Datalink Layer: Examples

4/21/2008

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Recap: Summary of MAC Protocols

How do you access a shared media?

channel partitioning, by time, frequency or code

random access, • ALOHA, S-ALOHA, CSMA, CSMA/CD

“taking-turns”• polling• token passing

Recap: Aloha Protocol

Behaviors of Aloha on a LAN a total of m stations fixed transmission rate p for a backlogged

station to transmit in a slot pa for each un-backlogged station

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Outline

Admin. and recap MAC Examples

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Example MAC Protocols Example MAC protocols

GSM Ethernet Wireless LAN Bluetooth

There are many more link technologies e.g., ATM, DOCSIS, FDDI, Frame relay, IEEE

802.5 Token Ring, PPP, WiMax, X.25, xDSL if you are interested, please see schedule

page for a link to a set of optional slides

Key factors: traffic services

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Outline

Admin. and recap MAC Examples

GSM

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1 2 3 4 5 6 7 8

935-960 MHz124 channels (200 kHz)downlink

890-915 MHz124 channels (200 kHz)uplink

frequ

ency

time

GSM TDMA frame

GSM time-slot (normal burst)

4.615 ms

546.5 µs577 µs

tail user data TrainingSguardspace S user data tail

guardspace

3 bits 57 bits 26 bits 57 bits1 1 3

GSM - TDMA/FDMA

S: indicates data or control

http://wireless.fcc.gov/uls/index.htm?job=home

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Many Types of Logical Channels Control channels

Broadcast control channel (BCCH)

• From base station, announces cell identifier, synchronization

Common control channels (CCCH)

• Paging channel (PCH): Base transceiver station (BTS) pages a mobile host (MS)

• Random access channel (RACH): MSs for initial access, using slotted Aloha

• Access grant channel (AGCH): BTS informs an MS its allocation

Dedicated control channels• Standalone dedicated control

channel (SDCCH): signaling and short message between MS and an MS

Traffic channels (TCH)

Example: call setup from an MS BTSMS

RACH (request signaling channel)

AGCH (assign signaling channel)

SDCCH (request call setup)

SDCCH (assign TCH)

SDCCH message exchange

Communication using TCH

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GPRS: GSM Data Services Using GSM, an MS can use a (logical) traffic channel to

send data data rate standardized at 9.6 kbps

General Packet Radio Service (GPRS) allocate multiple slots from the same frame; by reserving

different number of slots and using different coding scheme, an MS achieves different rate (kbps)

simplified signaling process: still uses a random channel to request frequency and time slot

Coding scheme

1 slot2 slots

3 slots

4 slots

5 slots

6 slots

7 slots

8 slots

CS-1 9.05 18.2 27.15 36.2 45.25 54.3 63.35 72.4

CS-2 13.4 26.8 40.2 53.6 67 80.4 93.8 107.2

CS-3 15.6 31.2 46.8 62.4 78 93.6 109.2 124.8

CS-4 21.4 42.8 64.2 85.6 107 128.4 149.8 171.2

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GPRS Signaling

PRACH: Pkt. Random Access Channel; PAGCH: Pkt. Access Grant Channel; PTCH: Pkt. Traffic ChannelUSF: uplink state flag

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UMTS: Enhancements of GSM UMTS (Universal Mobile

Telecommunications System) Use CDMA for channel partitioning

o less fragmented channelso additional requirement: allocate different

amount of bw to mobile stations

W-CDMA chipping rate: 5 MHz, 3.840 Mchip/s

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Orthognal Variable Spreading Factor (OSVF)

By assigning a code with a low spreading factor, a node receives higher bw.

1

1,1

1,-1

1,1,1,1

1,1,-1,-1

X

X,X

X,-X 1,-1,1,-1

1,-1,-1,1

1,-1,-1,1,1,-1,-1,1

1,-1,-1,1,-1,1,1,-1

1,-1,1,-1,1,-1,1,-1

1,-1,1,-1,-1,1,-1,1

1,1,-1,-1,1,1,-1,-1

1,1,-1,-1,-1,-1,1,1

1,1,1,1,1,1,1,1

1,1,1,1,-1,-1,-1,-1

SF=1 SF=2 SF=4 SF=8

SF=n SF=2n

...

