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Local Area Networks and Medium Access Control. Computer Networks. Dr C. C. Constantinou ( originally prepared by N.J.Flowers). Reading. Chapter 6 of Leon-Garcia and Widjaja In particular §6.1, §6.2, §6.4, §6.4 Self-study §6.5 Chapters 3 and 4 of Tanenbaum Chapter 5 of Kurose and Rose

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Computer networks

Local Area Networks

and Medium Access Control

Computer Networks

Dr C. C. Constantinou

(originally prepared by N.J.Flowers)



  • Chapter 6 of Leon-Garcia and Widjaja

    • In particular §6.1, §6.2, §6.4, §6.4

    • Self-study §6.5

  • Chapters 3 and 4 of Tanenbaum

  • Chapter 5 of Kurose and Rose

    • Chapter 6 is on the peculiarities of wireless LANs – optional reading, but really important these days

Basic network types

Basic network types

  • Switched networks – multiple networks connected via multiplexers and switches which direct (route) packets from source to destination – usually Wide Area Networks

  • Broadcast networks – data is received by all receivers. Usually Local Area Networks

  • This lecture will concentrate on Broadcast Networks – also called Multiple Access Networks since the medium is shared

Broadcast networks

Broadcast Networks

  • Advantages

    • No routing

    • Simple, flat addressing scheme, hence low overhead

    • Cheap and simple

  • Disadvantages

    • Not scalable

    • Need some control – like in a meeting

Broadcast networks1

Broadcast Networks

  • Radio communications – walkie-talkies

  • Satellite communications

  • Mobile telephones

  • Coaxial cable networks

  • Bluetooth (2.4GHz radio)

  • Topology can be Ring, Star or Bus

Data link protocols

Directly connected, wire-like

Losses & errors, but no out-of-sequence frames

Applications: Direct Links; LANs; Connections across WANs

Data Links Services

Framing 

Physical addressing

Error control 

Flow control

Multiplexing 

Link Maintenance 

Security: Authentication & Encryption




Ethernet LAN

IEEE 802.11 (Wi Fi) LAN



Data link


Data link









Data Link Protocols



  • With broadcast networks we can have collisions when two transmissions occur at the same time and interfere

  • We need a protocol to prevent or minimise collisions

  • This is a Medium Access Control protocol – MAC

  • All devices that share the medium are in the same broadcast domain

  • All devices need to agree on the MAC and be coordinated even if not involved in the current message on the network

Mac schemes

MAC Schemes

  • Two basic ways to control access

  • Random Access – like a meeting without a chairman. Collisions occur but the protocol does something to fix it

  • Scheduling – slots are allocated to each device in turn like a meeting with a chairman

What is a collision

What is a collision?

  • When two stations transmit at the same time - but we need to consider the propagation delay

  • Even if the channel is empty collisions can occur

  • For a collision B must transmit between 0 and tprop but A doesn’t detect collision until 2tprop (worse case)

Setup time

Setup time

  • A must wait at least 2tprop before it knows the channel is free – this is the negotiation or coordination time

  • If bit rate is R bps, then setup time uses 2tpropR bits – these are effectively wasted.

  • If the average packet length is L, the efficiency in use of the channel is

Random access mac

Random access MAC

  • Simplest form is just to transmit when desired – don’t listen for silence first

  • First system was ALOHA – University of Hawaii needed to connect terminals on different islands

  • Used radio transmitters that send data immediately – this gives no setup delay

  • Transmitters detect collision by waiting for a response – if a collision occurs, there will be data corruption and the receiver says ‘send again’

  • Collisions result in complete re-transmission

  • For light traffic, low probability of collision so re-transmissions are infrequent



  • Problem is a collision involves at least two devices – both need to re-transmit

  • If both devices re-transmit immediately (or after the same delay) another collision will occur…. and again…. and again

  • ALOHA requires a random delay after collision before re-transmission

  • Since devices don’t listen for silence before transmission this delay must allow one transmitter to complete its transmission – delay is long to ensure this

