16 Mbps Transfer Rate

The transfer rate for Token Ring is 4 Mbps for older systems or 16 Mbps for newer systems (1990 and newer). There are several products in development and available that will increase Token Ring's transfer rate using Switching Hubs and even faster transfer rates over existing cabling.
NOTE: 16 Mbps NIC cards will operate at both 16 and 4 Mbps speeds.
4 Mbps NIC cards will only operate at 4 Mbps.
To identify the speed of an unknown card, exam the integrated circuits on the card. There is only 1 chipset that implements IEEE 802.5's 4 Mbps standard for Token Ring. It was developed jointly by Texas Instruments and IBM. It is a 5 chip set and consists of:
TMS38051 Ring Interface Transceiver
TMS38052 Ring Interface Controller
TMS38010 Communications Protocol Processor
TMS38021 Protocol Handler for 802.5 Functions
TMS38030 DMA Controller between NIC and PC Bus 4 Mbps Token Ring NICs are usually full length expansion cards.
16 Mbps NICs have typically 1 large IC with 132 pins and several small ones. They are typically 1/2 length cards. The IC number is TMS380C16 for the Texas Instrument version or TROPIC for the IBM version or DP8025 for the National version.

IEEE 802.5 Topology

Token Ring is a Logical Ring / Physical Star topology. So far we've been only discussing the logical portion. Nodes on the network are physically connected via their NICs to a central concentrator or hub. The concentrator is called a MAU or MSAU both stand for MultiStation Access Unit. To avoid confusion with Ethernet MAUs, we will refer to a Token Ring hub as a MSAU (pronounced "M sow") or as a concentrator.

MSAUs

A Token Ring MSAU has connections to connect to the nodes and it also has special connections called Ring In and Ring Out to connect to other MSAUs

The Ring In connector is abbreviated RI and the Ring Out connector is abbreviated RO. The nodes (PCs) would be attached to connectors 1 to 8 for this 8 node MSAU.

45d. IEEE 802.5 Bus Arbitration (cont'd)

When the packet arrives at node G, node G reads the destination address and reads the information. Node G marks the information packet as read and passes it on.

Note: the Source and Destination addresses remain unchanged after passing through Node G. Node B is still the Source address and Node G is still the Destination address.
The packet continues around the ring, until it reaches the source address Node B. Node B checks to make sure that the packet has been read - this indicates that Node G is actually present. The information packet is erased. Node B then releases the token onto the ring.

Information marked READ is passed through the ring back to the Source - Node B
The information packet is called the Token Frame. The token is called the Token (sometimes referred to as the free token). This can be confusing. Remember, when we talk about a frame, we are talking about data/information. When talking about a token, we are talking about bus arbitration and permission to use the bus.

45d. IEEE 802.5 Bus Arbitration (cont'd)

The token is a special packet, that is circulated around the ring. It is read from one node than passed to the next node until it arrives at a node that needs to access the ring (transfer information/data). When a node receives the token, the node is allowed to send out its information packet.

Example: The token is circulating the ring, Node B needs to send some data to Node G. Node B waits for the token to come by. There is only one token allowed on the ring. When it receives the token, it can then send out its information packet. Node G is the destination address.

Node C receives the packet, reads the destination address and passes it on to the next node. Node D, E & F do likewise.

IEEE 802.4 Token Bus

An industrial version of Token Ring is standardized under IEEE 802.4 Token Bus. It is used in manufacturing process equipment for plant operation. It is used in automobile plants for computerized assembly. It uses a Logical Ring and a Physical Bus topology.

IEEE 802.5 Token Ring

IEEE 802.5 Token Ring standard is based on the IBM Token Ring network. Token Ring has been used mainly in large corporations and was considered in the past to be the only way to handle data communications in large networks (1000+) nodes.
Token Ring equipment is more expensive than Ethernet and is one of the reasons that Ethernet is more popular. The other reason is that Token Ring is much more complex bus arbitration method than CSMA/CD and few network personnel understand the full capabilities of Token Ring.

IEEE 802.5 Bus Arbitration

Token Ring is a token passing bus arbitration. A token is circulated on the ring. If a node on the ring needs to access the ring (transfer information), it claims the token.

Token Ring

STOP - You are now leaving Ethernet IEEE 802.3 Please fasten your seatbelts and place your trays in the fully upright position
Token Ring is a token passing bus arbitration topology for the Physical and Data Link Layers. It is a logical ring and a physical star topology.

Token Ring uses a token passing scheme for bus arbitration. A special packet is passed around the ring called a token. When a node requires access to the ring, the node claims the token and then passes its information packet around the ring. All nodes read the destination address and if it is not addressed for them, the information packet is then passed on to the next node. When the destination node reads the packet, it marks it as read and passes it on to the next node. When the information packet completely circulates the ring and arrives back at the source node, the source node releases the token back on to the ring.
Token Rings are not usually drawn as the above drawing indicates: a separate line between each node. They are usually represented as understood that separate paths exist between nodes and are drawn as in the figure to the right.

IBM Token Ring

Token Ring was originally developed by IBM for their PC LAN networks. It started out in 1969 as the Newhall Network, named after the originator of the token ring concept. IBM's Token Ring is the basis for the IEEE 802.5 standard Token Ring. They are very similar and have minor differences which we will cover.

Brouters (Bridge/Routers)

Brouters are protocol dependant devices. When a brouter receives a frame to be forwarded to the remote segment, it checks to see if it recognizes the Network layer protocol. If the Brouter does, it acts like a router and finds the shortest path. If it doesn't recognize the Network layer protocol, it acts like a bridge and forwards the frame to the next segment.

The key advantage to Brouters is the ability to act as both a bridge and a router. It can replace separate bridges and routers, saving money. This is, of course, provided that the Brouter can accomplish both functions satisfactorily.

EGRP - Exterior Gateway Routing Protocol

EGRP was created to solve many of the problems with RIP and has become the default routing protocol across the Internet. EGRP is an enhanced distance vectoring protocol, it uses up to 5 metrics (conditions) to determine the best route:
Bandwidth
Hop Count (Delay) - maximum of 255
Maximum Packet size
Reliability
Traffic (Load)
These routing metrics are much more realistic indicators of the best routes compared to simple hop counts

OSPF - Open Shortest Path First

OSPF is a link state premise, this means that it has several states of routers linked together in a hierarchical routing model:

The top of the root is the Autonomous Router, it connects to other autonomous systems (the Internet). The next is the Backbone Routers, which is the highest area in the OSPF system. Border routers are attached to multiple areas and run multiple copies of the routing algorithm. Last is internal routers which run a single routing database for one area.
Basically, by dividing the network into a routing hierarchy, substantial reduction of routing update traffic and faster route convergence results on a local basis. Each level has a smaller routing table and less to update.