Wired Lan Ethernet - DATA Link Layer
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Transcript of Wired Lan Ethernet - DATA Link Layer
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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
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Wired LAN
Ethernet
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Data Link
Layer
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Position of Data Link Layer
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LLC and MAC
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IEEE standards for LAN
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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
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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
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Multiple
Access
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Multiple Access Protocols
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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
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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?
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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
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Evolution of Random Access
Methods
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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
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ALOHA Network
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Pure ALOHA
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Slotted ALOHA
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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
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Random Access
MA
CSMA
CSMA/CD
CSMA/CA
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Collision in CSMA
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Persistence Strategies
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CSMA/CD Procedure
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CSMA/CA Procedure
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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
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Select
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Poll
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Token Passing Network
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Token Passing Procedure
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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
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In FDMA, the bandwidth is divided into
channels.
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In TDMA, the bandwidth is just one
channel that is timeshared.
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In CDMA, one channel carries all
transmissions simultaneously.
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Project 802
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Project 802
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Ethernet Address in
Hexadecimal Notation
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A Network with and without a
Bridge
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Collision Domain
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Switched Ethernet
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Fast Ethernet