Introduction to Computer Networks Introduction to Computer Networks

Token Ring This lesson presents an overview of the major Token Ring components, features, and functions.

Overview of Token Ring IBM's version of Token Ring was introduced in 1984 for the entire range of IBM computers and computing environments.IBM's version of Token Ring was introduced in 1984 for the entire range of IBM computers and computing environments. The goal of IBM's version of Token Ring was to facilitate a simple wiring structure using twisted- pair cable that connects a computer to the network through a wall socket, with the main wiring located in a centralized location.The goal of IBM's version of Token Ring was to facilitate a simple wiring structure using twisted- pair cable that connects a computer to the network through a wall socket, with the main wiring located in a centralized location. In 1985, the IBM Token Ring became an American National Standards Institute (ANSI)/IEEE standard.In 1985, the IBM Token Ring became an American National Standards Institute (ANSI)/IEEE standard.

Token Ring Features A Token Ring network is an implementation of IEEE standard A Token Ring network is an implementation of IEEE standard Their token-passing ring access method, distinguishes Token Ring networks from other networks.Their token-passing ring access method, distinguishes Token Ring networks from other networks.

Architecture The architecture of a typical Token Ring network begins with a physical ring.The architecture of a typical Token Ring network begins with a physical ring. However, in its implementation, a star-wired ring, computers on the network are connected to a central hub. The logical ring represents the token's path between computers.However, in its implementation, a star-wired ring, computers on the network are connected to a central hub. The logical ring represents the token's path between computers.

Token Ring Basics A Token Ring network includes the following features:A Token Ring network includes the following features: Star-wired ring topologyStar-wired ring topology Token-passing access methodToken-passing access method Shielded and unshielded twisted-pair (IBM Types 1, 2, and 3) cablingShielded and unshielded twisted-pair (IBM Types 1, 2, and 3) cabling Transfer rates of 4 and 16 MbpsTransfer rates of 4 and 16 Mbps Baseband transmissionBaseband transmission specifications802.5 specifications

Frame Formats (1) Access control indicates the frame's priority and whether it is a token or a data frame.Access control indicates the frame's priority and whether it is a token or a data frame. Frame control contains either MAC information for all computers or "end station" information for only one computer.Frame control contains either MAC information for all computers or "end station" information for only one computer.

Frame Formats (2) Frame check sequence contains CRC error-checking information.Frame check sequence contains CRC error-checking information. Frame status tells whether the frame was recognized, copied, or whether the destination address was available.Frame status tells whether the frame was recognized, copied, or whether the destination address was available.

How Token Ring Networking Works When the first Token Ring computer comes online, the network generates a token.When the first Token Ring computer comes online, the network generates a token. The token is a predetermined formation of bits that permits a computer to put data on the cables.The token is a predetermined formation of bits that permits a computer to put data on the cables.

How Token Ring Networking Works The token travels around the ring polling each computer until one of the computers signals that it wants to transmit data and takes control of the token.The token travels around the ring polling each computer until one of the computers signals that it wants to transmit data and takes control of the token. A computer cannot transmit unless it has possession of the token.A computer cannot transmit unless it has possession of the token.

How Token Ring Networking Works After the computer captures the token, it sends a data frame out on the network.After the computer captures the token, it sends a data frame out on the network. The frame proceeds around the ring until it reaches the computer with the address that matches the destination address in the frame.The frame proceeds around the ring until it reaches the computer with the address that matches the destination address in the frame.

How Token Ring Networking Works The destination computer copies the frame into its receive buffer;The destination computer copies the frame into its receive buffer; and marks the frame in the frame status field to indicate that the information was received.and marks the frame in the frame status field to indicate that the information was received.

How Token Ring Networking Works The frame continues around the ring until it arrives at the sending computer, where the transmission is acknowledged as successful.The frame continues around the ring until it arrives at the sending computer, where the transmission is acknowledged as successful. The sending computer then removes the frame from the ring and transmits a new token back on the ring.The sending computer then removes the frame from the ring and transmits a new token back on the ring.

How Token Ring Networking Works Token passing is deterministic, which means that a computer cannot force its way on to the network as it can in a CSMA/CD environment.Token passing is deterministic, which means that a computer cannot force its way on to the network as it can in a CSMA/CD environment. If the token is available, the computer can use it to send data.If the token is available, the computer can use it to send data. Each computer acts as a unidirectional repeater, regenerates the token, and passes it along.Each computer acts as a unidirectional repeater, regenerates the token, and passes it along.

