used by some people today.

The capacity of these disks; which was considered to be quite large at the time, was around 320 Kilobytes. The more modern varieties have a larger capacity with the high density varieties that can still be brought today having a standard capacity of 1.44 Megabytes.

Today most new machines do not even have a legacy floppy drive included which is probably just as well. They were not the most reliable of devices and by today's standards their storage capacity was miniscule. Today we might wonder just what use; if any, could a floppy disk be? Well back then computers were primarily code-oriented and the output of human "friendly" text was a considerable and notable achievement. Then we started to see a new and wonderful device being attached to the computer. It was called a keyboard.

Up until then input had been in the form of punch cards and tape. Except for the multi-mega resource endowed who were able to afford magnetic tape reels. But for the average enthusiast they keyboard and floppy disk drives were the rage. Then we saw another new device come into being; the monitor. Well; now things were really getting up to speed, we could read text (in monochrome of course) on a television type device. The world was really becoming a truly wondrous place.

Things only got better as hard disk drives that were “affordable” and compact enough for the smaller computer were mass produced. With capacities ranging up to 60 Megabytes these storage giants and they were big and heavy and most definitely not portable as we know the term today. Now we could load a program into the computer and store it on the internal hard drive and then whenever we wanted to start the machine would had the option to start the program stored on the hard drive and run with that.

As more programs were developed and the hard drive became larger we saw the introduction into mainstream; although still enthusiast computers the need to manage these programs and this need resulted in the creation of what I will loosely call the early disk operating systems. They were refined and added to as time progressed and the term for them became the abbreviation DOS which many of you may have heard of. It stands for Disk Operating System.

Initially this was either pre-installed or factory upgraded but later it became possible to install this disk operating system from floppy disk which had also evolved into larger than 360 Kilobyte capacity disks by then. In fact 720 Kilobyte disks were very popular by now. Well is it has a habit of doing time did not stand still and software progressed in its capabilities and size so that multiple disks were required to install the disk operating system and most other programs as well.

Fortunately manufacturers were introducing larger capacity hard drive and floppies were evolving into the 1.44 MB capacity disks we know today and all was well in the world. Software evolved further and became ever larger with ever increasing storage requirements.

Major corporations were using mainframes with large capacity magnetic reel storage which was wonderful from a storage capacity point of view but the manner in which it operated was linear which meant that much turning of tape was required to access various data when required. The answer of course was to schedule access to increase the efficiency of use but that had its limitations.

In the mean time some bright spark in the computer department was talking to the mother via long distance telephone when the thought occurred to them that if they could converse over such a long distance surely machines could do the same. Okay maybe not over long distance but across the room would be handy. So it's off to the communications department that our hero goes. Upon arrival he asked the guys down there the big question. The reply he got nearly made him wet his pants in excitement.

Machines have in fact been communicating quite well over long and short distances for quite some time now. “Go down to Wall Street and check it out for yourself” was the com guy's reply. No need; our hero knows that if he goes to executive accounts and finance there are machines there that are always in touch with the money men down-town. So it's off to finance that our hero goes.

Once there; merely observing the ticker tape machines was inspiration enough. Using the telephone lines was the inspiration that he had. So off to the lab and sometime later our hero has rigged-up a contraption using readily available parts; cheap was a prime motive here. And so the first basic modem came into being. At least we don't need to take disks out of one machine and transfer them to the machine next to it by hand any more.

With the passage of time this novelty become a storm and the military decided to get in on the act an established a special research project known as ARPA. The network they created was called ARPANET (not much in the way of imagination here). The important thing however; was that they created the protocols that permitted machines to communicate over very long distances with reasonable reliability.

The core of this set of protocols was the Transmission Control Protocol (TCP) which was responsible for getting the message through. With more devices joining the conversation it became obvious that some way of identifying each machine was needed. The identification of each machine on a machine to machine level had as with all electronic communications devices been achieved using a hardware address which we know as the Media Access Control address or MAC address and worked well for awhile.

