Post on 22-Jan-2018
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