ece544: communication networks-ii, spring 2011 d. raychaudhuri lecture 3 includes teaching materials...

ECE544: Communication Networks-II, Spring 2011

D. Raychaudhuri

Lecture 3

Includes teaching materials from J. Kurose, L. Peterson and ATM Forum tutorials

Today’s Lecture• Switched Networks

– Switching Concepts– Ethernet Switches

• Learning bridge• Spanning tree• Multicast

– Asynchronous Transfer Mode (ATM) Network• Overview• Virtual Circuit Switching• Virtual Circuit and Virtual Path• ATM AAL• ATM Quality of Service (QoS) • Leaky Bucket Algorithm• Switch Implementation

Intro to Switching• Build a large network by interconnecting a number of switches• Easily add new hosts• Switching Techniques

– Datagram or connectionless (Ethernet)• Unique address• No need to setup connection

– Virtual circuit or connection-oriented (ATM)• Set up connection and maintain connection state

– Source routing• Source specify the whole or partial route to the

destination

Ethernet Hub• Hub is just a repeater

– Receive signal from one port and broadcast to all other ports

• Extends max distance between nodes, but collisions are propagated – Individual segment collision domains become one

large collision domain• Cannot interconnect different LAN technologies,

e.g. 10BaseT & 100BaseT

hub

hubhub

hub

Bridges/LAN switches • Link layer device

– stores and forwards frames– examines frame header and selectively forwards frame

based on MAC dest address– when frame is to be forwarded on segment, uses the

corresponding MAC to access segment (e.g. CSMA/CD for Ethernet)

hub

hubhub

Switch

Bridges/LAN switches (Cont.)

• Interconnect multiple LANs, possibly even support different IEEE 802.x types, e.g. 802.3 and 802.5, 802.11, but NOT 802.x with ATM

BridgeIP

LLC

802.3 MAC 802.3 MAC 802.5 MAC

LLC

IP

LLC

802.5 MACLAN LAN

Token-ring

Bridge

Ethernet Hubs vs. Ethernet Switches• An Ethernet switch is a packet switch for

Ethernet frames • Buffering of frames prevents collisions • Each port is isolated and builds its own collision domain

– Break subnet into LAN segments• Host can directly connect to switch, no collision, full

duplex

• An Ethernet Hub does not perform buffering:• Collisions occur if two frames arrive at the same time.

HighS

peedB

ackplane

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

OutputBuffers

InputBuffers

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

Hub Switch

A Switched Enterprise Network

InternetRouter

Switch

Forwarding• Which port to forward a frame?

– Use forwarding database/table< MAC address, port, Time-to-

Live (TTL)>• How to build the forwarding

table???– A routing problem

Forwarding Table

Destination

Port Time-to-Live (TTL)

A 3 2

B 0 4

C 3 2

D 3 4

E 2 5

F 1 1

G 0 4

H 0 5

01

3

0

2

2

2

13 3

0

1Host A

Host B

Host H

Host F

Host G

Host EHost D

Host C

Switch 1

Switch 2

Switch 3

Transparent Bridges Three parts to transparent bridges:

(1) Learning of Addresses(2) Forwarding of Frames(3) Spanning Tree Algorithm

• Forwarding tables entries are set automatically with a simple heuristic:

The source address field of a frame that arrives on a port tells which host is reachable from this port.

– When a frame received, switch “learns” location of sender

– records sender/location pair in forwarding table with TTL = MAX_TTL

• TTL reset to MAX_TTL every time a frame with the same source addr is received to refresh the existing table entry

• Entry removed when TTL counts down to 0

Self Learning (Learning Bridges)

Port 1

Port 2

Port 3

Port 4

Port 5

Port 6

Src=x, Dest=y

Src=x, Dest=y

x is at Port 1y is at Port 5

Frame Forwarding/Filtering

When switch receives a frame:

index forwarding table using MAC dest address

if entry found for destinationthen{

if dest on the same port from which frame arrived then drop the frame (filtering)

else forward the frame on port indicated

}

else flood

Forward on all but the port on which the frame arrived

Example

Bridge 2

Port1

LAN 1

A

LAN 2

CB D

LAN 3

E F

Port2

Bridge 2

Port1 Port2

•Consider the following packets: (Src=A, Dest=F), (Src=C, Dest=A), (Src=E, Dest=C)

•What have the bridges learned?

• Consider the two LANs that are connected by two bridges.

