LAN STANDARDS
1.
802.2- (CSMA\CD) or
Ethernet
2. 802.4 – Token bus-based
3. 802.5 – Token ring-based
INTRODUCTION
The IEEE 802 committee came up with a bunch of
LAN standards collectively known as LANs as shown in Fig.1.
Figure.1 IEEE 802 Legacy LANs
ETHERNET (IEEE 802.3)
Ethernet refers to the family of LAN products covered by the IEEE
802.3 standard that defines the carrier sense multiple access collision detect
(CSMA/CD) protocol.
Each
Ethernet-equipped computer operates independently
of all other stations on the network: there is no central controller. All
stations attached to an Ethernet are connected to a shared signaling system,
also called the medium. To send data a station first listens to the channel,
and when the channel is idle the station transmits its data in the form of an
Ethernet frame, or packet.
After
each frame transmission, all stations on the network must contend equally for
the next frame transmission opportunity. Access to the shared channel is
determined by the medium access control (MAC) mechanism embedded in the
Ethernet interface located in each station. The medium access control mechanism
is based on a system called Carrier Sense Multiple Access with Collision
Detection (CSMA/CD).
As each
Ethernet frame is sent onto the shared signal channel, all Ethernet interfaces
look at the destination address. If the destination address of the frame
matches with the interface address, the frame will be read entirely and be
delivered to the networking software running on that computer. All other
network interfaces will stop reading the frame when they discover that the
destination address does not match their own address.
TOKEN RING (IEEE 802.5)
Originally, IBM developed
Token Ring network 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 on the same lines.
The term Token Ring is generally used to refer to both IBM's Token Ring
network and IEEE 802.5 networks.
Before going into the details of the Token Ring protocol,
let’s first discuss the motivation behind it. As already discussed, the medium
access mechanism used by Ethernet (CSMA/CD) may results in collision. Nodes
attempt to a number of times before they can actually transmit, and even when
they start transmitting there are chances to encounter collisions and entire
transmission need to be repeated. And all this become worse one the traffic is
heavy i.e. all nodes have some data to transmit. Apart from this there is no
way to predict either the occurrence of collision or delays produced by
multiple stations attempting to capture the link at the same time. So all these
problems with the Ethernet gives way to an alternate LAN technology, Token
Ring.
Token Ring and
IEEE802.5 are based on token passing MAC protocol with ring topology. They
resolve the uncertainty by giving each station a turn one by one. Each node
takes turns sending the data; each station may transmit data during its turn.
The technique that coordinates this turn mechanism is called Token passing; as
a Token is passed in the network and the station that gets the token can only
transmit. As one node transmits at a time, there is no chance of collision.
Stations are connected by point-to-point links using
repeaters. Mainly these links are of shielded twisted-pair cables. The
repeaters function in two basic modes: Listen mode, Transmit mode. A
disadvantage of this topology is that it is vulnerable to link or station
failure. But a few measures can be taken to take care of it.
Differences between Token Ring and IEEE 802.5
Both of these networks are basically compatible, although
the specifications differ in some ways.
IEEE 802.5 does not specify a topology,
although virtually all IEEE 802.5 implementations are based on the star
topology. While IBM's Token Ring network explicitly specifies a star, with all
end stations attached to a device called a Multi-Station Access Unit (MSAU).
IEEE 802.5 does not specify a media
type, although IBM Token Ring networks use twisted-pair wire.
There are few differences in routing
information field size of the two.
TOKEN RING OPERATION
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 immediately after a frame transmission is complete.
The information frame circulates around the ring until it
reaches the intended destination station, which copies the information for
further processing. The information frame makes a round trip 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 station in error-free form. Then the sending station inserts a
new free token on the ring, if it has finished transmission of its packets.
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. Token Ring networks are ideal for applications in which delay
must be predictable and robust network operation is important.
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.
RING MAINTENANCE
There are two error conditions that could cause the token
ring to break down. One is the lost token in which case there is no
token the ring, the other is the busy token that circulates endlessly.
To overcome these problems, the IEEE 802 standard specifies that one of the
stations be designated as ‘active monitor’. The monitor detects the lost
condition using a timer by time-out mechanism and recovers by using a
new free token. To detect a circulating busy token, the monitor sets a ‘monitor
bit’ to one on any passing busy token. If it detects a busy token with the
monitor bit already set, it implies that the sending station has failed to
remove its packet and recovers by changing the busy token to a free token.
Other stations on the ring have the role of passive monitor. The primary job of
these stations is to detect failure of the active monitor and assume the role
of active monitor. A contention-resolution is used to determine which station
to take over.
TOKEN BUS (IEEE 802.4)
Although Ethernet was widely used in the
offices, but people interested in factory automation did not like it because of
the probabilistic MAC layer protocol. They wanted a protocol which can support
priorities and has predictable delay. These people liked the conceptual idea of
Token Ring network but did not like its physical implementation as a break in
the ring cable could bring the whole network down and ring is a poor fit to
their linear assembly lines. Thus a new standard, known as Token bus, was developed,
having the robustness of the Bus topology, but the known worst-case behavior of
a ring. Here
stations are logically connected as a
ring but physically on a Bus and follows the collision-free token passing
medium access control protocol. So the motivation behind token bus protocol can
be summarized as:
·
The probabilistic nature of CSMA/ CD
leads to uncertainty about the delivery time; which created the need for a
different protocol
·
The token ring, on the hand, is very
vulnerable to failure.
·
Token bus provides deterministic
delivery time, which is necessary for real time traffic.
·
Token bus is also less vulnerable
compared to token ring.
FUNCTIONS OF A TOKEN BUS
It is the technique in which the station on bus or tree
forms a logical ring, that is the stations are assigned positions in an ordered
sequence, with the last number of the sequence followed by the first one as
shown in Fig.2. Each station knows the identity of the station following it and
preceding it.
Figure.2 Token Bus topology
A control packet known as a Token regulates the right
to access. When a station receives the token, it is granted control to the
media for a specified time, during which it may transmit one or more packets
and may poll stations and receive responses when the station is done, or if its
time has expired then it passes token to next station in logical sequence.
Hence, steady phase consists of alternate phases of token passing and data
transfer.
