ATM Adaptation Layer (AAL)• AAL is responsible for adaptation of information

of higher layers to the ATM cells (in the transmission direction) or adaptation of ATM cells into the information of the higher layer (receiver direction).

• AAL is subdivided into two sublayers:- SAR (Segmentation and Reassembly)- CS (Convergence Sublayer): Multiplexing, loss detection, timing recovery, message identification

ATM Adaptation Layer (AAL)• AAL provides a variety of services:

Class 1: Circuit Emulation with Constant Bit Rates (CBR). Voice of 64 kbps Fixed Bit Rate (Voice,Video)

Class 2: Connection-oriented service with Variable Bit Rates (VBR) and timing between source and destination. VBR Video & Audio

Class 3: Connection-Oriented Service. Data Transfer and Signaling ABR Traffic with no timingClass 4: Connectionless Data Service

SMDS, Ethernet, Internet, Data Traffic, No constraints.

Traffic ClassesA DCB

Yes No

Constant Variable

ConnectionlessConnection Oriented

1 2 ¾ or 5 ¾ or 5

TimingBetween

Source andDestination

Bit Rate

ConnectionMode

AAL

ExampleDS1, E1

N64 KbpsEmulation

Packet Video, Audio(Real Time)

FrameRelay

IP,Ethernet

General Structure of AALAAL-SAP

ATM-SAP

CONVERGENCE SUBLAYER(CS)

SAR SUBLAYER

PrimitivesAAL

ConvergenceSublayer

(CS)

Segmentatioin&

Reassembly(SAR)

Sublayer

Service Access Point

• Service Data Unit (SDU) crosses the SAP• PDU is data unit between peer layers

General Data Unit Naming ConventionAAL-SAP

AAL-SDU

CS-PDU PayloadCS-PDUHeader

CS-PDUTrailer

SAR-PDU PayloadSAR-PDUHeader

SAR-PDUTrailer

ATM-SAP

ATM-SDU

Cell information Field(Cell Payload)

CellHeader

PL-SAP

CS-PDUNo SAP is defined between CS and SAR

SAR-PDU

ATM CellATM Layer

Physical Layer

SegmentationAnd Reassembly(SAR) Sublayer

ConvergenceSublayer(CS)

AAL Interfaces

AAL

Segmentationof CS-PDU

Structure of AAL with SSCS and CPCS

Service SpecificConvergence Sublayer (SSCS)

AAL-SAP

ATM-SAP

Common PartConvergence Sublayer (CPCS)

Segmentation And Reassembly (SAR)

AAL-PDU Primitives

SSCS-PDUPrimitives

SAR-PDU Primitives

CPCS-PDUPrimitives

SSC

SC

PCS

C SSA R

AA

L C

omm

on P

art

(CP)

AA

L

AAL Type 1AAL 1 provides the foll. services to the AAL users:• Transfer of service date unit with a constant source bit-rate and their delivery with the same bit rate - Voice traffic 64kbps: as in N-ISDN to be transported over an ATM network. This service is called circuit emulation. In other words, how TDM type circuits can be emulated over ATM.

• CBR-Voice; CBR-Video (fixed (constant) bit rate video)

AAL Type 1• Transfer of timing information between source and destination.

• Transfer of structure information between source and destination; some users may require to transfer of structured data, e.g., 8 kHz structured data for circuit mode device for 64 kbps (N-ISDN). • Indication of lost or errored information which is not covered by AAL1, if needed.

AAL Type 1 (Cont.)The functions listed below may be performed in the AAL in order to enhance the layer service provided by the ATM layer:

• Segmentation and reassembly of user information

• Handling of cell delay variation to achieve constant rate delivery (playout buffer)

• Handling of cell payload assembly delay

• Handling of lost and misinserted cells (SN processing) Discarded

• Source clock frequency recovery at the receiver - 4 bit RTS is transferred by CSI - handling of timing relation for Asynchronous transfer (SRTS Synchronous Residual Time Stamp)• Monitoring of AAL-PCI (Protocol Control Information) for bit errors• Handling of AAL-PCI bit errors

AAL Type 1 (Cont.)

PCISAR-PDU HeaderCS-PDU HeaderCS-PDU Trailer

• Monitoring of the user information field for bit errors and possible corrective action - FEC maybe performed for high quality video or audio (124,128 Reed Solomon code)

AAL Type 1 (Cont.)

AAL Type 1 (cont.)

Receiver’s Responsibilities are as follows.

