Medium Access Control

Channel Allocation

• Static channel allocation in LANs and MANs

• FDMA, TDMA, CDMA• Dynamic channel allocation in LANs and

MANs• MAC protocols: with collisions, polling, token

Static Channel Allocation

• Delay for one fast channel:

• Delay for multiple FDM slower channels:

][E/1LC

D

NTNLNC

D

/])[E/(

1FDM

Poisson Process

• Probability of k arrivals in time t:

• Probability that packet duration exceeds t:

• Note that λ is the average packet arrival rate, and 1/μ is the average packet duration.

!)(][k

etkAPtk

tetP ]inarrivalno[

M/M/1 Queue

• Queue equations, pj(t) is the probability that the number of packets at time t in a queue is j

• The solution of the previous recursion for stationary probabilities to which pj(t) converge

)()()())(1()()()()1()(

11

100

tdtptdtptpdtdttptdtptpdtdttp

jjjj

jjj pp )/()/1()/(0

Delay

• Little’s formula for average delay E[D]

E[Q] is the average number of packets in a queue

• Delay is:

)/(][E][E

QD

11

/11]packets][[E D

Static Channel Allocation

• Delay for one fast channel:

where C is the channel bit-rate and E[L] is the average packet length.

• Delay for multiple FDM N times slower channels:

][E/1

1)/1(][E

LCD

][E/])[E/(

1][E FDM DNNLNC

D

Multiple Access Protocols

• ALOHA• Carrier Sense Multiple Access (CSMA)

protocols• CSMA/CD• CSMA/CA

• Collision-Free protocols• Reservation based • Token based

Pure ALOHA

Vulnerable period for the shaded frame.

ALOHA Throughput• Throughput is S=GPs, where Ps is the probability of successful transmission.• The k frames per f frame slots is

Ps=e-fG

• For pure ALOHA f=2, for slotted ALOHA f=1, so:

!)(][k

efGkPfGk

Pure and Slotted ALOHA

Throughput versus offered traffic for ALOHA systems.

Carrier Sense Multiple Access

• 1-Persistant CSMA• Nonpersistant CSMA• P-Persistant CSMA

CSMA with Collision Detection

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

Wireless LAN: CSMA-CA

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

DOCSIS (Data Over Cable Service Interface Specification)

Collision-Free Protocols:Reservations

The basic bit-map protocol.

Collision-Free Protocols: Bidding

The binary countdown protocol. A dash indicates silence.

Collision Free Protocols: Fiber Distributed Data Interface (FDDI)

• Station transmits only when it has a token• Timers count the time while the token is away• Two timers determine how much data a station

may transmit, so that the token delay is limited

Ethernet

• Ethernet, IEEE 802.3• 10Base (10Mbps)• Fast Ethernet (100Mbps)• Gigabit Ethernet

Ethernet MAC Sublayer Protocol

Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.Preamble-synchronization, Type-upper layer protocol,

Pad-to make the minimum packet size 64B

CSMA with Collision Detection

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

Back-Off Mechanism

• After a collision, user accesses medium with probability 1/W where W is the window size.

• With each collision W doubles.

Ethernet Performance

Collision detection can take as long as 2 .

Throughput of CSMA/CD (Ethernet)• Assume that requests form a Poisson process with rate g, T is time slot duration, and Tp is a packet duration. The throughput equals S=Tp/(Tp+I), where I is the average time between packet transmissions.• The probability of a packet transmission is equal to the probability that there is only one request in some previous time slot which is Ps=gTe-gT.• The average time between transmissions is

ssi

si

PTPPiTI /)1(

Throughput of CSMA/CD• The throughput is

• It tends to 0 when g increases . • Protocol is unstable like ALOHA.

1

1

p

gT

p

p

gTgTe

ITT

S

g

S 1 2

Throughput of CSMA/CD• The throughput is

• If p is the packet generation probability and k is the number of active users Ps=kp(1-p)k-1

• The maximum throughput is achieved for p=1/k and it is tends to e when k tends to infinity

ssp

p

PPTTT

S/)1(

Ethernet Performance

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

10Mbps Ethernet Cabling

The most common kinds of Ethernet cabling.

10Mbps Ethernet Cabling

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

Ethernet Cabling

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

10Mb Ethernet Coding

(a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

10 Mb Ethernet Collision Detection

10Base5 cabling, Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998

10 Mb Ethernet Collision Detection

10Base2 and 10BaseT cabling,Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998

Fast Ethernet

The original fast Ethernet cabling.

Fast Ethernet

• Auto negotiation enables communication with 10Mb Ethernet

• Manchester code → 4B/5B code• Full duplex mode is optional with using

PAUSE command

Switched Ethernet

A simple example of switched Ethernet.

