Advanced Token Ring

 


This page explores the more advanced subjects in Token Ring networking. Some of the areas addressed are Ring Extention, Bridging and Switching, and Jitter Removal. Each subject has its own section within this page.


Ring Extention

Sometimes, despite the best efforts of the designer, a Token Ring layout for a particular environment just can not be made to cover the distances necessary. This results in a situation where the ring must be extended somehow. Some ways of accomplishing this are:

Copper Ring Extenders
These are devices which act as a repeater at a MSAU's Ring In and Ring Out ports. They serve to amplify the electrical signal to its normal levels after it has lost some power due to normal attenuation in the cables and devices present on the network. They usually allow a cable run of 500 meters (1650 feet) at 4 Mbps or 100 meters (330 feet) at 16 Mbps. Note that so long as the cable between the extenders does not exceed the extenders' distance rating, that cable run does not count in our normal ARL calculations. It is possible to create a ring with an ARL of zero (0) if these units are used between all wiring closets.
 
Fiber Optic Ring Extenders
These are basically the same type of product as the Copper Ring Extenders, except that they use fiber optic cable instead of STP or UTP to interconnect the wiring closets. They generally allow a distance of up to 2000 meters (6600 feet) which does not count toward the ARL.
 
MSAU With Ring Port Repeater
This is an Active MSAU which has Copper or Fiber Ring Extenders built in to its Ring In and Ring Out ports. This provides the benefit of using stand alone extenders, but with more convenience and usually lower cost that a separate MSAU and Extender.

It is important to note that none of the above solutions will allow a ring which has more than 250 nodes on STP cable or 72 nodes on UTP cable. If more devices are needed on one network, refer to the Bridging & Switching section below.


Bridging & Switching

Whenever it is necessary to split a Token Ring network into two or more rings, a bridge or switch is necessary. Basically, both products perform similar functions, however a switch is more advanced and is preferable to a bridge. Below is an explanation of each:

Bridge

A bridge is a device which connects two physical networks and creates one logical network. It also functions to forward frames from one ring to another only if they need to be sent across. There are three types of bridges:
 
Source Routing - These bridges require that all nodes on all rings know the route to take to get a frame to a particular destination node, and include this information in every frame sent. The bridges do not learn which nodes are on which rings. Source Routing bridges can be a little faster than any other type as they do not need to look up the destination for each frame - the transmitting node handles this for the bridges. However, the overhead added by the source routing protocol negates this effect, and can result in a network which must carry far more traffic and in turn operates somewhat more slowly than networks built around other types of bridges.
 
Transparent - Transparent bridges do not require that the nodes on any ring know the route to take to get a frame to a particular destination node. These bridges learn the location of every node on the network, and forward frames which come in on one of their ports to destinations on another of their ports. They do not forward a frame if both the source and destination nodes are on the same bridge port. These bridges are the easiest to use, and usually result in the best performance, as they eliminate the overhead associated with source routing.
 
Source Route Transparent - These bridges are merely units which support both source routing and transparent bridging. This allows the user to use them for any Token Ring bridging application.
It is important to note that most bridges only have two ports. If it is necessary to interconnect more than two rings, than more than one bridge will be needed. This usually results in a situation where traffic must flow through, and take bandwidth from, several other rings between the tranmitting and recieving nodes. This can result in congestion on the network, and is the reason for the invention of Token Ring switches.
 
Token Ring Switches
A more advanced product than a simple bridge, a Token Ring switch is a device which interconnects multiple rings with a high speed backplane. It can be thought of as a multi-port bridge. These units usually are Source Route Transparent units, and can be used both with or without source routing protocol software on the individual nodes. They also normally allow the nodes to be connected to a 100 Mbps FDDI backbone. Token Ring switching is very similar to Ethernet switching, and one should consult our page on Ethernet switching for a quick primer on switched network fundamentals.

Jitter Removal

Occasionally, a Token Ring network is not limited by distance or the number of nodes, rather it runs into a problem of excessive jitter. Since Token Ring is a fully repeated network, with every node acting as a repeater, it is more likely to develop a problem with jitter than most other types of networks.