Monitoring the System The first computer to come online is assigned by the Token Ring system to monitor network activity.The first computer to come online is assigned by the Token Ring system to monitor network activity. The monitoring computer makes sure that frames are being delivered and received correctly.The monitoring computer makes sure that frames are being delivered and received correctly. It does this by checking for frames that have circulated the ring more than once and ensuring that only one token is on the network at a time.It does this by checking for frames that have circulated the ring more than once and ensuring that only one token is on the network at a time.

Beaconing The process of monitoring is called beaconing.The process of monitoring is called beaconing. The active monitor sends out a beacon announcement every seven seconds.The active monitor sends out a beacon announcement every seven seconds. If a station does not receive an expected announcement from its upstream neighbor, it attempts to notify the network of the lack of contact.If a station does not receive an expected announcement from its upstream neighbor, it attempts to notify the network of the lack of contact. It sends a message that includes its address, the address of the neighbor that did not announce, and the type of beacon. From this information, the ring attempts to diagnose the problem and make a repair.It sends a message that includes its address, the address of the neighbor that did not announce, and the type of beacon. From this information, the ring attempts to diagnose the problem and make a repair.

Recognizing a Computer When a new computer comes online on the network, the Token Ring system initializes it so that it can become part of the ring.When a new computer comes online on the network, the Token Ring system initializes it so that it can become part of the ring. This initialization includes:This initialization includes: Checking for duplicate addresses.Checking for duplicate addresses. Notifying other computers on the network of its existence.Notifying other computers on the network of its existence.

Hardware Components: The Hub Hardware for Token Ring networks is centered on the hub, which houses the actual ring.Hardware for Token Ring networks is centered on the hub, which houses the actual ring. A Token Ring network can have multiple hubs.A Token Ring network can have multiple hubs. In a Token Ring network, the hub is known by several names as:In a Token Ring network, the hub is known by several names as: MAU (Multistation Access Unit).MAU (Multistation Access Unit). MSAU (MultiStation Access Unit).MSAU (MultiStation Access Unit). SMAU (stands for Smart Multistation Access Unit).SMAU (stands for Smart Multistation Access Unit).

Hardware Components: The Hub Cables attach the individual clients and servers to the MSAU, which works like other passive hubs.Cables attach the individual clients and servers to the MSAU, which works like other passive hubs. The internal ring automatically converts to an external ring at each connection point when a computer is connected.The internal ring automatically converts to an external ring at each connection point when a computer is connected.

Hub Capacity An IBM MSAU has 10 connection ports. It can connect up to eight computers. Each ring can have up to 33 hubs.An IBM MSAU has 10 connection ports. It can connect up to eight computers. Each ring can have up to 33 hubs.

Hub Capacity Each MSAU-based network can support as many as 72 computers that use unshielded wire; or up to 260 computers that use shielded wire.Each MSAU-based network can support as many as 72 computers that use unshielded wire; or up to 260 computers that use shielded wire.

Built-in Fault Tolerance MSAUs were designed to detect when a NIC fails, and to disconnect from it.MSAUs were designed to detect when a NIC fails, and to disconnect from it. This procedure bypasses the failed computer so that the token can continue on.This procedure bypasses the failed computer so that the token can continue on. Therefore, a faulty computer or connection will not affect the rest of the Token Ring network.Therefore, a faulty computer or connection will not affect the rest of the Token Ring network.

Cabling The STP or UTP cable to a hub connects computers on a Token Ring network.The STP or UTP cable to a hub connects computers on a Token Ring network.

The Fate of Token Ring Networks Token Ring was losing market share to Ethernet.Token Ring was losing market share to Ethernet. Even though Ethernet is more popular, Token Ring is good at supporting mission-critical applications.Even though Ethernet is more popular, Token Ring is good at supporting mission-critical applications. Token Ring users today face lots of challenges:Token Ring users today face lots of challenges: Complexities, manageability, cost, space, congestion, and upgrading to high-speed technologies. Complexities, manageability, cost, space, congestion, and upgrading to high-speed technologies. The idea of switching is to provide high- performance, low-cost alternatives.The idea of switching is to provide high- performance, low-cost alternatives.