Broadband ISDN and ATM

발표자 : 박종민Jmpark@cs.chonbuk.ac.kr

Contents

1. Introduction to B-ISDN and ATM– The B-ISDN ATM Reference Model

2. Physical Layer in ATM– Virtual Circuits versus Circuit Switching

– Transmission in ATM Networks

– ATM Switches ( Knockout, Batcher-Banyan )

3. Data Link Layer in ATM

4. Network Layer in ATM– Cell Formats

– Connection Setup

– Routing and Switching

– Service Categories

– Quality of Service

– Congestion Control

– ATM LANs

5. ATM AAL Layer Protocols– Structure of the ATM Adaptation Layer

– AAL 1, AAL 2, AAL 3 / 4, AAL 5

– Comparison of AAL Protocols

– Service Specific Connection-Oriented Protocol(SSCOP)

1. Introduction to B-ISDN & ATM

• B-ISDN(Broadband Integrated Services Digital Network)

– A single physical network integrates variety of services

– Problems• QoS Requirements for services widely different

– Voice : Real time(low delay jitter), tolerates occasional losses– Data : Usually no real-time requirements, error-free, guaranteed delivery– Video : High bandwidth, low delay and jitter

• Traffic characteristics also widely different

• Certain applications require synchronization among multiple traffic streams

• ATM(Asynchronous Transfer Mode)

– Underlying technology that makes B-ISDN possible– Transmit all information in small, fixed size packet (ATM cell)

– Cell switching technology• Flexible and handle both constant(audio, video) and variable(data) rate traffic• Easier than traditional multiplexing techniques using fiber optics• Broadcasting(television distribution)

– Connection-oriented : Cell order is guaranteed

– Intended speed : 155 Mbps, 622 Mbps, gigabit speeds later• 155 Mbps(exact choice) : made for compatibility with AT&T’s SONET

B-ISDN ATM Reference Model

• Physical Layer(deals with physical medium)– Voltage, bit timing, various other issues.– Designed to be independent of the transmission medium

• ATM Layer(deals with cells and cell transport)– Defines the layout of a cell – Establishment and release of virtual circuits– Congestion control

• AAL Layer(ATM Adaptation Layer)– Allows users to send packets larger than a cell– Segments packets, transmits the cells individually, and reassembles then at the

other end

• User plane– Deals with data transport, flow control, error correction, and other user functions

• Control plane is concerned with connection management

2. Physical Layer in ATM

• B-ISDN service : Compromise between circuit and packet switching

• Virtual Circuits– Connection oriented

– Implemented internally with packet switching

– Connection Types• Permanent virtual circuits (No setup time)• Switched virtual circuits (Like telephone call)

– Circuit establishment• The route is chosen from source to destination• All switch along the way make table entries so they can route any packet on

that virtual circuit

Transmission in ATM Network

• Synchronous transmission mode

• Asynchronous transmission mode

– Not required that the stream of cells coming out of a computer be continuous(Gaps between the data cells are filled by idle cells)

– Does not standardize the format for transmitting cells

– All ATM links are point-to-point(unidirectional)

– Multicasting : Enter a switch on one line and exit it on multiple lines

ATM Switches

• General Model for an ATM cell switch

• ATM switches are generally synchronous– Cell arrive on the input lines asynchronously– Master clock marks the beginning of a cycle

• Common goal– Switch all cells with as low a discard rate as possible– Never reorder the cells on a virtual circuit

• Input queueing : head-of-line blocking

• Output queueing

The Knockout Switch

• Output queueing

Batcher-Banyan Switch

3. Data Link Layer in ATM

• TC(Transmission Convergence) sublayer

• Cell Transmission– takes a sequence of cells– add HEC to each one ( HEC : Header Error Control )– convert the result to bit stream– match the bit stream to the speed of the underlying physical transmission

system by inserting OAM cells as filler– OAM : Operation And Maintenance

• Cell Reception– takes an incoming bit stream– locates cell boundaries– verifies the header– processes the OAM cells– passes the data cell up to the ATM layer

• Locating the cell boundaries– HUNT : Shifting bits into the shift registers one at a time looking for a

valid HEC

– PRESYNCH : shifts in the next 424 bits(5-bytes) without examining them

4. Network Layer in ATM Network

• Connection-oriented layer– Basic element : virtual circuit(virtual channel)