As the factors of scaling-up the size of the networked computers became of more important it was soon realised that purely using MAC addresses for machines that were permanently connected in a smallish local network was not a problem. The troubles began when trying to connect each of these small networks on a sporadic periodic basis (only when needed).

So the idea of letting the machines on the smaller local network sort out their MAC addresses and using a different addressing structure to identify remote networks gained impetus. This was the beginnings of a logical addressing structure. Well as the years went by and more and more machines were being networked and more and more networks were being interconnected on a global scale this logical addressing structure evolved considerably.

The best part of it all was that the smart guys who created TCP did such a fine job that it has been used ever since and when the time came that the number of interconnected networks was getting to be humanly unmanageable manually some way to overcome this had to be found. We were also seeing a number of different architectures being developed by different groups more or less independently and in competition with one another. The result was that not all networks could connect with all of the other networks.

At around this point in time the number of home computers being sold was beginning to skyrocket and many who used networked computers at work desired the same for their home computers. Having already developed the Hypertext Transfer Protocol (HTTP) and the Hypertext Markup Language (HTML) academics and researchers could now confer with each other and here a greater degree of collaboration between these groups of individuals developed. Admittedly this may have at times been a little constrained and secretive but non-the-less it happened.

The military were also in the processes of extending and evolving their networks. Cutting the story short it became obvious that something had to be done to make everything capable of talking to everything else. It was at around this point that the Internet Architecture Board (IAB) and the Institute of Electrical and Electronics Engineers (IEEE) really came to the fore.

The IAB took responsibility for overseeing and formulating standards relating to the development of the Internet. The IEEE met in February of 1980 to discuss what needed to be done to produce uniformity if the Internet was going to truly develop.

They came to the realization that there were a number of different competing and incompatible network technologies and decided that due to the various different possible media available it was all too much for just one group to deal with and so they created a number of subcommittees each charged with the responsibility for the production of a set of standards covering a smaller area of technology that could be used by one and all. I guess they must have heard what the little general said “Divide and conquer”.

Thus we saw the formation of the 802 DOT committees and the standards that were produced by them have all evolved over time. Some are still widely in use today, some aren't, some have gone to better pastures and others are only just beginning to become mainstream while a few are yet to “arrive”. For example the network system originally developed by the Digital Corporation®, Intel® and the Xerox® Corporation was called Ethernet and the committee formed to deal with the standards for this type of network architecture was the 802.3 subcommittee.

Another was formed to handle wireless networking communications. This was the 802.11 subcommittee and just to illustrate the manner in which the number of changes that have occurred as these architectures have evolved over the years the 802.11 subcommittee has appended the 802.11 with a letter to distinguish one set of wireless networking technology standards from the others. The first was called 802.11a and we are currently seeing the finalisation and market implementations of the 802.11n standard today. That makes a total of 14 major standards of wireless networking technologies in less than 28 years.

Well now we get to the point where we have all sorts of networks and a variety of transmission media what is what and which do I use? These are the questions that we will begin to answer now.

Transmission Media

The cables used in cabled networks are usually either copper-based or fiber-optic transmission medium, the architecture, topology, protocol(s), and size of the network will determine which is chosen and what variety of each is preferential.

I would be amiss not to mention that there could be a mix and match of cabling and other transmission media such as wireless. Different segments of a network may consist of different transmission media. Generally it is easier to separate out segments with different transmission so as that each type of media and each category within that type of medium are contiguous for that local section of the network. There are many reasons as to why this is a good idea and I will be discussing them as we progress.

An obvious example would be when mixing copper-based cable networks with wireless or even fiber-optic cable network sections. It is not too difficult to understand how each of these different transmission media will propagate a signal in different ways and hence will have different specifications and physical and electronic requirements that need to be satisfied for an effective transmission to take place.

With respect to the various different varieties (or flavours if you will) of each type of media for now let us just accept that the same holds true and consider it to be fact. I will explain the reasons in greater detail later.

In Part three of the Network Cabling Guide we will dive into copper cabling technologies so see you soon.