The previous chapter discussed LAN media and the IEEE model and how the data link layer provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses. This chapter introduces Layer 2 LAN technologies. Ethernet, Fiber Distributed Data Interface (FDDI), and Token Ring are widely used LAN technologies that account for virtually all deployed LANs. 

In this chapter, you will learn about Ethernet, FDDI, and Token Ring, along with the IEEE specifications for each of these technologies. You will also learn about the LAN standards that specify cabling and signaling at the physical and data link layers of the OSI reference model. You will also be introduced to Layer 2 devices and basic Ethernet 10BASE-T troubleshooting.

IBM developed the first Token Ring network in the 1970s. It is still IBM's primary LAN technology, and is second only to Ethernet (IEEE 802.3) in terms of LAN implementation. The IEEE 802.5 specification is almost identical to, and completely compatible with, IBM's Token Ring network. The IEEE 802.5 specification was modeled after IBM's Token Ring and continues to shadow its ongoing development. The term Token Ring refers both to IBM's Token Ring and to IEEE's 802.5 specification. The chart in the main graphic compares and contrasts the two standards.

Tokens
Tokens are 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter. The start delimiter alerts each station to the arrival of a token, or data/command frame. This field also includes signals that distinguish the byte from the rest of the frame by violating the encoding scheme used elsewhere in the frame.

Access Control Byte
The access control byte contains the priority and reservation field, and a token and monitor bit. The token bit distinguishes a token from a data/command frame, and a monitor bit determines whether a frame is continuously circling the ring. The end delimiter signals the end of the token or data/command frame. It contains bits that indicate a damaged frame, and a frame that is the last of a logical sequence.

Data/Command Frames
Data/command frames vary in size depending on the size of the information field. Data frames carry information for upper-layer protocols; command frames contain control information and have no data for upper-layer protocols.

In data/command frames, a frame control byte follows the access control byte. The frame control byte indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information.

Following the frame control byte are two address fields that identify destination and source stations. As with IEEE 802.5, their addresses are 6 bytes in length. The data field follows the address field. The length of this field is limited by the ring token that holds the time, thus defining the maximum time a station may hold the token.

Following the data field is the frame check sequence (FCS) field. The source station fills this field with a calculated value dependent on the frame contents. The destination station recalculates the value to determine whether the frame has been damaged in transit. The frame is discarded if it has been damaged. As with the token, the end delimiter completes the data/command frame.

Token Passing
Token Ring and IEEE 802.5 are the primary examples of token-passing networks. Token-passing networks move a small frame, called a token, around the network. Possession of the token grants the right to transmit data. If a node that receives a token has no information to send, it passes the token to the next end station. Each station can hold the token for a maximum period of time, depending on the specific technology that has been implemented.

When a token is passed to a host that has information to transmit, the host seizes the token and alters 1 bit of it. The token becomes a start-of-frame sequence. Next, the station appends the information to transmit to the token and sends this data to the next station on the ring. There is no token on the network while the information frame is circling the ring, unless the ring supports early token releases. Other stations on the ring cannot transmit at this time. They must wait for the token to become available. Token Ring networks have no collisions. If early token release is supported, a new token can be released when the frame transmission has been completed.

The information frame circulates around the ring until it reaches the intended destination station, which copies the information for processing. The information frame continues around the ring until it reaches the sending station, where it is removed. The sending station can verify whether the frame was received and copied by the destination.

Unlike CSMA/CD networks, such as Ethernet, token-passing networks are deterministic. This means that you can calculate the maximum time that will pass before any end station will be able to transmit. This feature, and several reliability features, makes Token Ring networks ideal for applications where any delay must be predictable, and robust network operation is important. Factory automation environments are examples of predictable robust network operations.

Priority System
Token Ring networks use a sophisticated priority system that permits certain user-designated, high-priority stations to use the network more frequently. Token Ring frames have two fields that control priority - the priority field and the reservation field.

Only stations with a priority equal to, or higher than, the priority value contained in a token can seize that token. Once the token has been seized and changed to an information frame, only stations with a priority value higher than that of the transmitting station can reserve the token for the next network pass. The next token generated includes the higher priority of the reserving station. Stations that raise a token's priority level must reinstate the previous priority when their transmission has been completed.

Management Mechanisms
Token Ring networks use several mechanisms for detecting and compensating for network faults. One mechanism is to select one station in the Token Ring network to be the active monitor. This station acts as a centralized source of timing information for other ring stations and performs a variety of ring maintenance functions. The active monitor station can potentially be any station on the network. One of this station’s functions is to remove continuously circulating frames from the ring. When a sending device fails, its frame may continue to circle the ring and prevent other stations from transmitting their frames, which can lock up the network. The active monitor can detect these frames, remove them from the ring, and generate a new token.

The IBM Token Ring network's physical star topology also contributes to the overall network reliability. Active MSAUs (multi-station access units) can see all information in a Token Ring network, thus enabling them to check for problems, and to selectively remove stations from the ring whenever necessary. Beaconing - a Token Ring formula - detects and tries to repair network faults. When a station detects a serious problem with the network (e.g. a cable break) it sends a beacon frame. The beacon frame defines a failure domain. A failure domain includes the station that is reporting the failure, its nearest active upstream neighbor (NAUN), and everything in between. Beaconing initiates a process called autoreconfiguration, where nodes within the failure domain automatically perform diagnostics. This is an attempt to reconfigure the network around the failed areas. Physically, MSAUs can accomplish this through electrical reconfiguration.

Signal encoding is a way of combining both clock and data information into a stream of signals that is sent over a medium. Manchester encoding combines data and clock into bit symbols, which are split into two halves, the polarity of the second half always being the reverse of the first half. Remember that the Manchester encoding results in 0 being encoded as a high-to-low transition and 1 being encoded as a low-to-high transition. Because both 0's and 1's result in a transition to the signal, the clock can be effectively recovered at the receiver.

The 4/16 Mbps Token-Ring networks use differential Manchester encoding (a variation on Manchester encoding). Token-Ring uses the differential Manchester encoding method to encode clock and data bit information into bit symbols. A 1 bit is represented by no polarity change at the start of the bit time and a 0 bit is represented by a polarity change at the start of the bit time.

IBM Token Ring network stations (often using STP and UTP as the media) are directly connected to MSAUs, and can be wired together to form one large ring. Patch cables connect MSAUs to other MSAUs that are adjacent to it. Lobe cables connect MSAUs to stations. MSAUs include bypass relays for removing stations from the ring. -