• Examine the CRC and parity bit for error detection.• Correct single bit errors in SN field. If multiple bit errors in SN field, then declare invalid.• Reassemble the CS-PDU in correct sequence using SN-numbers.• Discard misinserted CS-PDUs and generate dummy information for missing CS-PDU.

AAL Type 1 (Cont.)

• Buffer the received CS-PDUs to compensate for cell delay variation (jitter) to achieve constant rate delivery. (PLAYOUT Buffer)• Clock frequency recovery (Handling of timing relationship for asynchronous circuit transport)• Monitoring and handling AAL-PCI (Protocol Control Information) SAR-PDU Header, SAR-PDU Trailer, CS-PDU Trailer are collectively called AAL-PCI.

STACK

ATM

Convergence Sublayer

- accepts 124-byte blocks from user- appends 4-byte FEC- writes to matrix “row”- forwards CS-PDU to SAR when 47 blocks (rows) have been written

Segmentation/Re-assembly Sublayer

- reads matrix “columns”(47bytes)- effect: interleaving

Adaptation Layer Type 1 (AAL1)

• Forward Error Correction

• No Retransmission

* (124,128) Reed-Solomon Code* Polynomial undefined* Corrects 2 errored bytes per row* Corrects 4 “erasure” bytes (knows position)* Uses interleaving

FEC in AAL1

Cell 1Byte 1

Cell 2Byte 1

Cell 1Byte 2

Cell 1Byte 47

Cell 2Byte 2

Cell 2Byte 47

Cell 124Byte 1

Cell 124Byte 2

Cell 124Byte 47

ReadingR-S Code with 4 byte FEC

Reed-Solomon Code recovers up to 4 lost cells in a block of 128.

AAL 1User Data Bit Stream

CPCS-PDU Payload

AAL-SAP

…SAR-PDUPayloadH SAR-PDU

PayloadH SAR-PDU

PayloadH

ATM-SAP

Cell Payload

1B 47B

48 Bytes

H Cell PayloadH Cell PayloadH…

53 Bytes5B

Higher Layers

CS

SAR

ATMLayer

AAL

SAR-PDU of AAL 1

Cell Header SN SNP SAR-PDU Payload

4 bits 4 bits

SAR-PDU Header

1 Octet 47 Octets

SAR-PDU (48 Octets)

SN (Sequence Number) for numbering of the SAR-PDUs

SNP (Sequence Number Protection) to protect the SN field

To detect lost or misinserted cells (Error Detection & Correction)

SAR-PDU Header of AAL 1

CSI Sequence Count

CRC Even Parity

13)( xxxG

13

)( xxxG

1 bit 3 bits 3 bits 1 bit

SN Field SNP Field

CSI Field• Sequence Count 0, .., 7 • CSI bit used to transfer timing info. or info. about data structure. • CSI values in cells 1,3,5,7 are inserted as a 4-bit timing value.

• In even numbered cells 0,2,4,6, CSI used to support blocking of info. from a higher layer.

• If CSI bit is set to 1 in a cell 0,2,4,6, then the first octet of SAR-PDU payload is a pointer that indicates the start of the next structured block within the payload of this cell and the next cell, i.e., 2 cells (0-1, 2-3, 4-5, 6-7) are created as containing a 1-octet pointer and a 93-octet payload and pointer indicates where in that 93 octet payload is the first octet of the next block of data.

P & Non-P FormatsAAL-1 CS uses a pointer to delineate the structure boundaries.

Supported by 2 types of CS_PDUs called non-P & P

Can be used only in SAR PDUs with even SN values (because SRT scheme uses the CSI bits in SAR PDUs with odd SN values)

SN SNP SAR-PDU Payload (User Data) Non P-format

1 Octet 47 Octets

SAR-PDU Header

SAR-PDU (48 Octets)

SAR-PDU Payload SN SNP

1 Octet 1 Octet 46 Octets

SAR-PDU Header

(CSI = 1)

Reserved for Pointer

P-format

SAR-PDU

Header

Structure

Pointer

Field

User

Data

P-Format

Sequence Counter

0,2,4,6

Reserved Bit Offset Field

7 Bits are the offset measured in Bytes between the end of the pointer field & start of the structured block in 93 bytes consisting of remaining 46 bytes in this CS-PDU & 47 Bytes of the next CS-PDU. This offset may range from 0-92.

7 Bits

SN even uses 1 Octet Pointer field to indicate the offset into the current payload of the first octet of a n*DSO payload.

Value of n may be as large as 92 in the P-format since pointer is repeated every other cell when supporting AAL 1.