Gigabit Ethernet

(a) A two-station Ethernet. (b) A multistation Ethernet.

Gigabit Ethernet

Gigabit Ethernet cabling.

Gigabit Ethernet

• Prioritization of fiber over copper• 4B/5B coding → 8B/10B coding• Full duplex mode is preferred with PAUSE

message• Carrier extension, and frame bursting

introduced in half-duplex mode

IEEE 802.2: Logical Link Control

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

A Sample HFC System

Secondary Hub

o o o o o oo o

o o o o

o o

HOME

o oo o

5-42 MHz 550 MHz 750 MHz

RF Spectrum on coax:return 80 broadcast channels 30 QAM channels

(~150 video channels)

Downstream: 500 MHz shared by ~50,000 (broadcast) 200 MHz by 1200 (narrowcast)

Upstream: ~37 MHz shared by 300

broadcast narrowcastnarrowcast

Fiber Node

up

b

n (4n/fiber)

Sheryl Woodward, AT&T Labs-Research

Justification for Using Shared Medium

• Equivalent circuit rate (ECR) on a cable with many users is the rate of a dedicated link that would provide the same e.g. average delay (similar results is obtained for 90th percentile page delay). By Shankar, Jiang and Mishra:

where tON is the transmission tim, and tOFF is the think time, r is the channel rate, tON/(tON+tOFF)<<1, on periods have an exponential distribution.

rt

ttMMtt

tr ECR11ECRON

OFFON

OFFON

ON

Justification for Using Shared Medium

• Let’s calculate how many users can be allocated one DOCSIS channel of 32Mbps to get the same experience as DSL user with dedicated rate of 2Mbps. According to traffic statistics page size is 68KB on average, and tOFF is 14.5s on average,

which is much more than 32/2=16 users. Price: high user speed.

800017.0

5.14017.03221

Mbps32/KB68s5.14Mbps32/KB68

M

DOCSIS MAC Protocol

• Traffic that is transmitted downstream to the users is controlled by CMTS (cable modem termination system) in headend. It polices and shapes the traffic, and perform algorithms such are WFQ and RED.• Users requests are resolved at headend, and they are informed about the resolution through the downstream channel. If there is a collision of requests, users repeat their requests according to exponential back-off mechanism, otherwise they send data in specified time slot(s).

QoS in DOCSISService QoS parameters Access Mode Applications

UGS Unsolicited grant size, interval, jitter

Isonchronous Videoconferencing, VoD, VoIP

UGS-AD Unsolicited grant size, interval, jitter;polling interval, jitter

Isonchronous, periodic request polling

VoIP with silence supression

rtPS Polling interval, jitter Periodic request polling, piggybacking reservation

VoIP

QoS in DOCSISService QoS parameters Access Mode Applications

nrtPS Polling interval, min reserved rate, max sustained rate, priority

Periodic request polling, piggybacking reservation, immediate access

Demanding FTP

BE min reserved rate, max sustained rate, priority

Normal, piggybacking reservation, immediate access

Telnet, FTP, WWW

CIR Unspecified Unspecified Unspecified

Performance for BE service in DOCSIS• Assume that requests form a Poisson process with rate g, T is time slot duration, and Tp is a packet duration. The throughput equals S=Tp/(Tp+I), where I is the average time between packet transmissions.• The probability of a packet transmission is equal to the probability that there is only one request in some previous time slot which is gTe-gT.• The average time between transmissions is

ssi

si

PTPPiTI /)1( 1

Performance for BE service in DOCSIS• The throughput is

• It tends to 0 when g increases . • Protocol is unstable like ALOHA.

1

1

p

gT

p

p

gTe

ITT

S

g

S 1 2

Wireless LANs

• Distributed coordination function (DCF)• Point coordination function (PCF)

The 802.11 Protocol Stack

Part of the 802.11 protocol stack.

The 802.11 MAC Sublayer Protocol

(a) The hidden station problem.(b) The exposed station problem.

Wireless LAN: CSMA-CA

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

The 802.11 MAC Sublayer Protocol

The use of virtual channel sensing using CSMA/CA.

The 802.11 MAC Sublayer Protocol

A fragment burst.

The 802.11 MAC Sublayer Protocol

Interframe spacing in 802.11.

The 802.11 Frame Structure

The 802.11 data frame.Address 3 and 4-for source and dest base stations, Seq-fragment sequence number,

Type-data or control, Subtype-RTS or CTS, MF-more fragments, More-more frames, W-WEP, O-frame sequence maintained

802.11 Services

• Association• Disassociation• Reassociation• Distribution• Authentication• Integration• Privacy