...

...

...

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Outline Admin. and recap Example MAC protocols

GSM• Channel partitioning (time, freq., code) and slotted

Aloha Ethernet

Outline

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Ethernet

“Dominant” LAN technology: First widely used LAN

technology Kept up with speed race: 10 Mbps, 100 Mbps,

1 Gbps, 10 Gbps

Metcalfe’s Ethernetsketch

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Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble: 8 bytes 7 bytes with pattern 10101010 followed by one byte with

pattern 10101011 (why the preamble?) Source and dest. addresses: 6 bytes Type: indicates the higher layer protocol, mostly IP but

others may be supported such as Novell IPX and AppleTalk)

CRC: CRC-32 checked at receiver, if error is detected, the frame is simply dropped

8 6 6 2 46-1500 (including padding) 4

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The Basic MAC Mechanisms of Ethernet

get a packet from upper layer;K := 0; n := 0; // K: control wait time; n: no. of

collisionsrepeat: wait for K * 512 bit-time; while (network busy) wait; wait for 96 bit-time after detecting no signal; transmit and detect collision; if detect collision stop and transmit a 48-bit jam signal; n ++; m:= min(n, 10), where n is the number of

collisions choose K randomly from {0, 1, 2, …, 2m-1}. if n < 16 goto repeat else give up

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Ethernet’s Exponential Backoff:

Goal: adapt retransmission attempts to estimated current load compared with CSMA, 1/2m can be considered

as p not a static p---adjusted using exponential

backoff• first collision: choose K from {0,1}; delay is K x 512

bit transmission times• after second collision: choose K from {0,1,2,3}…• after ten or more collisions, choose K from

{0,1,2,3,4,…,1023}

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Ethernet: From Bit to Electrical Signal

Use Manchester encoding One voltage change per bit

for a “1”, a voltage change from 1 to 0 for a “0”, a voltage change from 0 to 1

Example

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Ethernet Technologies: 10Base2

10: 10Mbps; 2: under 200 meters max cable length

Thin coaxial cable in a bus topology

Issues of such connectivity?

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10BaseT and 100BaseT

10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Hub to which nodes are connected by twisted

pair, thus “star topology” there is a bus inside the hub; boost signal from one port

to all other ports

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Interconnecting with hubs

Multiple hubs interconnect to form a larger Ethernet network extends max distance between nodes; more ports

Issue: individual segment collision domains become one large collision domain

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Ethernet Bridges

Link layer device stores and forwards Ethernet frames examines frame header and selectively

forwards frame based on MAC dest address segments become separate collision domains

bridge collision domain

collision domain

= hub

= host

LAN (IP network)

LAN segment LAN segment

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Bridge Forwarding

Key issue: How do determine to which LAN segment to forward frame?

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Ethernet Bridge Self Learning

A bridge has a bridge table Entry in bridge table:

(Node LAN Address, Bridge Interface, Time Stamp)

stale entries in table dropped (TTL can be 60 min)

Bridges learn which hosts can be reached through which interfaces when frame received, bridge “learns” location

of sender: incoming LAN segment records sender/location pair in bridge table

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Filtering/ForwardingWhen bridge receives a frame:

index bridge table using MAC dest addressif entry found for destination

then { if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated } else flood

forward on all but the interface on which the frame arrived

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Ethernet Bridge: Example

Suppose C sends frame to D and D replies back with frame to C.