AAL1

Unstructured Data Transfer

STD Mode(Structured Data Transfer)

DS1/E1(1.544Mbps) DS3/E3

(45Mbps)

n x DSO (64kbps) Service

(supports an octet structured n – DSO Service)

including timing SRTS Method

(4-bit RTS included in CSI Bit !!)One sent in (1,3,5,7)

CSI bit (in even SN values) for SDT to convey information about internal byte alignment structure of the user data bit stream.

Structured Data Transfer

• Kind of fractional DS1/E1 service where the user only requires an n*64kbps (DS0) connection where n can be small as 1 and as high as 24 for DS1 (T1) and 30 for E1.

• An n*64 kbps service generates blocks of n bytes which are carried in P and non-P format CS-PDUs.

• The beginning of a block is pointed to by the pointer in the 1-byte header of the CS-PDU-- > P format.

1 192 1 1 1 1 1 1 1192 192 192 192 192 192 192

193 175 18 193 165 28 193 147 46 193 137

p

47=376

SN=0CSI=1

P-Format

46=368

SN=1CSI=0

Non-P-Format

46=368

SN=2CSI=1

P-Format

47=376

SN=3CSI=0

Non-P-Format

0-1 = 93 Octets 2-3=93 Octets

Pointer indicates where in that 93 octet payload is the first octet of the next block of data.

No structured boundary, then use dummy offset value of 127.

EXAMPLE: STRUCTURED DATA TRANSFER

DS1 Signal

CS-PDUs

Unstructured Data Transfer

* The entire DS-1/E1 signal is carried over an ATM network.

• The DS-1 signal is received from user A which is packed bit-by-bit into the 47-byte non-P format CS-PDU which then becomes the payload of a SAR-PDU.

DS1 CIRCUIT EMULATION USING AAL 1

Transmitter uses AAL 1 operating in SRTS mode to emulate a DS 1 digital bit stream created by a video codec. DS1 frame has 193 bit frames that repeat 8000 times per second (192 user data bit + 1 framing bit). CS computes the RTS every 8 cell times and provides this to the SAR sublayer for insertion in the SAR header. 193 bit frames are packed into 47 octet SAR-PDUs by SAR layer. SAR then adds the SN, inserts the data from CS, computes CRC and parity over SAR header and passes 48-octet SAR-PDU to ATM layer.

octets

HeaderHeader

SAR-PDU

SAR-PDU

octets1

47

5

48

bits1

192

1

192

192

192

192

192

1

1

1

1Time

DS1 Signal SRTS CS SAR-PDUsATM Cells

HeaderHeader

SAR-PDU

SAR-PDU

octets1

47

5

48

HeaderHeader

SAR-PDU

SAR-PDU

octets1

47

5

48

RTS

EXAMPLE: UNSTRUCTURED DATA TRANSFER

Handling of Lost and Misinserted Cells in AAL1

• At the transmitter, CS provides SAR with a Sequence Count Value and a CSI associated with each SAR-PDU payload. Sequence Count Value starts with 0, and incremented sequentially and is numbered modulo 8.

• At the receiver, CS receives Sequence Count, CS indication from SAR, and check status of Sequence Count and CS indication. CS identifies SAR-PDU payload sequence SAR-PDU loss, and SAR-PDU misinsertion.

• CSI is used to transfer timing information and default value of CSI is “0”. 4 bit RTS is sent in odd-sequence-numbered PDUs (1,3,5,7) in SRTS approach.

Handling of Lost and Misinserted Cells in AAL1(Cont.)

Remark:

• For each SAR-PDU, SAR receives a sequence number (SN) value from CS.

• At the receiver, SAR passes the SN to CS. The CS may use these SNs to detect lost or misinserted SAR-PDU payloads.

• SAR protects the SN value and CSI against bit errors. It informs the CS when SN value and the CSI are in error and cannot be corrected.

• Transmitter computes the CRC value across the 4 bits of SAR-PDU header and inserts into CRC field. CRC contains the remainder of the division (mod 2) by polynomial of the product multiplied by the contents of SN field.

• After completing the above operations, transmitter inserts the even parity bit. 7 bit code word protected.

3x1xx 3

TIMING (CLOCK) RECOVERY TECHNIQUES IN AAL 1

1. Adaptive Clocking in AAL 1

(No Network clock is available).

2. Synchronous Residual Time Stamp Approach (SRTS)

(Global Network Clock is available)

Common network reference clock is not available!!!

1. Adaptive Clocking (Receiver)

Cells PLAYOUT BUFFER

Used for Transfer

Delay Variable

Receiver reads info. with a local

clock.

Receiver writes received infofield in this buffer.

PLL (Phase Lock Loop)Provides local clock.

