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ACCESS PROTOCOL
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The fourth area in which
LANs are classified is according to the protocol that governs the
way individual stations access the transmission medium. Any number
of protocols can be developed, but the following three are the most
common:
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- CSMA/CD
This stands for Carrier Sense Multiple Access with Collision
Detection. It is also referred to as Listen While Talk (LWT).
Under this protocol, all nodes attached to the network monitor/
’listen’ to the transmission medium at all times. When a station
needs to transmit data, it waits until the medium is free and
then transmits. If two or more nodes transmit at the same time,
a collision results. Each node detects the collision and waits
for a random amount of time and then tries again to retransmit.
Since each station waits for a different amount of time, probability
that the collision will occur the second time is very low.
- CSMA/CA
This stands for Carrier Sense Multiple Access with Collision
Avoidance. It is similar to CSMA/CD except that all nodes
implement an algorithm that helps to avoid collisions rather
than simply detect, wait and then retransmit.
- TOKEN PASSING
This protocol is commonly used on ring and bus-structured networks.
A special message packet, called the Token is passed from one
node to the next node around the ring. When a node receives
the token, it either transmits a message if it has a message
to send, by capturing the token and then releases it for next
node after transmission is over, or it passes the unused token
to the next node on the ring. Each station receives one chance
to transmit during the time that it takes for the token to circulate
the ring.
The CSMA/CD protocol is well suited for use with bus-structured
networks. It works well because of the typically high transmission
speed of the LAN and because a very small percentage of the
total transmission capacity is used. When traffic on the LAN
increases, collisions begin to occur for a large number of messages
(packets), and hence performance of the LAN degrades.
With the token-passing protocol, a higher percentage of the
total capacity of the transmission medium can be used. However,
each station must wait for its turn before transmitting, and
thus token passing may be less efficient than the CSMA/CD protocol
on the networks that use a very low percentage of the total
transmission capacity.
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| LAN STANDARDS |
Some sort of the standardization
is always necessary for any technology or product line to be economically
and technically feasible. There are many different conceptual i.e.
logical correspondences and overall organizational details that
are not evident from physical examination of the system and physical
levels i.e. cable type, connector size, number of connections, and
so on at which standardization models can be constructed.
IEEE 802 STANDARDS is an important set of standards for local
area networks. It has been documented by the Institute of Electrical
and Electronics Engineers (IEEE). The IEEE LAN standards deal with
layers 1 and 2 of the OSI reference Model (i.e. physical, and Data
Link Layers). It divides the OSI link layer into two sub layers-Logical
Link Control (LLC) and Medium Access Control (MAC).
The MAC sub player performs the access functions for the particular
access control method employed by the network. Typical access control
methods are CSMA/CD and token passing, discussed earlier. The logical
link control sub player performs functions comparable to conventional
data link protocols. These include framing, addressing and error
control.
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LAN COMPONENTS
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| There are many devices
used in a LAN environment. A few are briefly explained below: |
- Network interface unit (NIU): This is an interface
unit, inside the node, which connects nodes to the LAN. This
is also known as LAN card or the Network Interface Card (NIC).
- Repeater: Repeaters are dumb (non-intelligent) devices,
which regenerate the signals received at its input, and make
it available to one or more output ports. It operates at the
OSI layer1; the physical layer. It is used to extend the maximum
transmission distance of the LAN media. A repeater connects
two similar LAN segments.
- Bridge: It is a device, which can be used to connect
two LANs at the data link layer of the OSI model. One can interconnect
Ethernet, token bus and token ring LANs together using a bridge.
This divides LANs into different collision domains and hence
increases the total bandwidth of the LAN.
- Switch: A switch is essentially a multiport device,
which forwards packets from one port to another. In case of
10Mbps Ethernet switch, each port supports dedicated 10Mbps
bandwidth. Ethernet switch is fast emerging as a replacement
of the traditional thick backbone and best way to improve performance
of the network.
- Router: A router is a multiport/ multiprotocol device,
which connects two different types of network at the OSI level3,
the network layer. It can be used to connect a LAN to a WAN.
For example, an X.25 packet switching network can be linked
with an Ethernet LAN by means of a router.
- Gateways: A gateway is a device, which connects two,
or more networks, which use entirely different architecture.
It operates at the application level (highest level). For example,
an Ethernet network can be connected to an SNA or a DNA network
with the help of a gateway.
- Terminal server: It is a multiport device, which connects
many asynchronous dumb terminals to the LAN. It can be considered
a gateway.
- Server: It is normally a powerful computer, which acts
as a resource center for other (generally smaller) computers
on the network. It provides services such as databases or software
libraries to other computer (clients) on the LAN.
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