UNIT II
Medium access – CSMA – Ethernet – Token ring – FDDI - Wireless LAN – Bridges and Switches
Token Ring:
The Token Ring network was originally developed by IBM
in the 1970s. It is still IBM's primary local-area network (LAN) technology.
The related IEEE 802.5 specification is almost identical to and completely
compatible with IBM's Token Ring network. In fact, the IEEE 802.5 specification
was modeled after IBM Token Ring, and it continues to shadow IBM's Token Ring
development. The term Token Ring generally is used to refer to both IBM's Token
Ring network and IEEE 802.5 networks.
Token Ring Operation
Token Ring and IEEE 802.5 are two principal
examples of token-passing networks (FDDI is the other). Token-passing networks
move a small frame, called a token, around the network. Possession of the token
grants the right to transmit. If a node receiving the 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.
If a station possessing the token does have
information to transmit, it seizes the token, alters 1 bit of the token (which
turns the token into a start-of-frame sequence), appends the information that
it wants to transmit, and sends this information to the next station on the
ring. While the information frame is circling the ring, no token is on the
network (unless the ring supports early token release), which means that other
stations wanting to transmit must wait. Therefore, collisions cannot occur in
Token Ring networks. If early token release is supported, a new token can be
released when frame transmission is complete.
The information frame circulates the ring until
it reaches the intended destination station, which copies the information for
further processing. The information frame continues to circle the ring and is
finally removed when it reaches the sending station. The sending station can
check the returning frame to see whether the frame was seen and subsequently
copied by the destination.
Unlike CSMA/CD networks (such as Ethernet),
token-passing networks are deterministic, which means that it is possible to
calculate the maximum time that will pass before any end station will be
capable of transmitting.
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. After the
token is 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 pass around the network. When the next token is generated,
it includes the higher priority of the reserving station. Stations that raise a
token's priority level must reinstate the previous priority after their transmission
is complete.
Fault-Management Mechanisms
Token
Ring networks employ several mechanisms for detecting and compensating for
network faults. For example, one station in the Token Ring network is selected
to be the active monitor. This station, which potentially can be any station on
the network, acts as a centralized source of timing information for other ring
stations and performs a variety of ring-maintenance functions. One of these
functions is the removal of continuously circulating frames from the ring. When
a sending device fails, its frame may continue to circle the ring. This can
prevent other stations from transmitting their own frames and essentially can
lock up the network. The active monitor can detect such frames, remove them
from the ring, and generate a new token.
Frame Format
Token Ring and IEEE
802.5 support two basic frame types: tokens and data/command frames. Tokens are
3 bytes in length and consist of a start delimiter, an access control byte, and
an end delimiter. Data/command frames vary in size, depending on the size of
the Information field. Data frames carry information for upper-layer protocols,
while command frames contain control information and have no data for
upper-layer protocols.
Token Frame Fields
The three token frame fields illustrated in Figure 9-3 are summarized in the descriptions that follow:
Start delimiter -
Alerts each station of the arrival of a token (or data/command frame). This
field 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 -
Contains the Priority field (the most significant 3 bits) and the Reservation
field (the least significant 3 bits), as well as a token bit (used to
differentiate a token from a data/command frame) and a monitor bit (used by the
active monitor to determine whether a frame is circling the ring endlessly).
End delimiter -
Signals the end of the token or data/command frame. This field also contains
bits to indicate a damaged frame and identify the frame that is the last in a
logical sequence.
Data/Command Frame Fields
Data/command frames have the same three fields as Token Frames, plus several others. The Data/command frame fields illustrated in Figure 9-3 are described in the following summaries:
- Start delimiter - Alerts each station of the arrival of a token (or data/command frame). This field 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 - Contains the Priority field (the most significant 3 bits) and the Reservation field (the least significant 3 bits), as well as a token bit (used to differentiate a token from a data/command frame) and a monitor bit (used by the active monitor to determine whether a frame is circling the ring endlessly).
- Frame-control bytes - Indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information.
- Destination and source addresses - Consists of two 6-byte address fields that identify the destination and source station addresses.
- Data - Indicates that the length of field is limited by the ring token holding time, which defines the maximum time a station can hold the token.
- Frame-check sequence (FCS) - Is filed by the source station with a calculated value dependent on the frame contents. The destination station recalculates the value to determine whether the frame was damaged in transit. If so, the frame is discarded.
- End Delimiter - Signals the end of the token or data/command frame. The end delimiter also contains bits to indicate a damaged frame and identify the frame that is the last in a logical sequence.
- Frame Status - Is a 1-byte field terminating a command/data frame. The Frame Status field includes the address-recognized indicator and frame-copied indicator.
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