• Assume host n is transmitting a frame F with unknown destination.

What is happening?• Bridges A and B flood the frame

to LAN 2.• Bridge B sees F on LAN 2 (with

unknown destination), and copies the frame back to LAN 1

• Bridge A does the same. • The copying continues

Danger of Loops

LAN 2

LAN 1

Bridge BBridge A

host n

F

F F

FF

F F

Spanning Trees / Transparent Bridges

• A solution is to prevent loops in the topology

• IEEE 802.1d has an algorithm that organizes the bridges as spanning tree in a dynamic environment– Note: Trees don’t have loops

• Bridges that run 802.1d are called transparent bridges

LAN 2

Bridge 2

LAN 5

LAN 3

LAN 1

LAN 4

Bridge 5

Bridge 4Bridge 3

d

Bridge 1

Spanning Tree Protocol (STP)• Each bridge has a unique ID (MAC addr + priority level)• Select the bridge with the smallest ID as the root of the spanning

tree, called “root bridge”– All the ports on the root bridge are active (forwards the frames)

• Each bridge determines the minimum-cost path from itself to the root and nodes which of its port is on the path (root port)– Link cost: the cost of traversing a single network segment (link)– Path cost: the sum of the costs of the segments (links) on the path

• an administrator can configure the cost of traversing a particular segment (link)

• E.g. set the cost for every segment to 1, the path cost is a count of the number of bridges along the path.

– Root path cost: the cost of the minimum-cost path from this bridge to the root

– Root port: the port connecting to the minimum-cost path on this bridge

– Breaking ties: When multiple paths from a bridge are min-cost paths, choose the path using the neighbor bridge with the lower bridge ID.

– If the multiple ports connects this bridge and the neighbor bridge on the root path, choose the port with the lowest port ID as the root port.

Spanning Tree Protocol (Cont.)

• Select a single “designated bridge” and its designated port on each LAN segment– Designated bridge: the bridge on that LAN segment

with the minimum-cost path to the root. Only designated bridge allowed to forward frames to and from this LAN segment.

• If two or more bridges have the same root path cost, choose the one with the lowest bridge ID

– Designated port: the port connecting the designated bridge to this LAN segment

• If the designated bridges has two or more ports attached to this LAN, choose the port with the lowest port ID

• Any port that is not a root port or a designated port is blocked.

Spanning Tree Protocol (Cont.)

• Bridges exchange messages to configure the bridge (Configuration Bridge Protocol Data Unit, CBPDUs) to cut the loop and build the tree.– Source addr: port MAC addr, Dest. addr: STP multicast address – <sending bridge ID, root bridge ID, root path cost>

• At the beginning, each bridge considers itself to be the root, sends CBPDU identifying itself as root

• Upon receiving a CBPDU, check if the new path is better– if better, update its STP record, forward the message after

updating the root path cost in the message – After stabilization, only the root bridge generates new CBPDUs

regularly, others stops generate CBPDUs once learning it is not a root

• From a non-root port, receives a CBPDU indicating it is not the designated bridge for that segment, goes to blocking state

• BPDU is still received in blocking state.

Spanning Tree Protocol Example

B6B4

B1

B2

B7

B5B3

1. B3 receives (B2,0,B2)2. Since 2<3, B3 accepts B2 as root3. B3 adds 1 to distance advertised

by B2 and sends (B2,1,B3) to B54. Meanwhile B2 accepts B1 as

root because it has lower ID and sends (B1,1,B2) to B3

5. B5 accepts B1 as root and sends (B1,1,B5) towards B3

6. B3 accepts B1 as root and it notes that both B2 and B5 are closer to the root than it is.

7. B3 stops fowarding messages on both its interfaces

1. B3 receives (B2,0,B2)2. Since 2<3, B3 accepts B2 as root3. B3 adds 1 to distance advertised

by B2 and sends (B2,1,B3) to B54. Meanwhile B2 accepts B1 as

root because it has lower ID and sends (B1,1,B2) to B3

5. B5 accepts B1 as root and sends (B1,1,B5) towards B3

6. B3 accepts B1 as root and it notes that both B2 and B5 are closer to the root than it is.

7. B3 stops fowarding messages on both its interfaces

AA BB

CC

DDFF

KK

EE

II

JJ

HHGG

Virtual LAN• Group the stations in a broadcast domain, regardless of their

physical location. • A VLAN ID (VID) in the frame• A frame is not forwarded/broadcasted from one VLAN to

another VLAN• Each VLAN establishes its own spanning tree• Assign a port to one or multiple or all VLANs (static or

dynamic)