Bridge receives frame from C to D notes in bridge table that C is on interface 1 because D is not in table, bridge sends frame into

interfaces 2 and 3

frame received by D

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Bridge Learning: Example

D generates frame for C, sends Bridge receives frame

notes in bridge table that D is on interface 2 bridge knows C is on interface 1, so selectively

forwards frame to interface 1

C | 1

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Bridges Spanning Tree For increased reliability, desirable to have redundant,

alternative paths from source to dest With multiple paths, cycles result - bridges may

multiply and forward frame forever Solution: organize bridges in a spanning tree by

disabling subset of interfaces

Disabled

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Bridges vs. Routers both store-and-forward devices

routers: network layer devices (examine network layer headers) bridges are link layer devices

routers maintain routing tables, implement routing algorithms

bridges maintain bridge tables, implement filtering, learning and spanning tree algorithms

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Routers vs. Bridges

Bridges + and - + Bridge operation is simpler+ Bridge tables are self learning

- All traffic confined to spanning tree, even when alternative bandwidth is available

- Bridges do not offer protection from broadcast storms (flooding of packets)

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Routers vs. Bridges

Routers + and -+ arbitrary topologies can be supported

+ provide protection against broadcast storms- require IP address configuration (not plug and

play)- require higher packet processing

bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

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Gbit Ethernet and Ethernet Switches Gbit Ethernet typically

use Ethernet switches Essentially a multi-interface

bridge layer 2 (frame) forwarding,

filtering using LAN addresses

Switching: A-to-A’ and B-to-B’ simultaneously, no collisions

cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame

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Not an atypical LAN (IP network)

Dedicated

Shared

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Summary: Comparison

hubs bridges routers switches

traffi c isolation

no yes yes yes

plug & play yes yes no yes

optimal routing

no no yes no

cut through

yes no no yes

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Outline Admin. and recap Example MAC protocols

GSM• Channel partitioning and slotted Aloha

Ethernet• Random MAC protocol (CSMA/CD + Exponential

backoff) Wireless LAN

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802.11 – Traffic Services and Access Methods

Two types of traffic services Asynchronous Data Service (mandatory)

• exchange of data packets based on “best-effort”• implemented by random access

Time-Bounded Service (optional)

Two types of coordination function (aka MAC) DCF (Distributed Coordination Function) PCF (Point Coordination Function)

• access point polls

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IEEE 802.11 Wireless LAN

Basic Service Set (BSS) (a.k.a. “cell”) contains: wireless station (WS) access point (AP): base

station BSS’s combined to form

distribution system (DS) Two operation modes:

infrastructure mode• everything through AP

peer-to-peer mode• called ad hoc network

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Random Access Carrier Sense in 802.11

BA C

The hidden-terminal problem A is sending to B, but C cannot receive from

A • Friis Law (power decay proportional to distance squared)

Therefore C sends to B, without detecting the transmission from A to B

In summary, A is “hidden” for C

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The Exposed Terminal Problem

BA C D

B is sending to A, C intends to send to D C senses an “in-use” medium, thus C waits But A is outside the radio range of C,

therefore waiting is not necessary In summary, C is “exposed” to B Implication: false carrier sense

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Summary of Problems of Wireless MAC

How to achieve carrier sense? in Ethernet, we use carrier sense to avoid and

detect potential collision for wireless networks, the hidden-terminal,

and the exposed-terminal problems make carrier sense (i.e., listen before talk) neither sufficient nor necessary

• not detected transmission at the sender does not imply no current transmission to the receiver

• detected transmission at the sender does not imply transmission will cause collision

How to integrate random access (DCF) and taking turns (PCF)?

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Basic Solution: Using RTS/CTS to Address the Carrier Sense Problem Short signaling packets---virtual carrier

sense RTS (request to send) and CTS (clear to

send)• to avoid collision at the receiver, any station who

hears a CTS should not transmit• frames need to contain sender address, receiver

address, transmission duration

BA CCTSCTS DEFRTSRTS

Example: A sends to B

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Basic Solution: Using Inter Frame Spacing to Prioritize Access Different inter frame spacing (IFS): if the required IFS of a

type of message is short, the type of message has higher priority SIFS (Short Inter Frame Spacing)

• highest priority, for ACK, CTS, polling response PIFS (Point Coordination Function Spacing)

• medium priority, for time-bounded service using PCF DIFS (Distributed Coordination Function Spacing)