  • Likelihood of collision is increased after each collision

Collision limit

Collision limit

  • For lightly loaded network, get very few collisions so throughput is high

  • As traffic increases, more and more collisions generate more and more collisions…… wasting bandwidth

Collision dominated

Collision dominated

  • For heavily loaded networks the collisions cause collisions and every packet takes many attempts to get through – intimately network becomes collision dominated and throughput goes down to zero

  • Peak throughput is 18.4% of channel capacity

Slotted aloha

Slotted ALOHA

  • Need to reduce collisions to improve throughput

  • Slotted ALOHA constrains stations to transmit in specific synchronised time slots

  • Time slots are all the same and packets occupy one slot

  • All devices share the slots – collisions are reduced since the can only occur at the start of the slot – cannot have a collision half way through a transmission

  • A ‘Don’t interrupt me once I’ve started’ protocol !

Slotted aloha1

Slotted ALOHA

  • Better performance under light load than pure ALOHA

  • Maximum throughput is 36.8%

Aloha problem

ALOHA problem

  • Channel bandwidth is wasted due to collisions

  • We can reduce collisions by avoiding transmissions that are certain to cause a collision

  • ALOHA transmits without first without listening to check the channel is free

  • Can sense the medium for presence of a signal before transmitting

  • Carrier Sense Multiple Access – CSMA MAC scheme

Computer networks


  • Station A transmits – as other stations detect the signal, they defer any transmissions

  • After tprop station A has captured the channel

  • Vulnerable period is t= tprop

Csma when to stop waiting

CSMA – when to stop waiting

  • If the channel is busy, station wishing to transmit waits until what happens?

  • 1-Persistent CSMA

    • Wait until channel is free and transmit immediately – but it’s likely more than one transmitter is waiting so a collision is likely

    • ‘Greedy’ access mechanism resulting in high collision rate

Computer networks

  • Non-persistent CSMA

    • Stations wanting to transmit sense the channel

    • If busy, they re-schedule another sense for later

    • Re-scheduling method is called the Backoff algorithm

    • If channel is free at re-sense, transmit, else re-schedule again

    • Since stations do not persist in sensing the channel and ‘come back later’ for another look, collisions are reduced

    • The drawback is the re-sense may be scheduled for a lot longer than needed – channel may be free before backoff algorithm times out so efficiency is lower than 1-Persistent CSMA

Computer networks

  • p-Persistent CSMA

    • A combination of 1-Persistent and Non-Persistent

    • Stations wanting to transmit sense the channel

    • If busy, they continuously re-sense until it becomes idle

    • With a probability p, the station transmits immediately

    • With a probability 1-p, the station re-schedules another sense (often delay is tprop)

    • Note - delay is from channel becoming free – with Non-Persistent the delay was from first sense time

Advantages of p persistent

Advantages of p-Persistent

  • Efficiency is good since there is a probability p of instant transmission when channel is free – the higher p the better (ultimately p=1 becomes 1-Persistent CSMA)

  • Probability p of two devices transmitting causing a clash – the lower p the better (ultimately p=0 becomes 0-Persistent or Non-Persistent CSMA)

  • …. hence the value of p is a compromise and depends on many factors

Csma performance

CSMA performance

  • CSMA is sensitive to propagation delay and other factors – typical performance 53 to 81% - better than ALOHA (18 to 37%)



Csma and aloha problem

CSMA and ALOHA problem

  • Both CSMA and ALOHA collisions involve an entire packet – the collision is not detected until the entire packet is sent

  • E.g. a 1500 bit packet, collision occurs after 10 bits, the remaining 1490 bytes are still sent and will be corrupted

  • The receiver will detect this (via a checksum) and respond with a Negative Acknowledgement (NAK) and the data will be sent again

  • This is inefficient – the last 1490 bits are a waste of channel capacity

Csma cd


  • Better channel usage if we detect the collision when it occurs rather than waiting until the end of the packet