B1 B2

Host A

Host DHost C

Host B

VLAN 100 VLAN 100

VLAN 200 VLAN 200

• Overview• Virtual Circuit Switching• Virtual Circuit and Virtual Path• ATM AAL• ATM Quality of Service (QoS) • Leaky Bucket Algorithm• Switch Implementation

Asynchronous Transfer Mode (ATM) Network

Asynchronous Transfer Mode (ATM) Network

ATM Introduction

• 1990’s standards for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network (BISDN) architecture

• Goal: integrated, end-end transport of carry voice, video, data

• meeting timing/quality of service (QoS) requirements of voice, video (versus Internet best-effort model)

• “next generation” telephony: technical roots in telephone world

• packet-switching (fixed length packets, called “cells”) using virtual circuits

ATM VisionATM Vision

• ATM network moves cells (fixed length packets) with low delay and low delay variation at high speeds

• Devices at ends translate (e.g., segment and reassemble) between cells and original traffic

The Ultimate Integrated Services The Ultimate Integrated Services NetworkNetwork

The Ultimate Integrated Services The Ultimate Integrated Services NetworkNetwork

DataData Voice VideoVideo

Voice

Voice

DataDataDataData

VideoVideo

VideoVideo

ATMATMNetworkNetwork

ATM Basic ConceptsATM Basic Concepts• Negotiated Service Connection

– End-to-end connections, called virtual circuits

– Traffic contract

• Virtual circuit based switching– Dedicated capacity

• Cell Based– Small, fixed length

AA

Negotiated Service Connection

Negotiated Service Connection

Parameters Traffic

Characteristics Peak Cell Rate Sustainable

Cell Rate

Quality of Service Delay Cell Loss

Traffic Contract

Virtual Connection 1-QOS A

Virtual Connection 1-QOS B

Virtual Connection 1-QOS b

ATM System ArchitectureATM System Architecture

DataDataCellCell

VideoVideoCellCell

VoiceVoiceCellCell

AA

The ATM CellThe ATM Cell

• Small Size (low delay, but high overhead)– 5 Byte Header– 48 Byte Payload

• Fixed Size (easy switch implementation, but padding overhead)

• Header contains virtual circuit information• Payload can be voice, video or other data

types

HeaderHeader

5 Bytes 48 Bytes48 Bytes

PayloadPayload

AA

ATM Adaptation Layer (AAL)ATM Adaptation Layer (AAL)

• Only at edge of ATM network (end system)• Roughly analogous to Internet transport layer• Provides mapping Of applications (IP or native ATM

applications) to ATM service of the same type• Segments/Reassembles into 48 Payloads• Hands 48 Byte Payloads To ATM Layer

48 Bytes48 Bytes

AALAAL TypesTypes11 Circuit EmulationCircuit Emulation

-Constant Bit Rate (CBR) -Constant Bit Rate (CBR)

Low Bit Rate Voice (Real Time)Low Bit Rate Voice (Real Time)

-Variable Bit Rate (VBR)-Variable Bit Rate (VBR)

Time Invariant DataTime Invariant Data

““Simple” DataSimple” Data

22

3/43/4

55

AA

ATM LayerATM Layer

• Adds/Removes Header To 48 Byte Payload– Header Contains Connection Identifier, multiplexes 53

Byte cells into virtual connections, • ATM’s “Network” layer

– Transport cells across ATM network (analogous to IP network layer, but very different strategy and services than IP network layer)

– Signaling, cell switching, routing

}}

48-Byte48-BytePayloadsPayloadsFrom AALFrom AAL 5-Byte Header5-Byte Header

Header Contains Virtual Header Contains Virtual PathPath

and Channel Identifiersand Channel Identifiers

53-Byte Cell53-Byte CellTo Physical LayerTo Physical Layer

AA

Physical LayerPhysical Layer

Speed MatchingSpeed Matchingand Framingand Framing

CableCablePlantsPlants

Wide Range of Wide Range of SpeedsSpeeds

LAN, MAN, LAN, MAN, WAN WAN CompatibilityCompatibility

Uses Existing Uses Existing MediaMedia

Twisted PairTwisted Pair

CoaxCoax

FiberFiber -Multimode -Multimode

-Single Mode-Single Mode

TransmissionTransmissionFrameFrame

AA

ATM PHY: Two SublayersATM PHY: Two SublayersTransmission Convergence Transmission Convergence