– Unusual for a connection-oriented protocol• Does not provide any acknowledgements• Used for real-time traffic(audio, video)

– Guarantee : cells sent will never arrive out of order

• Supports a two-level connection hierarchy

Cell Formats

• UNI(User-Network Interface)– Define boundary between a host and an ATM network

(between the customer and the carrier)

• NNI(Network-Network Interface)– Define boundary between two ATM switches

GFC : General Flow Control PTI : Payload TypeVPI : Virtual Path Identifier CLP : Cell Loss PriorityVCI : Virtual Channel Identification HEC : Header Error Check

40 bits

40 bits

• GFC : flow control or priority• VPI : select a particular virtual path• VCI : select a particular virtual circuit• PTI : define the type of payload

• CLP : set by host to differentiate between high-priority traffic and low-priority traffic• HEC : checksum over the header

Payload type Meaning

000 User data cell, no congestion, cell type 0

001 User data cell, no congestion, cell type 1

010 User data cell, congestion experienced, cell type 0

011 User data cell, congestion experienced, cell type 1

100 Maintenance information between adjacent switches

101 Maintenance information between source and destination switch

110 Resource Management cell(used for ABR congestion control)

111 Reserved for future function

Connection Setup

• Messages used for connection establishment and release

• ATM address– 20 bytes(based on OSI addresses)

• Byte 1 : indicate which of three formats the address is in• Bytes 2-3 : country(alternatively, international organization)• Bytes 4 : format of the rest of the address

3-byte(authority), 2-byte(domain), 2-byte(area), 6-byte(address)

– 15 digit decimal ISDN telephone number

Message Meaning when sent by host Meaning when sent by network

SETUP Please establish a circuit Incoming call

CALL PROCEEDING I saw the incoming call Your call request will be attempted

CONNECT I accept the incoming call Your call request was accepted

CONNECT ACK Thanks for accepting Thanks for making the call

RELEASE Please terminate the call The other side has had enough

RELEASE COMPLETE Ack for release Ack for release

Connection setup in an ATM network

Connection release

Routing and Switching

• Route on VPI field, but not VCI field– Exception : route on VCI at the final hop in each direction

• Advantage– No new routing decisions have to be made

– Routing of individual cells is easier when all VCs for a given path are always in the same bundles

– Easier to switch a whole group of VCs

– Easier for carriers to offer closed user groups(private network) to corporate customers

• Examples : Rerouting a VP reroutes all of its VCs

Service Categories

• The ATM service categories

• Characteristics of the ATM service categories

Class Description Example

CBR Constant bit rate T1 circuit

RT-VBR Variable bit rate : real time Real-time videoconference

NRT-VBR Variable bit rate : non-real time Multimedia email

ABR Available bit rate Browsing the Web

UBR Unspecified bit rate Background file transfer

Service characteristic CBR RT-VBR NRT-VBR ABR UBR

Bandwidth guarantee Yes Yes Yes Optional No

Suitable for real-time traffic Yes Yes No No No

Suitable for bursty traffic No No Yes Yes Yes

Feedback about congestion No No No Yes No

Quality of Service

• Some of the quality of service parameters

Parameter Acronym Meaning

Peak cell rate PCR Maximum rate at which cells will be sent

Sustained cell rate SCR The long-term average cell rate

Minimum cell rate MCR The minimum acceptable cell rate

Cell delay variation tolerance CDVT The maximum acceptable cell jitter

Cell loss ratio CLR Fraction of cells lost or delivered too late

Cell transfer delay CTD How long delivery takes(mean and maximum)

Cell delay variation CDV The variance in cell delivery times

Cell error rate CER Fraction of cells delivered without error

Severely-errored cell block ratio SECBR Fraction of blocks garbled

Cell misinsertion rate CMR Fraction of cells delivered to wrong destination

• The probability density function for cell arrival times

Congestion Control

• Admission Control.– Must describe the traffic to be offered and the service expected.

– Check to see if it is possible to handle this connection.

– If no route can be located, the call is rejected.

– Denying admission should be done fairly.