  • Carrier Sense Multiple Access with Collision Detection - CSMA-CD

  • Performed by the transmitting station listening to itself and if what it hears is different from what it sends then there is a collision

  • If this occurs, transmitter sends a short jamming signal which notifies all stations there has been a collision – without this the receiver will not know there has been a collision and will continue to listen

  • Then the transmission is aborted and a re-try scheduled

Protocol without a chairman csma cd

Protocol - Without a chairman = CSMA-CD

  • One person speaks, all others listen

  • Before someone speaks, they check that nobody else is talking, then they talk

  • If two people start talking at the same time, both stop and apologise, and one of them re-starts talking

  • Multiple Access – MA

  • Carrier Sense – CS

  • Collision Detect - CD

Mac the scheduling approach

MAC – the scheduling approach

  • Previous MAC’s have been random access

  • Simple to implement, good performance EXCEPT under heavy load – collision dominated

  • Scheduling Systems are a way of controlling access to the media – like a meeting with a chairman

  • Each station has a reserved slot when it can transmit so no collisions

  • Disadvantage is some stations may not want to transmit and this slot is wasted

Reservation systems

Reservation Systems

  • To overcome this, we have a special timeslot where devices say if they want to talk – this is a minislot within the reservation interval

Reservation systems1

Reservation Systems

  • Listeners pickup the reservation packet and can work out who said what in subsequent packets

  • Talkers also know when to talk since they also pickup the reservation packet r

  • Time between r and next r is a frame

  • Wasted bandwidth is only length of r per frame – the larger the frame, the higher the efficiency. Typically 95% for 20 packets per frame



  • Reservation requires stations make explicit reservation ahead of time

  • Polling is where stations take turn to access the medium

  • The right to access is then passed to the next station via some mechanism

  • This does not occur in fixed time slots – the access control mechanism is flexible

Computer networks

  • Centrally controlled polling

    • A master controller sends a polling message to one station, this then sends the data (which may be nothing) and finishes with a go-ahead message

    • Central controller then polls the next station – this may be round-robin or some otherorder

Token passing networks

Token Passing Networks

  • Another way of polling – the right to access is a token that is passed from one station to the next (no central controller)

  • When listening, devices copy data from input to output hence passing everything along

  • When transmitting, devices receive data coming in, modify or add to it and send this on to the next station

How to transmit

How to transmit

  • A station that wants to transmit waits for a free token

  • The ‘free token’ is the polling message that allows access to the medium

  • Station then modifies the token to say the medium is no longer free, adds its data and sends this on

  • This full packet eventually reaches the destination where it is read

  • Packet must be removed from the ring – either:

    • Receiver does this and does not forward the packet

    • Receiver marks the token as read and sends it on – the transmitter then removes the packet. This is an acknowledgment that the packet was read OK

Token re insertion

Token re-insertion

  • After transmission is complete, a new free token needs to be re-inserted

  • Most common form is whoever removed the full packet re-inserts a new free token

  • Another problem – since devices re-generate the data, what if device is switched off during this? Free token is lost…

  • Normally there is a nominated controller that re-starts the ring if the token is lost

Typical mac efficiencies

Typical MAC Efficiencies

Normalized Delay-Bandwidth Product

Propagation delay

Time to transmit a frame

  • If a<<1, then efficiency close to 100%

  • As a approaches 1, the efficiency becomes low

CSMA-CD (Ethernet) protocol:

Token-ring network

a΄= latency of the ring (bits)/average frame length

Typical delay bandwidth products

Typical Delay-Bandwidth Products

  • Max size Ethernet frame: 1500 bytes = 12000 bits

  • Long and/or fat pipes give large a

Mac protocol features

MAC protocol features

  • Delay-bandwidth product

  • Efficiency

  • Transfer delay

  • Fairness

  • Reliability

  • Capability to carry different types of traffic

  • Quality of service

  • Cost

Mac delay performance

MAC Delay Performance

  • Frame transfer delay

    • From first bit of frame arrives at source MAC

    • To last bit of frame delivered at destination MAC

  • Throughput

    • Actual transfer rate through the shared medium

    • Measured in frames/sec or bits/sec

  • Parameters

    R bits/sec & L bits/frame

    X=L/R seconds/frame

    l frames/second average arrival rate

    Load r = l X, rate at which “work” arrives

    Maximum throughput (@100% efficiency): R/L fr/sec

Normalized delay versus load


Transfer delay






Normalized Delay versus Load

E[T] = average frame

transfer delay

  • At low arrival rate, only frame transmission time

  • At high arrival rates, increasingly longer waits to access channel

  • Max efficiency typically less than 100%

X = average frame

transmission time

Dependence on rt prop l

a > a




Transfer Delay







Dependence on Rtprop/L

Comparison of mac approaches

Comparison of MAC approaches

  • Aloha & Slotted Aloha

    • Simple & quick transfer at very low load

    • Accommodates large number of low-traffic bursty users

    • Highly variable delay at moderate loads

    • Efficiency does not depend on a


    • Quick transfer and high efficiency for low delay-bandwidth product

    • Can accommodate large number of bursty users

    • Variable and unpredictable delay

Comparison of mac approaches1

Comparison of MAC approaches

  • Reservation

    • On-demand transmission of bursty or steady streams

    • Accommodates large number of low-traffic users with slotted Aloha reservations

    • Can incorporate QoS

    • Handles large delay-bandwidth product via delayed grants

  • Polling

    • Generalization of time-division multiplexing

    • Provides fairness through regular access opportunities

    • Can provide bounds on access delay

    • Performance deteriorates with large delay-bandwidth product

What is a lan

What is a LAN?

Local area means:

  • Private ownership

    • freedom from regulatory constraints of WANs

  • Short distance (~1km) between computers

    • low cost

    • very high-speed, relatively error-free communication

    • complex error control unnecessary

  • Machines are constantly moved

    • Keeping track of location of computers a chore

    • Simply give each machine a unique address

    • Broadcast all messages to all machines in the LAN

  • Need a medium access control protocol

Typical lan structure



Typical LAN Structure

  • Transmission Medium

  • Network Interface Card (NIC)

  • Unique MAC “physical” address

Ethernet Processor


Medium access control sublayer

Medium Access Control Sublayer

  • In IEEE 802.1, Data Link Layer divided into:

  • Medium Access Control Sublayer

    • Coordinate access to medium

    • Connectionless frame transfer service

    • Machines identified by MAC/physical address

    • Broadcast frames with MAC addresses

  • Logical Link Control Sublayer

    • Between Network layer & MAC sublayer

Mac sub layer


IEEE 802

Network layer

Network layer

802.2 Logical link control


Data link










Token Ring




Various physical layers



MAC Sub-layer

Logical link control layer



Reliable frame service


Unreliable Datagram Service

















Logical Link Control Layer

  • IEEE 802.2: LLC enhances service provided by MAC

Logical link control services

Logical Link Control Services

  • Type 1: Unacknowledged connectionless service

    • Unnumbered frame mode of HDLC

  • Type 2: Reliable connection-oriented service

    • Asynchronous balanced mode of HDLC

  • Type 3: Acknowledged connectionless service

  • Additional addressing

    • A workstation has a single MAC physical address

    • Can handle several logical connections, distinguished by their SAP (service access points).

Llc pdu structure

LLC PDU Structure


1 or 2 bytes

1 byte



SAP Address


SAP Address



Source SAP Address

Destination SAP Address




7 bits

7 bits


Examples of SAP Addresses:

06 IP packet

E0 Novell IPX

FE OSI packet

AA SubNetwork Access protocol (SNAP)

I/G = Individual or group address

C/R = Command or response frame

Encapsulation of mac frames

IP Packet


LLC Header



MAC Header


Encapsulation of MAC frames

What is used in real networks

What is used in real networks?

  • Next lecture will cover practical aspects:

    • Ethernet types

    • Packet structure

    • Broadcasts

    • Routers/bridges

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