computer networks unit-7

8/7/2019 Computer Networks UNIT-7

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C o n f i d e n t i a l

Course : BCA

Semester : IV

Subject Code : BC 0048

Subject Name : Computer Networks

Unit number : 7

Unit Title : Medium Access Control Sub Layer

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Unit-7 Medium Access Control Sub Layer

Medium Access Control Sub Layer

Objectives

After going through the presentation, you should be able to:

Describe static and dynamic channel allocations.

Discuss Multiple access protocols

Describe IEEE standards

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Lecture outline

Introduction

The channel allocation problem

Multiple access protocols

IEEE standards

Summary

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Medium Access Control Sub Layer





Introduction

The data link layer is overloaded, it is split into MAC and LLC sub layers.

MAC sub-layer is the bottom part of the data link layer.

Medium access control is often used as a synonym to multiple access

protocol, since the MAC sub layer provides the protocol and control

mechanisms that are required for a certain channel access method.

The MAC layer is essentially important in local area networks (LAN¶s),

many of which use a multi-access channel as the basis for

communication. WAN¶s in contrast use a point to point networks.

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We can classify the channels as static and dynamic.

The static channel is where the number of users are stable and the traffic

is not bursty.

When the number of users using the channel keeps on varying the

channel is considered as a dynamic channel. The traffic on these dynamic

channels also keeps on varying.

Static channels allocation :

The usual way of allocating a single channel among the multiple users is

frequency division multiplexing (FDM). If there are N users, the

bandwidth allocated is split into N equal sized portions.

FDM is simple and efficient technique for small number of users.

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Dynamic channels allocation in LAN¶s and MAN¶s :

When the number of users using the channel keeps on varying the

channel is considered as a dynamic channel.

The traffic on these dynamic channels also keeps on varying. For

example:

In most computer systems, the data traffic is extremely bursty. We see

that in this system, the peak traffic to mean traffic ratios of 1000:1 are

common.

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There are many protocolsfor allocating a multiple access channel.

They are

1. Pure or Unslotted Aloha

2. Slotted or Impure ALOHA

3. CSMA Protocol

4. CSMA/CD Protocol

1. Pure or Unslotted Aloha :

The ALOHA network was created at the University of Hawaii in 1970

under the leadership of Norman Abramson.

The Aloha protocol is an OSI layer 2 protocol for LAN networks with

broadcast topology.

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1. Pure or Unslotted Aloha : ( CONTINUED..)

Figure : Vulnerable period for the node: frame

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1.Pure or Unslotted Aloha : ( CONTINUED..)

The Aloha protocol is an OSI layer 2 protocol used for LAN.

A user is assumed to be always in two states: typing or waiting. The

station transmits a frame and checks the channel to see f it was

successful.

If so the user sees the reply and continues to type. If the frame

transmission is not successful, the user waits and retransmits the frame

over and over until it has been successfully sent.

The throughput is

We get for G = 0.5 resulting in a maximum throughput of 0.184, i.e.

18.4%.

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GeG P GS

2

0..

!!






2. Slotted or Impure ALOHA :

An improvement to the original Aloha protocol was Slotted Aloha.

It is in 1972, Roberts published a method to double the throughput of an

pure ALOHA by uses discrete timeslots.

His proposal was to divide the time into discrete slots corresponding to

one frame time. This approach requires the users to agree to the frame

boundaries.

To achieve synchronization one special station emits a pip at the start of

each interval similar to a clock. Thus the capacity of slotted ALOHA

increased to the maximum throughput of 36.8%.

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2. Slotted or Impure ALOHA :( continued«)

In slotted aloha a station can send only at the beginning of a timeslot,

and thus collisions are reduced.

In this case, the average number of aggregate arrivals is G arrivals per

2X seconds. This leverages the lambda parameter to be G. The maximum

throughput is reached for G = 1.

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3. CSMA Protocol :

Carrier Sense Multiple Access (CSMA) is a probabilistic Media Access

Control(MAC) protocol in which a node verifies the absence of other

traffic before transmitting on a shared physical medium, such as an

electrical bus, or a band of electromagnetic spectrum.

Different CSMA protocols are

1. Non-Persistent CSMA :

Non-persistent CSMA is less greedy. The algorithm is given below:

Sense the channel.

IF the channel is idle, THEN transmit.

If the channel is busy, THEN wait a random amount of time and start

over.

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3. CSMA Protocol : ( continued..)

2. 1-Persistent CSMA :

1-Persistent CSMA is selfish. The algorithm is given below:

Sense the channel.

IF the channel is idle, THEN transmit.

IF the channel is busy, THEN continue to listen until channel is idle. Nowtransmit immediately.

3. P ± Persistent CSMA :

p ± persistent CSMA is a slotted approximation. The algorithm is given

below:

Sense the channel.