• Resource Reservation.– Reserving resources in advance, usually at call setup time.

– Bandwidth can be reserved by having the SETUP message along each line it traverses.

• Rate-Based Congestion Control.– CBR and VBR : No dynamic congestion control are possible( real-time ).

– UBR : Nobody cares.

– ABR : How congestion should be detected, signaled, and controlled ?

• Rate-based solution– After every k data cells, transmits a special RM cell– RM(Resource Management) cell

• Travels along the same path as the data cells• Contains ER(Explicit Rate).• When it gets the destination, it is examined, updated, and sent back

– Congestion control mechanism• Overloaded switches generate RM cells and ship them back to the sender• Overloaded switches set the middle PTI bit

ATM LANs

• How to provide connectionless LAN service over the ATM network– Introduce a connectionless server into the network

– Every host has a (potential) ATM virtual circuit to every other host

LES : LAN Emulation Server

BUS : Broadcast/Unknown Server

5. ATM ALL Layer Protocol

• AAL : ATM Adaptation Layer

• Goal– Provide useful services to application programs– Shield application programs from the mechanical processing

• Original service classes supported by AAL(now obsolete)

• Defined four protocols to handle services(AAL 1 ~ AAL4)

Structure of the ATM Adaptation Layer

• ATM adaptation layer

• Convergence sublayer– Provide the interface to the application

– Service specific part

– Common part

• SAR(Segmentation And Reassembly) sublayer– Sender : add header and trailers to the data

– Receiver : reassemble the cells into message

Convergence sublayer (service specific part)

Convergence sublayer (common part)

Segmentation reassembly sublayer

AAL 1

• Used for transmitting class A traffic– Simple, connection-oriented, real-time constant bit rate

– Ex. Uncompressed audio and video

• CS sublayer– Break up the input message or stream into 46- or 47- byte unit

– Does not have any protocol header

• Cell format– SN(Sequence Number), SNP(Sequence Number Protection: checksum)

– P cell : used when message boundaries must be preserved

AAL 2

• Used for transmitting class B traffic– Rate can vary strongly in time

– Message boundary preserved

– Ex. Compressed audio or video

• As in AAL1, the CS sublayer does not have a protocol header• Cell format

– 1-byte header, 2-byte trailer, 45-byte data bytes

– IT(Information Type) : indicate that cell is the start, middle, or end

– LI(Length Indicator) : tells how big the payload is

AAL 3/4

• Protocols for classes C and D

• Stream or message mode– Message mode : Boundaries are preserved

– Stream mode : Boundaries are not preserved

• Multiplexing of several sessions onto one virtual circuit

• AAL 3 / 4 CS sublayer message format– CPI(Common Part Indicator) : Message type

– Btag, Etag : Used to frame message

– BA size, Length : Payload length

• Cell format : ST(Segment Type), MID(Multiplexing ID)

AAL 5

• SEAL(Simple Efficient Adaptation Layer)

• Service– Reliable service, Unreliable service– Unicast and Multicast(not guaranteed) are supported– Supports message mode and stream mode

• CS sublayer– UU(User to User) : available for a higher layer for its own purpose

• Sequencing, multiplexing

– Length : Payload length(not counting padding)

• SAR sublayer : does not add headers or trailers

• Some difference between the various AAL protocols

Comparison of AAL Protocols

Item AAL 1 AAL 2 AAL 3/4 AAL 5

Service class A B C/D C/D

Multiplexing No No Yes No

Message delimiting None None Btag/Etag Bit in PTI

Advance buffer allocation No No Yes No

User bytes available 0 0 0 1

CS padding 0 0 32-Bit word 0-47 bytes

CS protocol overhead(bytes) 0 0 8 8

CS checksum None None None 32 Bits

SAR payload bytes 46-47 45 44 48

SAR protocol overhead(bytes) 1-2 3 4 0

SAR checksum None None 10 Bits None

SSCOP

• AAL protocols does not provide for simple end-to-end reliable transport connections

• SSCOP– Service Specific Connection-Oriented Protocol

– Used for control, not for data transmission

– Fundamentally a dynamic sliding window protocol• No piggybacking• Periodically, sender asks to send back the bit map giving the window status