SublayerSublayer

Physical Layer Medium Dependent Physical Layer Medium Dependent SublayerSublayer

• Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below– Specific to the PMD– Cell delineation– Cell rate decoupling, inserting idle (empty) cells when no data cells

to send (with “unstructured” PMD sublayer)• Physical Layer Medium Dependent Sublayer (PMD): depends on

physical medium being used– Probably use existing standards and technology– Medium, line code, connectors

ATM Physical Layer (Cont.)

Physical Medium Dependent (PMD) sublayer

• SONET/SDH: transmission frame structure (like a container carrying bits);– bit synchronization;– bandwidth partitions (TDM);– several speeds: OC3 = 155.52 Mbps; OC12 =

622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps

• TI/T3:transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps

• unstructured: just cells (busy/idle)

155 Mbps, SONET STS-3c/SDH STM-1

155 Mbps, SONET STS-3c/SDH STM-1

99

RRoowwss

270 columns270 columns

9 bytes9 bytes

MaintenanceMaintenanceandand

operationsoperations

1 Synchronous1 SynchronousPayload EnvelopePayload Envelope

(1 column of overhead)(1 column of overhead)

• 9 260 8/125 sec = 149.76 Mbps payload

. . .

125125 secsec

AA

ATM: network or link layer?

• Vision: provide the end-to-end transport: “ATM from desktop to desktop”– ATM is network technology

• Reality: used to connect IP backbone routers – “IP over ATM”– ATM as switched link layer, connecting IP

routers

ATM InterfacesATM Interfaces

PrivatePrivateUNIUNI

PrivatePrivateNNINNI

PublicPublicUNIUNI

ATMATMDXIDXI

B-ICIB-ICI

UNI User Network InterfaceNNI Network Node InterfaceB-ICI BISDN Inter-Carrier InterfaceATM DXI Data eXchange InterfaceFUNI ATM Frame Based UNI Interface

Metropolis Data Metropolis Data Services Inc.Services Inc.

Country Wide Country Wide Carrier ServicesCarrier Services

PublicPublicNNINNI

FUNIFUNI

FUNIFUNI

D

ATM UNI CellATM UNI Cell

CLP = Cell Loss PriorityCLP = Cell Loss Priority

5 Byte Header5 Byte Header

48 Byte Payload48 Byte Payload

Virtual Channel Virtual Channel IdentifierIdentifier

77 66 55 44 33 22 11 00

Payload Type Payload Type IdentifierIdentifier CLPCLP

Generic Flow Generic Flow ControlControl

Virtual Path Virtual Path IdentifierIdentifier



Virtual ChannelVirtual ChannelIdentifierIdentifier

Header ErrorHeader ErrorCheckCheck

PayloadPayload(48 bytes)(48 bytes)


ATM NNI CellATM NNI Cell


5 Byte Header5 Byte Header

48 Byte Payload48 Byte Payload


77 66 55 44 33 22 11 00


Virtual Path IdentifierVirtual Path Identifier







Generic Flow ControlGeneric Flow Control

• Used for UNI only - Not NNI• Currently undefined

• Set to 0000B

• Proposed future uses– Flow control– Shared media multiple access

77 66 55 44 33 22 11 00






CLPCLP




Payload Type Payload Type IdentifierIdentifier

B

Payload Type Identifier (PTI)

• Bit 3: Used to discriminate data cells from operation, administration, maintenance cells.

• Bit 2: Used to indicate congestion in data cells (Bit 3 = 0)– Set by Switches– Source and Destination Behavior Defined for Available

Bit Rate Flow Control VCC’s

• Bit 1: Carried transparently end-to-end in data cells– Used by AAL5


77 66 55 44 33 22 11 00






CLPCLP




CC

Cell Loss PriorityCell Loss Priority• Cells with bit set (CLP =1) should be

discarded before those with bit not set (low priority)

• Can be set by the terminal• Can be set by ATM switches for internal

network control– Virtual channels/paths with low quality of

service– Cells that violate traffic management

contract• Key to ATM Traffic Management

77 66 55 44 33 22 11 00






CLPCLP





Header Error CheckHeader Error Check• Header error control

– Detection mode:• Protects header only (all five bytes)• Discards cell when header error

– Correction mode (optional): Correct 1 bit errors else discard when error detected

• Reduced cell loss in face of single bit errors• Reduced error detection for multiple bit errors

• Cell delineation for SONET, SDH, etc...• Recalculated link-by-link because of VPI/VCI value

changes

77 66 55 44 33 22 11 00






CLPCLP





B

Why 53 Bytes?Why 53 Bytes?

