Medium Access Control. Channel Allocation Static channel allocation in LANs and MANs FDMA, TDMA,...

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/

1

LCD

NTNLNC

D

/])[E/(

1FDM

Poisson Process

• Probability of k arrivals in time t:

• Probability that interarrival time exceeds t:

!

)(][

k

etkAP

tk

tetP ]inarrivalno[

M/M/1 Queue

• Queue equations


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

)()()1()(

11

100

tdtptdtptpdtdttp

tdtptpdtdttp

jjjj

jj pp )/(0

Delay

• Little’s formula

• Delay is:

)/(

][E][E

Q

D

1

1

/1

1]packets][[E D

Static Channel Allocation

• Delay for one fast channel:


][E/

1

LCD

NTNLNC

D

/])[E/(

1FDM

Dynamic Channel Allocation

1. (a) Single channel (b) Multiple channel

2. (a) Collision (b) Collision-free

3. (a) Continuous Time.(b) Slotted Time.

4. (a) Carrier Sense.(b) No Carrier Sense.

Multiple Access Protocols

• ALOHA

• Carrier Sense Multiple Access (CSMA) protocols

• CSMA/CD• CSMA/CA

• Collision-Free protocols• Reservation based • Token based

Pure ALOHA

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

Pure ALOHA

Vulnerable period for the shaded frame.

ALOHA Throughput

• Throughput is S=GP0, where P0 is the probability of successful transmission.• The k frames per f frame slots is

P0=e-fG

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

!

)(][

k

efGkP

fGk

Pure and Slotted ALOHA

Throughput versus offered traffic for ALOHA systems.

Carrier Sense Multiple Access

• 1-Persistant CSMA

• Nonpersistant CSMA

• P-Persistant CSMA

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various random access protocols.

CSMA with Collision Detection

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

Wireless LAN: CSMA with Collision Avoidance

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

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

Limited-Contention Protocols

Acquisition probability for a symmetric contention channel.

Adaptive Tree Walk Protocol

The tree for eight stations.

WDMA Networks

• Stations may be connected to the optical star coupler or onto the optical ring and utilize multiple wavelengths.

• Stations are equipped with the tunable/fixed transmitters and tunable/fixed receivers.

• Stations are tuned to the common control channel where they compete for the medium.

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.

CSMA with Collision Detection

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

Performance of Non-Persistant CSMA/CD

Rom and Sidi, Multiple Access Protocols, Springer Verlag, 1990

Performance of 1-Persistant CSMA/CD


Comparison of CSMA and CSMA/CD


Ethernet Performance

Collision detection can take as long as 2 .

Ethernet Performance

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

Back-Off Mechanism

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

• With each collision W doubles.

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

ttMM

tt

tr

ECR11ECR

ON

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

5.14017.0

32

21

Mbps32/KB68

s5.14Mbps32/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 DOCSIS

Service 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 . Exercise: Find the mapximum value of S in terms of T and Tp, and plot graphs of S versus g, and different T/Tp. • Protocol is unstable like ALOHA.

1

1

p

gT

p

p

gT

e

IT

TS

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.


The use of virtual channel sensing using CSMA/CA.


A fragment burst.


Interframe spacing in 802.11.

The 802.11 Frame Structure

The 802.11 data frame.

802.11 Services

• Association• Disassociation• Reassociation• Distribution• Integration• Privacy• Data delivery

Broadband Wireless

• Fixed wireless

• Larger bandwidth

The 802.16 Protocol Stack

The 802.16 Protocol Stack.

The 802.16 Physical Layer

The 802.16 transmission environment.

The 802.16 Physical Layer

Frames and time slots for time division duplexing.


Service Classes

• Constant bit rate service

• Real-time variable bit rate service

• Non-real-time variable bit rate service

• Best efforts service

The 802.16 Frame Structure

(a) A generic frame. (b) A bandwidth request frame.

Bluetooth

• Used for communication of the equipment in the house, office

• Interferes with IEEE 802.11

Bluetooth Architecture

Two piconets can be connected to form a scatternet.

Bluetooth Applications

The Bluetooth profiles.

The Bluetooth Protocol Stack

The 802.15 version of the Bluetooth protocol architecture.

The Bluetooth Frame Structure

A typical Bluetooth data frame.

Data Link Layer Switching

• Bridges from 802.x to 802.y• Local Internetworking• Spanning Tree Bridges• Remote Bridges• Repeaters, Hubs, Bridges, Switches, Routers, Gateways• Virtual LANs

Data Link Layer Switching

Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN.

Bridges from 802.x to 802.y

Operation of a LAN bridge from 802.11 to 802.3.

Bridges from 802.x to 802.y (2)

The IEEE 802 frame formats. The drawing is not to scale.

Local Internetworking

A configuration with four LANs and two bridges.

Spanning Tree Bridges

Two parallel transparent bridges.

Spanning Tree Bridges (2)

(a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree.

Remote Bridges

Remote bridges can be used to interconnect distant LANs.

Repeaters, Hubs, Bridges, Switches, Routers and Gateways

(a) Which device is in which layer.

(b) Frames, packets, and headers.

Repeaters, Hubs, Bridges, Switches, Routers and Gateways (2)

(a) A hub. (b) A bridge. (c) a switch.

Virtual LANs

A building with centralized wiring using hubs and a switch.

Virtual LANs (2)

(a) Four physical LANs organized into two VLANs, gray and white, by two bridges. (b) The same 15 machines organized into two VLANs by switches.

The IEEE 802.1Q Standard

Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not.

The IEEE 802.1Q Standard (2)

The 802.3 (legacy) and 802.1Q Ethernet frame formats.

Summary

Channel allocation methods and systems for a common channel.

Medium Access Control. Channel Allocation Static channel allocation in LANs and MANs FDMA, TDMA,...

Documents

Transcript of Medium Access Control. Channel Allocation Static channel allocation in LANs and MANs FDMA, TDMA,...