Medium Access Control Layer

7/29/2019 Medium Access Control Layer

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Computer Communications

Lectures 11-13

The Medium Access Control Sub-Layer

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The Channel Allocation Problem

• Consider a broadcast channel (sometimes also called multi-access or

random access channel)

• How to allocate the channel among multiple users?

• The channel allocation protocol reside in medium access control (MAC)

sub-layer within the data link layer

MAC Sub-layer



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3

Static Channel Allocation

• Frequency Division Multiplexing (FDM)

• Time Division Multiplexing (TDM)

• Both are inefficient when:

– Number of users large and time-varying

– Traffic bursty

• Compare with Statistical Multiplexing and Packet-Switching

4

Dynamic Channel Allocation Model

• Station Model

• Single Channel assumption

• Collision assumption

• Continuous vs. Slotted Time

• Carrier Sense vs. No Carrier Sense



3

5

Multiple Access Protocols

• ALOHA

• Carrier Sense Multiple Access Protocols

• Ethernet

• Wireless LAN Protocols

• IEEE 802.11 based Wireless LANs

• Collision-Free Protocols

• Limited-Contention Protocols

• Wavelength Division Multiple Access Protocols

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Pure ALOHA

• A user transmits whenever there is data to send

In pure ALOHA, frames are transmitted at completely arbitrary times.



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7

Pure ALOHA (2)

Vulnerable period for the shaded frame.

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Pure ALOHA vs. Slotted Aloha

• S = G * P0

Throughput versus offered traffic for ALOHA systems.



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9

Carrier Sense Multiple Access (CSMA)

• Sense (listen to) the carrier (channel) and “wait” if busy

• 1-persistent CSMA

– Wait until channel is idle and transmit (i.e., with probability 1)

– Upon collision, retry after a random wait period

– Performance worsens with increasing propagation delay

• p -persistent CSMA

– Assumes slotted time

– Wait until channel is idle and transmit with probability p

– If collision or someone else grabs the channel, retry after a random waitperiod

• Non-persistent CSMA

– If channel busy, wait a random period before retrying

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Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols.



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11

CSMA with Collision Detection (CSMA/CD)

• Abort transmissions as soon as collision is detected

• Basis of Ethernet LAN

• Inherently half-duplex

• Inefficient when large propagation delays and short frames

CSMA/CD can be in one of three states: contention,transmission, or idle.

Ethernet

Required Reading: Tanenbaum (section 1.5.3 andchapter 4)



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Original Ethernet and IEEE 802.3 Standard

• Thick coaxial “multidrop” cable up to 2.5km long (with repeaters every 500meters)

• Connect up to 256 computers

• 2.94Mbps line speed

• CSMA/CD

– Abort transmissions as soon as collision is detected and “jam” the cable to alertothers

• DIX Standard for 10Mbps Ethernet (1978)

• IEEE 802.3 Standard (1983): DIX Standard with two minor modifications

Architecture of the original Ethernet (1976).

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Ethernet Cabling

The most common kinds of Ethernet cabling.

Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.



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15

Ethernet Cabling (2)

Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

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Manchester Encoding

• Differentiate idle and busy periods

• Sender-receiver synchronization

(a) Binary encoding, (b) Manchester encoding,

(c) Differential Manchester encoding.



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Ethernet Frame Structure

• Addressing mechanism supports group (multicast/broadcast) addressesand distinguishing local from global addresses

• Minimum frame length requirement (64 bytes) – To allow proper detection of collisions

– Distinguish valid frames from noise

Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.

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Need for Minimum Frame Length in Ethernet

• Minimum frame length = 500 bits rounded to 512 bits (64 bytes)

– Worst case round trip propagation delay on 2.5km cable with 4 repeaters = 50 microseconds

– Network speed = 10Mbps each bit takes 100 nano seconds

• Higher network speed greater minimum frame length or shorter maximumcable length

Collision detection can take as long as 2 .τ



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Binary Exponential Backoff (BEB) Algorithm

• Mechanism to dynamically adapt number of contending nodes

• After i collisions, choose a random number of slots between 0 and 2i -1before retrying

– After 10 collisions, randomization interval is frozen at 1023 slots

– Give up retrying after 16 collisions

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Ethernet Performance

• Channel efficiency = P / (P + 2τ

/A) longer the cable, longer the

contention interval and lower the channel efficiency

– P: mean frame transmission time

– 2 : slot duration (worst case round-trip propagation delay)

– A: probability that some station acquires the channel in a given slot

• Channel efficiency = 1 / (1+2BLe / c F) for optimal case of e contention slots per frame

– F: frame length

– B: network bandwidth

– L: cable length

– c: signal propagation speed

Efficiency of Ethernet at 10Mbps with 512-bit slot times.



11

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IEEE 802.2: Logical Link Control

• Hide differences between various 802 networks by providing a singleformat and interface to network layer

• Can support error control and flow control over 802 networks

– Three service options: unreliable datagram, acknowledged datagram and

reliable connection-oriented

• Based on HDLC

(a) Position of LLC. (b) Protocol formats.

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Switched Ethernet

• Switch: high-speed backplane + 4-32 plug-in line cards each with 1-8

connectors/ports

A simple example ofswitched Ethernet.

• Handling simultaneous transmissions from hosts connected to same

plug-in card

– Ports connected together to form an “on-card LAN” each card a separate

collision domain

» Handle collisions within a card and retransmissions using CSMA/CD with BEB

– Buffer at each input port each port a separate collision domain

» No collisions



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Fast Ethernet (IEEE 802.3u, 1995)

• 100Mbps – 10 times faster yet backward compatible with earlier

Ethernet

– Supports flow control

– Encoding schemes different from Manchester encoding used

• Use hubs or switches

The original fast Ethernet cabling.