64 + 564 + 5 32 + 432 + 4

48 + 548 + 5

• Compromise reached in ITU-TS Study Group XVIII in June 1989

Queuing Delay Advantage of Small Cells

••••••

100 byte message100 byte message

100 other active100 other activeconnectionsconnections 45 Mbps45 Mbps

Delay and delay variation are small for small messages e.g., a digitized voice sample

But high header overhead

High overheadHigh overhead Wait for other cellsWait for other cells

Just fits in one cellJust fits in one cell

Payload (bytes)Payload (bytes)

MaxMax

DelayDelay

(ms)(ms)

0022446688

10101212

11 5050 100100 150150 200200 250250 300300

AA

Packetization Delay Advantage of Small Cells

Packetization Delay Advantage of Small Cells

Percent Overhead and Packetization DelayPercent Overhead and Packetization Delayfor 64 Kbps Voicefor 64 Kbps Voice

Payload (Bytes)Payload (Bytes)

00

2020

4040

6060

8080

100100

00 2020 4040 6060 808000

22

44

66

88

1010DelayDelay

OverheadOverhead

Dela

y (

ms)

Dela

y (

ms)

% O

verh

ead

% O

verh

ead

Virtual Circuit Switching• Establish connection (virtual circuit) before any data is sent

– Permanent Virtual Circuit (PVC), manually or setup signaling initiated by the network administrator,

• Long lasting connections, e.g. “permanent” coonections for two IP routers

– Switched Virtual Circuit (SVC), setup using signaling by one of the hosts

• Dynamically set up on per-call basis– Negotiate QoS (bandwidth, delay, etc)– link,switch resources (bandwidth, buffers) may be allocated to VC:

to get circuit-like performance

• Each switch on source-destination path maintains “connection state” for each passing connection– Incoming interface, incoming virtual circuit identifier (VCI),

outgoing interface, outgoing VCI, reserved bandwidth, buffer, delay…

• Tear down• Forwarding: each cell/packet carries VC identifier (not

destination ID)

Virtual Paths and Virtual Channels

Physical LinkPhysical Link

Virtual PathVirtual Path

Virtual ChannelVirtual Channel

77 66 55 44 33 22 11 00






CLPCLP





• Bundles of Virtual Channels are switched via Virtual Paths – Better scalability (i.e. more capable of growing to

large numbers of circuits)

ATM VCs

• Advantages of ATM VC approach:– QoS performance guarantee for connection

mapped to VC (bandwidth, delay, delay jitter)

• Drawbacks of ATM VC approach:– Inefficient support of datagram traffic– one PVC between each source/dest pair) does

not scale (N*2 connections needed) – SVC introduces call setup latency, processing

overhead for short lived connections

Switched Virtual CircuitsSwitched Virtual Circuits

ATM SwitchATM Switch

Signalling ChannelSignalling Channel(VPI/VCI = 0/5)(VPI/VCI = 0/5)

Signalling ChannelSignalling Channel(VPI/VCI = 0/5)(VPI/VCI = 0/5)

• Switch and terminal exchange signalling messages using the predefined signalling channel, VPI/VCI = 0/5

CallCallProcessingProcessing

B

Permanent Virtual CircuitsPermanent Virtual Circuits

• Long setup time (especially with human intervention) means that connections are left active for long periods of time e.g., days, weeks

• VPI/VCI tables setup in terminals and switches

NetworkNetworkManagementManagement

SystemSystem

VPI/VCIVPI/VCI

VPI/VCIVPI/VCIVPI/VCIVPI/VCI

VPI/VCIVPI/VCI

B

Virtual ConnectionsVirtual Connections

PortPort VPI/VCIVPI/VCI PortPort VPI/VCIVPI/VCI 11 0/370/37 33 0/760/76 11 0/420/42 55 0/520/52 22 0/370/37 66 0/220/22 22 0/780/78 44 0/880/88