• lowest priority, for asynchronous data service

random direct access if medium is free DIFS

t

medium busy SIFSPIFS

DIFS DIFS

next framecontention

Access point access if medium is free DIFS

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Basic Control Flow of RTS/CTS

Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium) after waiting for DIFS

Receiver acknowledges via CTS after SIFS (if ready to receive) CTS reserves channel for sender, notifying possibly hidden

stations; any station hearing CTS should be silent for NAV

Sender can now send data at once

t

DIFS

data

defer access

otherstations

receiver

senderdata

DIFS

new contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

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802.11: RTS/CTS + ACK

802.11 adds ACK in the signaling to improve reliability implication: to avoid conflict with ACK, any station hearing RTS

should not send for NAV thus a station should not send for NAV if it hears either RTS and

CTS

Note: RTS/CTS is optional in 802.11, and thus may not be always turned on---some network interface cards turn it on only when the length of a frame exceeds a given threshold

t

SIFS

DIFS

data

ACK

defer access

otherstations

receiver

senderdata

DIFS

new contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

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802.11: PCF for Polling

tNAV

polledwirelessstations

point coordinator

NAV

PIFSD

USIFS

SIFSD

contentionperiod

contention free periodmediumbusy

D: downstream poll, or data from point coordinatorU: data from polled wireless station

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802.11 - Frame Format

Before the MAC header are an 80-bit preamble of alternating 0 and 1 for clock sync. a physical layer header (PLCP) which is always transmitted at 1

Mbps, including signaling fields such as sending rate Duration ID: NAV The four addresses are used to encode various addresses

e.g., Addr 1 is always the recipient address (i.e., the immediate recipiet of the frame), Addr 2 is always the transmitter addr

CRC: check sum

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802.11 Frame Control Field

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Outline Admin. and recap Example MAC protocols

GSM• Channel partitioning and slotted Aloha

Ethernet• Random MAC protocol (CSMA/CD + Exponential

backoff) Wireless LAN

• Random MAC protocol (CSMA/CA + RTS/CTS) + Polling

Bluetooth

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Bluetooth Design Objective

Design objective: a cable replacement technology to connect a small number of devices 1 Mb/s range 10+ meters single chip radio + baseband (means digital part)

• low power • low price point (target price $5)

Traffic Services SCO: Synchronous connected link (fixed periodical

traffic) ACL: Asynchronous connectionless link

Bluetooth

Nodes in Bluetooth form piconet: one master and upto 7 slaves Each radio can function as a

master or a slave SCO: a slave reserves with

the master a slot for a synchronous connected link

ACL: The master polls slaves for asynchronous connectionless traffic

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A piconet

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Bluetooth Links

Coexistence of Bluetooth and 802.11

Bluetooth shares the same freq. range as of 802.11

There are can be multiple piconets in close range, causing inteference (how about multiple 802.11?)

Question: how to share among piconets and with 802.11?

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Bluetooth Frequency Hopping

Divide spectrum into 79 frequencies

Master conducts pseudorandom frequency hopping

The slaves follow the pseudorandom jumping sequence of the master

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Bluetooth Frequency Hopping

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MAC: Summary In practical protocols, various MAC

techniques are often combined to achieve objectives GSM

• Channel partitioning and slotted Aloha Ethernet

• Random MAC protocol (CSMA/CD + Exponential backoff)

Wireless LAN• Random MAC protocol (CSMA/CA + RTS/CTS) +

Pollingo Bluetooth

o Time partitioning, polling, and random hopping

For physical layer, please see the optional slides linked on the schedule page

Backup

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Comparisons of Different Ethernet Standards

10Base2

10BaseT

100BaseT

1000BaseT

Bandwidth

10 Mbps

10 Mbps

100 Mbps

1000 Mbps

Topology Bus Star Star Star

Min frame size (not including preamble)

64 byte 64 byte 64 byte 64 byte (min slot time 512 byte, packet bursting)

Network Diameter

~200m ~2km ~200m ~200m