We continue our look into the world of the Network Operating System (NOS) by examining Ring, Tree and Mesh topologies. Since I discussed much about the Ring Topology last time, I will include the mesh Topology here to conclude this short series of articles introducing network operating system topologies.
Both the peer-to-peer network operating system model and client/server network operating system model have seen deployment implementations using a these topologies.
While looking at the Direct Link Topologies we learned how they rapidly become disproportionately expensive once more than three devices need to be connected. Officially, this is a type of Mesh Topology, as you will soon learn. So let us to it starting with the various Ring Topologies.
The Ring Topology Network
With a Ring Topology network, each node (Servers, Workstations, and Peripherals) connects directly to both of its immediately adjacent nodes. The cable circles around the nodes connecting each in turn before eventually connecting to the first node again.
Now we have all nodes connected in one complete self-contained loop and this is the reason that this type of enclosed loop physical connectivity structure is a Ring Topology.
In the previous article entitled “Network Operating System Star Topology”, I discussed the topic of physically star-wired logical ring topologies. I also introduced IBM's proprietary Token Ring Topology. I shall add a little more as this is the best way of explaining the issues concerned with Ring Topologies.
Token Ring
IBM developed the Token Ring protocol in the mid-1980s. As the name Token Ring (see Figure TR 1) suggests a closed loop (or circle), wiring schema connects all member machines of the Token Ring network segment. This “ring” must remain intact for communications to be possible. If the “ring” breaks then no all machine-to-machine communication is possible.
In the previous article “Network Operating System Star Topology”, we learned that a ring topology is very difficult to install and configure. It also poses the issue of multiple single-points-of-failures. In order to address these issues IBM created a special network device they called a Multi Station Access Unit (MSAU). Now each node connects directly to a centrally placed Multi Station Access Unit (MSAU).
Externally the MSAU looks just like any other hub or switch. Internally at the hardware level however, the Multi-Station Access Unit (MSAU) contains wiring which allows information to pass from one device to another in a logical circle or ring. This reduced the multiple single-points-of-failures to just one, the MSAU.
Technically speaking IBM's Token Ring protocol uses a star-wired ring topology with the Token Ring Multi-Station Access Unit (MSAU) is the heart of the Token Ring universe. The signal travels from node to MSAU, then to the next node, then back to the MSAU, then to the next node and then back to the MSAU and so and on it goes.
Token Ring Network Media Access
Probably the most defining feature of the Token Ring network is the manner in which machines gain access to the transmission medium. The access method involved in a Token Ring network uses token-passing access.
A special “electronic token” (special frame) is circulated around the network. Each node will take this “token” frame from the media and if not wanting to transmit the node will place the “token” frame back onto the transmission medium.
When an empty token reaches a computer that wishes to transmit, the computer attaches the data to the “token” and then places the entire package (“token” and data) back onto the transmission medium.
The “token” with the data then circulates until it eventually arrives at the intended destination node, which then captures the data. If it needs to reply and can do so immediately it does so.
If not the computer that just received the “token” and data package, places an empty “token” onto the transmission medium and so the whole process goes on.
One of the biggest advantages of this system was that it made collisions impossible. There would never be two machines attempting to communicate at the once since only one machine could hold the “token” at a time.
Token Ring Topology Summary
- Transmission Media - Star-wired ring using twisted pair or fiber optic cable
- Transmission speeds of 4 Mbps or 16 Mbps
- In Token Ring Networks, the signal travels around the network from one computer to another in a Logical Ring
- Due to the increasing popularity of Ethernet, the use of Token Ring has decreased
Fiber Distributed Data Interface (FDDI)
FDDI is another ring topology but unlike Token Ring, it uses dual counter-rotating rings.