Connection TableConnection Table

3737 4242

3737 7878

7676

5252

2222

8888

11

22

33

44

55

66

VideoVideo DataData

Voice

VideoVideo

DataData

Voice

VideoVideo

DataData

Voice

VideoVideo

VideoVideo

VideoVideo

77 66 55 44 33 22 11 00






CLPCLP





Call Control SignallingCall Control SignallingCall control protocol is used to establish, maintain, and clear

virtual channel connections between a user and networkCall control protocol is used to establish, maintain, and clear

virtual channel connections between a user and network

UserUser NetworkNetwork

UNIUNI

UNI or NNIUNI or NNI

UNI or NNIUNI or NNI

VirtualVirtualChannelChannelConnectionsConnections

InterfaceInterface

Call ControlSignalling

Call Control

Signalling

Setting Up a Call - 1Setting Up a Call - 1

SetupSetup

CallCallProceedingProceeding

AA BB

A wants to communicate with BA wants to communicate with B

• Setup message– Call reference– Called party address– Calling party address– Traffic characteristics– Quality of service

• Call proceeding message– Call reference– VPI/VCI

B


• Internal network processing– Resource

availability checking

– Virtual channel or path routing

– Function of the Network Node Interface (NNI)

SetupSetup


B


SetupSetup


SetupSetup

Call Call ProceedingProceeding

• Setup message– Call reference– Called party address– Calling party address– Traffic characteristics– Quality of service– VPI/VCI

Call ProceedingCall Proceeding Call referenceCall reference

Called user Called user deciding to accept deciding to accept callcall

B


SetupSetup


SetupSetup


Connect AckConnect Ack

Connect Connect • Connect message

– Call reference– Indicates call

acceptance• Connect Acknowledge

– Call referenceB


SetupSetup


SetupSetup



Connect Connect


Connect Connect

• Calling party informed that call is available for user information exchange

B

Bandwidth NegotiationBandwidth Negotiation

Setup Setup (20 Mb/s)(20 Mb/s)

UNIUNI NNINNI UNIUNI

Connect Connect (10 Mb/s)(10 Mb/s)





NNINNI

• Supports 212 Virtual Paths• Supports virtual connection

routing– Distribution of topology

information– Distribution of resource

availability information• Public version being

standardized by ITU TS• Private version specified by

ATM Forum Technical Working Group


Cell FormatCell Format


77 66 55 44 33 22 11 00









C

Address to End StationAddress to End Station

PrivatePrivateUNIUNI PublicPublic

UNIUNI

ATM End ATM End System System Address Address (AESA) (AESA) FormatFormat

Native Native E.164 orE.164 or

AESAAESA

Private ATM SwitchPrivate ATM Switch

Public ATM

Network

ATM Service CategoriesATM Service Categories• Constant Bit Rate (CBR)

– Continuous flow of data with tight bounds on delay and delay variation• Real-Time Variable Bit Rate (rt-VBR)

– Variable bandwidth with tight bounds on delay and delay variation • Non-Real-Time Variable Bit Rate (nrt-VBR)

– Variable bandwidth with tight bound on cell loss• Available Bit Rate (ABR)

– guarantee minimum– Flow control on source with tight bound on cell loss

• Unspecified Bit Rate (UBR)– No guarantees (i.e., best effort delivery)

Service Model

Guarantees? Congestion feedbackBandwidth Loss Order Timing

CBR Constant rate yes yes yes No congestion

VBR Guaranteed rate

yes yes yes No congestion

ABR Guaranteed minimum rate

no yes no yes

UBR none no yes no no

AAL1: Adaptive Clock MethodAAL1: Adaptive Clock Method

Water MarkWater Mark

Speed up Speed up bit clockbit clock

Slow down bit Slow down bit clockclock

Received CellsReceived Cells

Substitute Substitute CellsCells

Continuous Bit Continuous Bit StreamStream

• AAL1: for constant bit rate (CBR) services, e.g. circuit emulation

• Bit stream rate is independent of ATM network and (theoretically) can be any value

• Cell delay variation is critical to buffer sizing and bit clock jitter

Reconstructing the bit streamReconstructing the bit stream

B

User 1User 1 User 2User 2 User 3User 3 User 1User 1 User 3User 3

ATM CellATM Cell

Cell HeaderCell Header(5 Octets)(5 Octets)

AAL2 HeaderAAL2 Header(3 Octets)(3 Octets)

AAL2 PayloadAAL2 Payload(variable)(variable)

