Data Communication and Computer Networks

[EEE 314]

Mubashir Husain RehmaniMubashir Husain Rehmani

Assistant Professor

Department of Electrical Engineering

COMSATS Institute of Information Technology,

Wah Cantt, Pakistan

Wired LAN

Ethernet

Data Link

Layer

Position of Data Link Layer

LLC and MAC

IEEE standards for LAN

Data Link Layer

� Data Link Layer consists of two sub-layers

� Logical Link Control (LLC) Layer

� Medium Access Control (MAC) Layer

� We may not have a dedicated link (channel) for � We may not have a dedicated link (channel) for

communication

� Upper layer (LLC) is responsible for

� Data link control (flow or error control)

� Lower layer (MAC) is responsible for

� Medium access resolution

Multiple Access to a Link

� Multicast link or Broadcast link

� Nodes are connected with single link

� We need multiple access protocol

To coordinate access to the link� To coordinate access to the link

� Two devices do not send data at the same time

� Two devices do not interrupt each other

� A device do not monopolize the link

Multiple

Access

Multiple Access Protocols

Random Access

� Also called ‘contention methods’

� No device is superior to another device

� No station permits another station to send data

� At each instance, when a station has data to send, it willAt each instance, when a station has data to send, it will

make a decision by itself on whether or not to send

� This decision is based on the state of the medium

� Why we call Random Access or Contention Methods?

� There is no scheduled time for a station to transmit

� Transmission is random among the stations

� No rules say which station should send next

� Stations compete with each other

Random Access

� In Random Access, each station has a right to send

� However, if more than one station tries to send, collision

will occur

� In order to avoid this collision problem, each station will

answer these questions

� When can the station access the medium?

� What can the station do if the medium is busy?

� How can the station determine the success or failure of the transmission?

� What can the station do if there is an access conflict?

ALOHA Protocol

� Random access evolved from ALOHA Protocol

� It uses a simple procedure called ‘Multiple Access (MA)’

� MA is further improved by forces the station to sense the MA is further improved by forces the station to sense the

medium before transmitting

� Called Carrier Sense Multiple Access (CSMA)

� CSMA evolved into two methods:

� CSMA/CD: Carrier Sense Multiple Access / Collision Detection

� Tells the station what to do when a collision is detected

� CSMA/CA: Carrier Sense Multiple Access / Collision Avoidance

� Tries to avoid the collision

Evolution of Random Access

Methods

ALOHA Protocol

� Developed at University of Hawaii in early1970

� Original ALOHA protocol is called as Pure ALOHA

� Slotted ALOHA was invented to improve the performance of Pure � Slotted ALOHA was invented to improve the performance of Pure ALOHA

ALOHA Network

Pure ALOHA

Slotted ALOHA

Some Questions� A pure ALOHA network transmits 200-bit frames on a shared

channel of 200 kbps. What is the requirement to make this frame collision-free?

� A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the throughput if the system (all stations together) produces

� 1000 frames per second

� 500 frames per second

� 250 frames per second

� A slotted ALOHA network transmits 200-bit frames using a shared channel with a 200-kbps bandwidth. Find the throughput if the system (all stations together) produces

� 1000 frames per second

� 500 frames per second

� 250 frames per second

Random Access

MA

CSMA

CSMA/CD

CSMA/CA

Collision in CSMA

Persistence Strategies

CSMA/CD Procedure

CSMA/CA Procedure

Controlled Access

• Stations consult each other to find which station has the

right to send

• A station can not send unless it has been authorized by

other stations

Reservation: make reservation before sending data

Polling: Select and Poll functions are used

Token Passing: Network is organized in logical ring

other stations

Select

Poll

Token Passing Network

Token Passing Procedure

Channelization

Channelization is a multiple-access method in which the

available bandwidth of a link is shared in time, frequency, or

through code, between different stations.

FDMA

TDMA

CDMA

In FDMA, the bandwidth is divided into

channels.

In TDMA, the bandwidth is just one

channel that is timeshared.

In CDMA, one channel carries all

transmissions simultaneously.

Project 802

Project 802

Ethernet Address in

Hexadecimal Notation

A Network with and without a

Bridge

Collision Domain

Switched Ethernet

Fast Ethernet