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Gigabit Ethernet (IEEE 802.3z, 1998)

• 1Gbps – 10 times faster than fast Ethernet yet backward compatible with allexisting Ethernet standards

– Supports flow control

– Uses a different set of encoding schemes

• 10-gigabit Ethernet also standardized in 2002 (IEEE 802.3ae)

(a) A two-station

Ethernet.

(b) A multistation

Ethernet.

Gigabit Ethernet cabling.



13

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Retrospective on Ethernet

• Simple reliable, cheap and easy to maintain

• Flexible

• Interworks easily with TCP/IP (both IP and Ethernet connectionless)

• Evolved without need for software changes

Wireless LANs and IEEE 802.11 Standard

Required reading: Tanenbaum (section 1.5.4, section4.2.6 and section 4.4)



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Wireless LAN Protocols

• CSMA does not work when all nodes are not within range of each other

– Need to know interference at receiver, but know only about interference atsender with CSMA

• Hidden node problem: two non-neighboring nodes simultaneously

transmit to a common neighbor, causing a collision

• Exposed node problem: a node unnecessarily getting blocked from

transmitting due to an on-going transmission from a neighbor

A wireless LAN. (a) A transmitting. (b) B transmitting.

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Wireless LAN Protocols (2)

• MACA (Multiple Access with

Collision Avoidance) protocol

– Alleviates hidden station

problem, but does not completelyeliminate it

– Collisions still possible (e.g., RTScollisions)

» Retry after random wait periodbased on BEB algorithm

• MACAW (MACA for Wireless)

– Add ACKs and carrier sense

– To improve fairness, apply backoffalgorithm for each source-destinationpair

– To improve performance:

» congestion info exchange betweennodes

» decrease sensitivity of backoff algorithm to temporary problems

The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A.



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IEEE 802.11 Wireless LAN Standard

• Challenges

– Identifying a suitable, worldwidefrequency band

– Sufficient bandwidth and economicviability

– Protect users’ privacy

– Impact of host mobility

– Coping with limited battery life

– Dealing with human safety issues

• Several inherent differences with

Ethernet

– CSMA is not sufficient in wireless

LANs – Presence of multipath fading

– Need for host mobility support

– Dealing with mobility-unawaresoftware

• 802.11 standard (1997)

– 2.4GHz ISM band

– 1Mbps or 2Mbps(DSSS/FHSS/Infrared)

• 802.11b standard (1999)

– 2.4GHz ISM band

– 11Mbps (HR-DSSS)

• 802.11a standard (1999)

– 5GHz ISM band

– 54Mbps (OFDM)

• 802.11g (2001)

– 2.4GHz ISM band – 54Mbps (OFDM)

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IEEE 802.11 Wireless LAN Standard (2)

(a) Infrastructure mode. (b) Ad hoc mode.

A multicell 802.11 network.



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The 802.11 Protocol Stack

Part of the 802.11 protocol stack.

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The 802.11 MAC Sublayer Protocol

• Distributed Coordination Function

(DCF)

– Distributed

– Compulsory

– CSMA with Collision Avoidance(CSMA/CA)

» When channel busy, set a randombackoff counter but keep it frozenwhile channel busy

» No carrier sensing whiletransmitting

» Upon frame loss due to collisions orchannel errors, retry after random

wait period based on Ethernet BEBalgorithm

– Virtual carrier sensing

» RTS-CTS mechanism

» Network Allocation Vectors (NAVs)

• Point Coordination Function (PCF)

– Centralized: base station or AccessPoint (AP) controls intra-cell channelallocation

– Optional

– AP uses a polling mechanism

» Periodic beacon frames (10-100/second) contain systemparameters (e.g., clock synchronization) and invitation to joinpolling service

» Poll frames

» Polling frequency, polling order and

service priorities not specified instandard

– Also useful for power management

– Can co-exist with DCF via inter-framespacing mechanism in 802.11



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The 802.11 MAC Sublayer Protocol (2)

Virtual channel sensing mechanism.

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• Stop-and-wait protocol applied to each fragment

• Fragment size not set by the standard

A fragment burst.



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Interframe spacing in 802.11.

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The 802.11 Frame Structure

• Data, management and control frames

• Management frames similar to data frames, but with one less APaddress

• Control frames even shorter with only one or two addresses, no Data or

Sequence fields

The 802.11 data frame.



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802.11 Services

• Distribution (Inter-Cell) Services for managing cell membership andinteracting with nodes outside a cell

– Association

– Disassociation

– Reassociation

– Distribution

– Integration

• Station (Intra-Cell) Services for activity within a single cell

– Authentication

– Deauthentication

– Privacy

– Data Delivery

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Collision-Free Protocols

• Assume:

– Exactly N stations, each with a unique address from 0 to N-1

– Propagation delay negligible

The basic bit-map protocol (comes under the category of reservation

protocols).



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Collision-Free Protocols (2)

The binary countdown protocol. A dash indicates silence.

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Limited-Contention Protocols

• Adapt assignment of stations to slots according to load

Acquisition probability for a symmetric contention channel.



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Adaptive Tree Walk Protocol

The tree for eight stations.

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Wavelength Division Multiple Access Protocols

• Dynamic variants of FDM/TDM methods

• Assume global synchronization

Wavelength division multiple access.

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Medium Access Control Layer