(Content) Filling level ofthe buffer is used to controlthe frequency of the local clock.

CONTROL is performed by continuously measuring the fill level around its median position & by using this measure to drive the PLL providing the local clock.

The content level of the buffer may be maintained within an upper limit and lower limit to present buffer overflow and underflow.

Underflow => PLL slowed down Overflow=> PLL speeded up

Network

DataFIFO Data

Terminal

Filling Level

Local Clock Jitter

Filter PLL.

Adaptive Clocking in AAL 1

Synchronous Residual Time Stamp (SRTS) Approach

BASIC IDEA: Convey a measure of the frequency difference between the reference clockand source clock. Network reference clock is available, source clock is not syncronized!

Sender NETWORK

Common Network Clock

Receiver

Difference between the localand network clocks.

Local Clock

TIMESTAMPCSI fieldSequence # field

Odd # ofsegments

Differencebetween 2

clocks

Transport this info. in odd numberedCells (CSI Field) to destination

LocalClock

Source Transmitter

(Assumed) • Common Network clock is available

• Source (local) clock is not synchronized with it.

• SRTS method conveys a measure of the frequency difference between the derived network reference clock and the source (local) clock.

• The derived network reference clock is determined from the frequency of the network clock divided by some integer.

• Within a time interval of N “source clock cycles” suppose there are M cycles of the derived “network reference clock”.

• There is a nominal value Mnom (fixed and known for the service) and the actual value of M may vary anywhere within a certain range (Mmin & Mmax) around this nominal value Mnom.

• The actual value of M will be the sum of Mnom and a residual part.

• By transmitting the residual part, the receiver has enough info to construct the source clock.

t

Tolerance

Source Frequency (fs)

Source clock

N cycles

T seconds

t

Derived Network

Frequency (fnx)

M M Mmin nom max

y y

24

Residual value M

Mnom

1

N

Sample&

Holdfs

4 BitCounter

Ct

1

X

fn

fnx

Network Reference clock frequency fn is divided by x such that

1 < fnx/fs < 2

4 Bit SRTS encoded in

CSI bit for SAR-PDUs

with Sequence

Numbers 1,3,5,7

• Source clock fs is divided by N to sample the 4-bit counter Ct driven by the network clock fnx once every N = 3008 = 47 x 8 x 8 bits generated by the source.

• This sampled counter output 4 bits (residual part) is transmitted as the SRTS in SAR-PDU. • It is sent in the CSI bits of SAR-PDUs which have odd SN values.

• The even SAR-PDUs have CSI=0.

• The CSI bits in SAR-PDUs with even SN values are used in SDT. • The method can accept a frequency tolerance for source frequency of 200 parts per million (ppm).

Ct, x, Mnom, N, fn are available at the destination and the clock value can be recovered accordingly!!!!

• For low bit rate communications, e.g., for compressed voice traffic.

• Main Idea: multiplex many users within a single ATM VCC, where each user’s information (SDT) is carried in variable length packets with a header (3 octets) identifying the user channel with control information. (kind of variable ATM cell)

• In the minicell header, the field for user identification has 8 bits limiting the number of AAL 2 users sharing a VCC to 256.

• Short and variable length payload.

• User packet multiplexing

Minicell Header 3 octets

Payload (1-64) octets SDU

AAL 2

WHY AAL 2?

AAL 1 needs not be filled with full 47 bytes. e.g., to transmit digitized voice at a rate of 1 byte every 125 sec, filling a cell with 47 bytes means collecting samples for 5.875 msec. If this delay before transmission is unacceptable, we send partially filled cells waste of bandwidth!!!

STRUCTURE OF AAL TYPE 2AAL SAP

ServiceSpecific

ConvergenceSublayer(SSCS)

CommonPart

Sublayer(CPS)

ATM SAP

ATMLayer

PHY SAP

• Transfer of Service Data Unit with a Variable Bit Rate• Transfer of timing information between source and destination• Indication of lost or errored information which is not covered by AAL 2

AAL-SDU

SSCS-PDUTrailer

UserPacket

SSCS-PDUHeader

SSCS-PDU AAL

ATM

CPS-SDU

CPS PacketCPS-Packet

HeaderCS-Packet

Payload

CPS-PDUHeader CPS-Packet CPS-

Packet PAD

CPS-PDU (48 octets)

ATMHeader ATM Cell Payload

ATM Cell

StartField

CPS-PACKET FORMAT

CPS-Packet(45 octets default64 octets optimal)

CPS-INFOCID| PPT| LI UUI HECCPS-Packet Header

(3 octets)CPS-Packet Payload

(Variable length)

CID: Channel Identifier (8 bits):

•0: Not used •1: Reserved for Layer Management (AAL2 ANP packets) •2-7: Reserved•8-255: ID of SSCS entity (valid CID values to identify channels)

CPS-PACKET FORMAT (Contd)

• CID helps to multiplex multiple AAL2 users/streams (channels) onto a single VCC (ATM connection).• Each channel is identified by the CID.• A channel is bidirectional and has the same CID value.* CID field supports up to 248 individual users per VCC.

