The Token Ring network was
originally developed by IBM in the 1970s. It is still IBM's primary
local area network (LAN) technology and is second only to Ethernet/IEEE
802.3 in general LAN popularity. 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. This chapter addresses both Token Ring and IEEE 802.5.
Token
Ring and IEEE 802.5 networks are basically
compatible, although the specifications differ in minor ways. IBM's
Token Ring network specifies a star, with all end
stations attached to a device called a multistation
access unit (MSAU). In contrast, IEEE 802.5 does not specify a
topology, although virtually all IEEE 802.5 implementations are based on
a star. Other differences exist, including media type (IEEE 802.5 does
not specify a media type, although IBM Token Ring
networks use twisted-pair wire) and routing
information field size. Figure 9-1 summarizes IBM Token
Ring network and IEEE 802.5 specifications.
Figure 9-1: Although
dissimilar in some respects, IBM's Token Ring Network
and IEEE 802.5 are generally compatible.
IBM Token Ring network stations are
directly connected to MSAUs, which can be wired together to form one
large ring (see Figure 9-2). Patch cables connect MSAUs to adjacent
MSAUs, while lobe cables connect MSAUs to stations. MSAUs include bypass
relays for removing stations from the ring.
Figure 9-2: MSAUs can be
wired together to form one large ring in an IBM Token
Ring network.
Token Ring
and IEEE 802.5 are two principal examples of token-passing
networks (FDDI being 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 one bit of the
token, which turns the token into a start-of-frame sequence, appends the
information 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 able to transmit. This feature and
several reliability features, which are discussed in the section "Fault-Management
Mechanisms" later in this chapter, make Token Ring networks ideal
for applications where delay must be predictable and robust network
operation is important. Factory automation environments
are examples of such applications.
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.
Token Ring networks employ several
mechanisms for detecting and compensating for network faults. One
station in the Token Ring network, for example, 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.
The IBM Token Ring network's star
topology also contributes to overall network reliability. Because all
information in a Token Ring network is seen by active MSAUs, these
devices can be programmed to check for problems and selectively remove
stations from the ring if necessary.
A Token Ring algorithm called beaconing
detects and tries to repair certain network faults. Whenever a station
detects a serious problem with the network (such as a cable break), it
sends a beacon frame, which defines a failure domain. This domain
includes the station 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
in an attempt to reconfigure the network around the failed areas.
Physically, the MSAU can accomplish this through electrical
reconfiguration.
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. Both formats are shown in Figure
9-3.
Figure 9-3: IEEE
802.5 and Token Ring specify tokens and data/command frames.
The three token frame fields illustrated
in Figure 9-3 are summarized in the descriptions that follow:
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:
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