AAL2• AAL2: variable bit rate (VBR) services, e.g. MPEG video• Emulation small payload to reduce packetization delay small payload to reduce packetization delay

– One cell can carry data from multiple usersOne cell can carry data from multiple users

B

AAL3/4AAL3/40 - 655350 - 65535 BytesBytes

BytesBytes44 4 4

CPCSCPCSHeaderHeader

CPCSCPCSTrailerTrailer

• AAL3/4 for data (e.g. IP datagrams)

• 44 Bytes of Data per Cell• Type: the first, last, middle or single cell• SEQ: sequence # of the cell (to detect the cell loss)• Message Identifier (MID) multiplex several PDUs

onto a single virtual connection• Len: length = # of bytes of PDU in the cell• CRC-10: Checking per Cell

22

DataData

D

Protocol Data Unit (PDU)

Common Part Convergence Sublayer (CPCS)

Protocol Data Unit (CS-PDU)

4444

User DataUser DataMMIIDD

22

LLEENN

SSEEQQ

TTYYPPEE

CCRRCC

4444

User DataUser DataMMIIDD

22

LLEENN

SSEEQQ

TTYYPPEE

CCRRCC

22

4444MMIIDD

22

LLEENN

SSEEQQ

TTYYPPEE

CCRRCC

Padding

22

BytesBytes

data

Segmentation and Reassembly (SAR) sublayer

AAL Header

AAL Trailer

PadPad

AAL 5AAL 50 - 655350 - 65535

2222 44 BytesBytes

BytesBytes

Not drawn to scaleNot drawn to scale

Error detection fieldsError detection fields

. . .. . .

Last cell flagLast cell flag

• 48 Bytes of Data per Cell

• Uses a paidload type identifier (PTI) bit in the ATM header to Indicate Last Cell

• Only One PDU at a Time on a Virtual Connection

• CRC-32: Per PDU CRC for error checking

4848

4848

0

1

DataData0-470-47

00LLeenn

CCRRCC

C

PDU

CS-PDU

77 66 55 44 33 22 11 00






CLPCLP





ATM AddressingATM Addressing• Public networks

– E.164 numbers (telephone numbers)– Up to 15 digits

• Private networks– 20 byte address– Format modeled after OSI NSAP (Network

Service Access Point)– Mechanisms for administration exist– Hierarchical structure will facilitate virtual

connection routing in large ATM networks– MAC address will be encapsulated within

NSAPB

ATM End System Address (AESA)ATM End System Address (AESA)

Selector (Not used by Network for Routing)Selector (Not used by Network for Routing)

PrivatePrivateUNIUNI

AESAAESAFormatFormat Network-SuppliedNetwork-Supplied EndEnd

System-System-SuppliedSupplied

SELSEL

SELSEL

4747 SELSELHO-DSPHO-DSP End System IDEnd System ID

4545 E.164 NumberE.164 Number HO-DSPHO-DSP End System IDEnd System ID SELSEL

3939 SELSELDCCDCC HO-DSPHO-DSP End System IDEnd System ID

IDCIDC

Higher Order Domain- Specific Part (HO-DSP)-routing field

Data Country Code (DCC)Data Country Code (DCC)

International Code Designator (ICD)International Code Designator (ICD)

E.164 Private AddressE.164 Private Address

IP over ATM• Classic IP

– Layer 3 “networks” – connect LANs– MAC (802.3) and IP addresses

• IP over ATM– Replace LAN segments with

ATM network– ATM addresses, IP

addresses

TransportIPEthPhy

Appl

TransportIP

AALATM

Appl

Phy

Phy

ATM

Phy

ATM

TransportIP

EthPhy

Appl

IP

Eth

Phy Phy

ATM

AAL

IP

Eth

PhyPhy

ATM

AAL

IP-over-ATM (Cont)• Packet journey in IP-over-ATM network

– at Source Host (IP-over-ATM router):• IP layer maps between IP and ATM dest address

– IP packet into ATM AAL5 PDUs– from IP addresses to ATM addresses just like

IP addresses to 802.3 MAC addresses (ARP)• passes datagram to AAL5• AAL5 encapsulates data, segments cells, passes

to ATM layer – ATM network: moves cell along VC to destination– at Destination Host (IP-over-ATM router): :

• AAL5 reassembles cells into original datagram• if CRC OK, packet is passed to IP