AAL2

ATM

PHY

AAL2

ATM

PHY

ATM Network

A B C D A’ B’ C’ D’

AAL2 can multiplex several data streams

AAL-SAP

CSP

SSCSSSCS

SSCSCID=Z

CID=YCID=X

ATM-SAP

Functional model of AAL2 (sender side)

CPS-PACKET FORMAT (Contd)• Packet Payload Type (2 bits): serves 2 functions:

* When PPT =/ 3, the CPS packet is serving a specific application, such as carrying voice data, or carrying an ANP packet.

* When PPT=3, the CPS packet is serving an AAL network management function associated with the management of the channel identified in the CID field.

CPS-PACKET FORMAT (Contd)* LI: Length Indicator (6 bits) * LI specifies the number of octets (minus 1) in the variable length user payload. * LI Coding: One less than CPS-Packet payload length CPS-Packet payload length = LP => LI = LP -1

* CPS-INFO: Information (variable size: (min. 1- max. 45 or 64 octets)) 45 means that exactly one CPS packet fits inside the 48 octet ATM cell payload.

CPS-PACKET FORMAT (Contd)* UUI: User-to-User Information (5 bits): Allows the functions of an SSCS to be specific according to a purpose.

UUI serves two purposes:

• To convey specific info transparently between CPS users, SSCS entities or layer management.• To distinguish between SSCS entities and layer management users.

Codepoints: 0-27 SSCS entities 28-29 Future use 30-34 Layer management

CPS-PACKET FORMAT (Ctd)

5 bit CRC : Generator Polynomial x5+x2+1 (excluding CPS packet payload and error correction). Detectable 1 and 2 bit errors.

HEC: Header Error Control (5 bits)

CPS-PDU FORMAT

SN P CPS-Packet CPS-Packet PADOSF

CPS-PDU Payload (47 octets)

Start Field (STF) indicates the position of the first packet

CPS-PDU (48 octets)CPS-PDU Header

OSF: Offset Field (6 bits) 6 bit pointer => Position Indication of first CPS-packet (starting point of the next CPS packet header within the cell)

Values: 0-40: First CPS packet boundary (0=Next to OSF)

47-63: No CPS packet boundarySN: Sequence Number (1 bit): mod 2 (value 1 or 0)P: Parity (1 bit) : Odd parity for STFPAD: Padding (0-47 octets)

ATMHeader

Packets are streamed into successive payloads

Cell Period

Padding: All 0’sFirst

Packet

CPS Packet

ATM Cell

Pointer in OSF points to find start of a CPS packet in cell

• OSF identifies the starting point of the next CPS packet header within the cell. • If more than one CPS packet is present in a cell, then AAL2 uses the LI in the CPS packet header to compute the boundary of the next packet.

CPS Packets

ATMLayer

User 1 User 2 User 3 User 4 User 5

48 bytes 48 bytesATMHeader

ATMHeader

EXAMPLE

PURPOSE: Accommodation of low bit rate (below 64 kbps) and delay sensitive applications into ATM networks, e.g., cellular systems.

Requirements: Short Cell Assembly Time and High Efficiency.

Rt VBR Sources

CPS PDUsSAR

STF STF

16 16 16 16 16

16161616163 3 3 3 3

1 119 19 199 10 18

…

…

…

…

PACKET CAN STRADDLE CELLS!!

H Packet 1 Packet 2 H ket 3 Packet 4Pac- Packe-

ATM Cell 1 ATM Cell 2

AAL Negotiation Procedures (ANP)• This is the function that provides the dynamic allocation of AAL2 channels on demand.

• This function is carried out by an AAL2 layer management entity at each side of an AAL 2 link.

• This layer management entity uses the services provided by AAL2 through a SAP for the purpose of transmitting and receiving ANP messages.

• These messages are carried on a dedicated AAL2 channel with CID=1, and they control the assignment, removal and status of an AAL2 channel.

• The following types of messages have been defined: Assignment request, assignment confirm, assignment denied, removal request, removal confirm, status poll, and status response.

Open Questions:

• Timing mechanisms???• Error correction schemes?

FEC but with QoS considerations!!