IF the channel is idle, THEN with probability p transmit and with

probability (1-p) delay for one time slot and start over.

IF the channel is busy, THEN delay one time-slot and start over.

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4. CSMA/CD Protocol :

In computer networking, Carrier Sense Multiple Access with Collision

Detection (CSMA/CD) is a network control protocol in which a carrier

sensing scheme is used.

A transmitting data station that detects another signal while transmitting

a frame, stops transmitting that frame, transmits a jam signal, and then

waits for a random time interval.

The random time interval also known as "backoff delay" is determined

using the truncated binary exponential backoff algorithm. This delay is

used before trying to send that frame again. CSMA/CD is a modification

of pure Carrier Sense Multiple Access (CSMA).

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4. CSMA/CD Protocol : ( continued..)

Collision detection is used to improve CSMA performance by terminating

transmission as soon as a collision is detected, and reducing the

probability of a second collision on retry.

CSMA/CD can be in anyone of the following three states as shown in

figure.1. Contention period 2. transmission period 3. Idle period

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IEEE standards

IEEE has standardized a number of LAN¶s and MAN¶s under the name of

IEEE 802.

Few of the standards are given below

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IEEE standards

Ethernets :

Ethernet was originally based on the idea of computers communicatingover a shared coaxial cable acting as a broadcast transmission medium.The methods used show some similarities to radio systems, althoughthere are major differences, such as the fact that it is much easier todetect collisions in a cable broadcast system than a radio broadcast.The common cable providing the communication channel was likened tothe ether and it was from this reference that the name "Ethernet" was

derived.The most kinds of Ethernets used were with the data rate of 10Mbps.

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IEEE standards

Fast Ethernet :

Fast Ethernet was introduced in 1995 and remained the fastest version of

Ethernet for three years before being superseded by gigabit Ethernet.

Fast Ethernet is a collective term for a number of Ethernet standards that

carry traffic at the nominal rate of 100 Mbit/s, against the original

Ethernet speed of 10 Mbit/s.

Ex :

100BASE-T is any of several Fast Ethernet standards for twisted pair

cables.

100BASE-TX (100 Mbit/s over two-pair Cat5 or better cable),

100BASE-T4 (100 Mbit/s over four-pair Cat3 or better cable, defunct),

100BASE-T2 (100 Mbit/s over two-pair Cat3 or better cable, also

defunct).

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IEEE standards

Gigabit Ethernet :Gigabit Ethernet (GbE or 1 GigE) is a term describing various

technologies for transmitting Ethernet packets at a rate of a gigabit per

second, as defined by the IEEE 802.3-2005 standard.

Gigabit Ethernet was the next iteration, increasing the speed to 1000

Mbit/s.

Different gigabits Ethernet are

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Name medium

1000BASE-T unshielded twisted pair

1000BASE-SX multi-mode fiber

1000BASE-LX single-mode fiber

1000BASE-CX balanced copper cabling

1000BASE-ZX single-mode fiber





IEEE standards

IEEE 802.3 Frame format :The frame format of IEEE 802.3 is shown below

Preamble field :

Each frame starts with a preamble of 8 bytes. Each containing bit

patterns ³10101010´.

Address field

The frame contains two addresses, one for the destination and for

the sender. The length of address field is 6 bytes.

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IEEE standards

IEEE 802.3 Frame format : ( continued«)

SOF:

This field is 1 byte long and is used to indicate the start of the frame.

Length:

This field is of 2 bytes long. It is used to specify the length of the data

in terms of bytes that is present in the frame. Data :

The length of this field ranges from zero to a maximum of 1500 bytes.

This is the place where the actual message bits are to be placed.

Pad:

If the data field is less than 46 bytes then the pad field comes into

picture. Such that total data and pad field must be equal to 46bytes

minimum. If the data field is greater than 46 bytes then pad field is not

used.

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IEEE standards

IEEE 802.3 Frame format : ( continued«)

Checksum:

It is 4 byte long. It uses a 32-bit hash code of the data. If some data

bits are in error, then the checksum will be wrong and the error willbe detected. It uses CRC method and it is used only for error

detection and not forward error correction.

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Summary

The data link layer split into MAC and LLC sub layers. MAC sub-layer is

the bottom part of the data link layer.

MAC sub layer provides the protocol and control mechanisms that are

required for a certain channel access method.

There are many algorithms for allocating a multiple access channelwhich are Pure or Unslotted Aloha , Slotted or Impure ALOHA ,

CSMA Protocol , CSMA/CD Protocol

There are standards defined for the LAN and MAN called as IEEE 802

standards.

The most important standards are Ethernet, token bus, token ring

Wireless local are network, wireless personal area network, wireless

sensor networks etc.

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