Ethernet Switching vs. Virtual Circuit Switching

• No connection setup (connection less)

• Packet carries dest. addr.• Switching based on globally

unique MAC address• a host does not know

whether the network is capable of delivering the packet when it sends the packet

• Each packet is forwarded independently and may be out of order

• A switch and link failure might not have any serious effect if it is possible to find an alternate route

• Establish connection state before sending any data (connection oriented)– Setup latency, processing

overhead, scalability (capability to grow to a large network)

• Packet/cell carries VCI• Switching based on incoming

port + VCI (unique per port)– VCI changed at the output

port• Negotiate the QoS parameters

and allocate resources (buffer, bandwidth) to VC– If no enough resource, reject

the connection request– QoS performance guranteed

for connection (bandwidth, delay, delay jitter)

• Each cell is routed along the established connection in order

• If a switch or a link fail, tear down the old connection and establish a new connection

Many ATM ideas adopted in IP networks called MPLS (more later)

70

Today’s Homework• Peterson & Davie, Chap 3, (same numbers for

both 3rd and 4th eds)-3.1 -3.5 -3.7-3.8-3.13 -3.26 -3.30

• Download and browse ATM UNI4.0 spec and relate contents to today’s lecture

71

Additional ATM Slides for QoS Lecture

Quality-of-ServiceQuality-of-Service

• Problem: Providing quality of service– How should ATM network resources be allocated to

ensure good performance including preventing congestion, e.g., how many virtual channels should be assigned to a particular transmission link?

• Solution: Traffic Management– Specify the traffic "contract" on each virtual

channel/path– Route (including rejecting setup request) each virtual

channel/path along a path with adequate resources (Admission Control)

– Mark (via Cell Loss Priority bit) for loss all cells that violate the contract (Traffic Policing)

77 66 55 44 33 22 11 00






CLPCLP





Quality of Service Parameters

• Mean cell rate• Peak cell rate• Burst size• End to end transit delay• Acceptable Cell Delay Variation (CDV)

(Forwards, Backwards)• Cumulative CDV (Forwards, Backwards) • Cell Loss Ratio (Forwards, Backwards)

Leaky Bucket Algorithm

• Provides traffic shaping:– smooth bursty

arrivals• Input bursty. Output

rate controlled.

• Provides traffic policing: Ensure that users are sending traffic within specified limits– Excess traffic

discarded or admitted with CLP = 1

Fixed Fixed transmission transmission

raterateSmoothed flowsSmoothed flows

FIFO FIFO QueuQueu

ee

Varying flowsVarying flows

LuckyLuckyBuckeBucke

tt

Overflow, Overflow, Non-conformingNon-conforming

Leaky Bucket Algorithm (Cont)

• A counter associated with each user transmitting on a connection – Counter = Counter - 1 periodically, until 0– Counter = Counter +1 whenever a cell arrives at the

bucket– If the counter > a threshold, cell is not conforming

• Tag the cell with CLP=1 (drop if no capacity in network), or • discards the cell immediately.

• the counter decrementing rate determines the average bandwidth

• the value of the threshold measures the burstiness

• User specifies leaky bucket parameters when connection set up and is expected to conform to the traffic contract when sending data

Generic Cell Rate Algorithm (GCRA)

• Equivalent to the leaky bucket algorithm.• A counter scheme based on two

parameters GCRA(I,L)– Increment parameter: I

• Inter-cell time = 1/PCR (peak cell rate)• Affects cell rate

– Limit parameter: L• Leaky bucket size L+I, rate 1• affects cell bursts One unit One unit

leak per leak per unit of unit of timetime

L + L + II

cell arrivalcell arrival

L

I

Last cell arrival

Theoretical Arrival Time (TAT)

Time I-L L

I

OKNO OK

Generic Cell Rate Algorithm (GCRA)• For a sequence of cell

arrival times, {tk}, determines which cells conform to the traffic contract

• “Leaky bucket”– A cell that would

cause the bucket to overflow is non-conforming

Cell arrival at t

t > TAT (late)? TAT=t

t < TAT-L (too early)?

TAT=TAT+I

Conforming Cell

NON-Conforming Cell

Yes

YesNo

No

Last cell arrival

Theoretical Arrival Time (TAT)

Time I-L L

I

OKNO OK

ece544: communication networks-ii, spring 2011 d. raychaudhuri lecture 3 includes teaching materials...

Documents