Atm adaptation layerl

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AALs DESCRIPTION

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ATM Adaptation Layer

Are considered here the following AALs:

AAL1
AAL3/4
AAL5

These AALs will be described through the header and trailer appended to the User's SDUs (Service Data Unit). Each sublayer, SAR (Segmentation & Reassembly) and CPCS (Common Part Convergence Sublayer), generates its own PDU by appending its header and trailer (if present) to the SDU of the previous layer according to the OSI model (see "basic concepts" section). With this approach, not only the services of each sublayer will be presented but also the generated overheads quantified in terms of wasted bandwidth.

AAL1

The SAR and CPCS PDUs are detailed in the following tables

AAL1 SAR - PDU OVERHEAD

HEADER SDU (CPCS -PDU) 1 octet - 2%

SN SNP

CSI SC CRC P

1bit 3bit 3bit 1bit 47 octets

SN: Sequence Number - CSI: Convergence Sublayer Indication - SC: Sequence Counter - SNP: Sequence Number Protection - CRC: Cyclic Redundancy Check - P: Parity

AAL1 CSCP-PDU OVERHEAD

SDT (CSI=0) odd PDUs SDU 1 octet every two SDUs 1%

47 octets

even PDUs (SC:even) Pointer SDU

8bit 46 octets

SRTS (CSI=1) even PDUs (SC: even) SDU 0 %

47 octets

odd PDUs (SC:odd) RTS SDU

CSI (SAR) 47 octets

SDT: Structured Data Transfer (N*64 Kbps) - SRTS: Synchronous Residual Time Stamp - P: Pointer - RTS: Residual Time Stamp

AAL1 is intended for CES (Circuit Emulation Services) with two operation modes:

SRTS for timing synchronization between source and destination. The timing information (phasing between source and network clock) is carried via the Residual Time Stamp (RTS) signal. This "4 bit" signal is encoded in CSI bit for SC count=1,3,5,7. It is refreshed every 8 cells.
SDT for transporting multiple 64 Kbps channels (from 1 to 92) on the same ATM VP/VC. To delineate the block of N*octets of the N channels, an 8 bit pointer, inserted into the ATM cell, locates the first octet of the block (see example below).

1	2	----	23	24	25	----	46	47	Cell Count	P
									Cell Count	P
1	2	----	23	1	2	----	23	1	1
P	2	----	23	1	2	----	23	1	2	24
2	3	----	1	2	3	----	1	2	3
P	3	----	1	2	3	----	1	2	4	23

AAL 3/4

This layer is intended for data services, connection oriented or not. It's mainly used for the SMDS service.
The AAL PDUs for the SAR and CPCS sublayers are described below

AAL 3/4 SAR PDU OVERHEAD

HEADER SDU (CPCS-PDU) TRAILER 4 octets 8 %

ST SN MID LI CRC

2bit 4bit 10bit 44 octets 6bit 10bit

ST: Segment Type (BOM, COM, EOM, SSM) - SN: Sequence Number
MID: Multiplex Identification - LI: Length Indicator
BOM, COM, EOM: Beginning, Continuation, End Of Message
SSM: Single Segment Message

AAL 3/4 CPCS-PDU OVERHEAD

HEADER SDU PAD TRAILER 8 octets (8+PAD)/L %

CPI BTag BAsize AL ETag L

1 B 1 B 2 B 1-65535 B 0-3 B 1 B 1 B 2 B

Acronyms:
CPI: Common Part Indicator - BTag: Beginning Tag - BAsize: Buffer Allocation Size
AL: 32 bit Alignment - Etag: Ending Tag - L: PDU Length - B: Byte

ST Byte allows segmentation and reassembly of the CPCS PDU. The Beginning cell is marked with BOM, the following cells with COM and the final one with EOM. If the cell contains the complete frame, SSM is used. The Sequence Number checks the continuity of cell flows.
The MID field allows up to 1024 user connections to be multiplexed on the same VP/VC. It is assigned within a BOM or SSM marked cell.
LI: number of SAR-SDU octets within the SAR-PDU. The value is 44 in BOM and COM segments, variable in EOM or SSM since the CPCS-PDU last segment may not completely fill the cell.
CRC detects errors in the SAR-PDU
CPI indicates which unit (bit or Byte) is used by the BAsize field
BTag-Etag: the same values are inserted by the sender and used by the receiver as an additional error check
PAD is used to align the CPCS-PDU to a multiple of 32 bit.

L encodes the length of the CPCS-SDU field to remove the PAD at the receiver side

AAL5

It is the most common used AAL layer to transport data. The overhead is reduced as a minimum as underlined hereafter.

ATM cell HEADER SAR-PDU SAR-OVERHEAD

PT

5 Bytes 48 Bytes 0 Byte

PT: Payload Type

CPCS - PDU OVERHEAD

SDU PAD TRAILER 8 Bytes (8+PAD)/L %

UU CPI L CRC

1-65535 B 0-47 B 1 B 1 B 2 B 4 B

PAD: Padding - UU: User Information - CPI: Common Part Indicator
L: Length of the CPCS-PDU - CRC: Cyclic Redundancy Check - B: Byte

There is no SAR header or trailer. The segmentation and reassembly is done simply by using the PT field of the ATM layer header (refer to ATM layer cell structure), although it intermingles the services of two distinct layers, which is not very clean but saves overhead.

PAD is used to make the overall CPCS-PDU a multiple of 48 Bytes ; ATM cell payloads will then be completely full
L encodes the length of the CPCS-SDU field to remove the PAD at the receiver side
CRC allows detection of errors on the complete CPCS-PDU
CPI is used to align the trailer with a 64 bit word
UU is AAL information transparently exchanged between the end-to-end AAL layers

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	ATM Adaptation Layer

AAL1 SAR - PDU					OVERHEAD
HEADER				SDU (CPCS -PDU)	1 octet - 2%
SN		SNP
CSI	SC	CRC	P
1bit	3bit	3bit	1bit	47 octets
SN: Sequence Number - CSI: Convergence Sublayer Indication - SC: Sequence Counter - SNP: Sequence Number Protection - CRC: Cyclic Redundancy Check - P: Parity

AAL1 CSCP-PDU				OVERHEAD
SDT (CSI=0)	odd PDUs	SDU		1 octet every two SDUs 1%
		47 octets
	even PDUs (SC:even)	Pointer	SDU
		8bit	46 octets
SRTS (CSI=1)	even PDUs (SC: even)	SDU		0 %
		47 octets
	odd PDUs (SC:odd)	RTS	SDU
		CSI (SAR)	47 octets
SDT: Structured Data Transfer (N*64 Kbps) - SRTS: Synchronous Residual Time Stamp - P: Pointer - RTS: Residual Time Stamp

AAL 3/4 SAR PDU						OVERHEAD
HEADER			SDU (CPCS-PDU)	TRAILER		4 octets 8 %
ST	SN	MID		LI	CRC
2bit	4bit	10bit	44 octets	6bit	10bit
ST: Segment Type (BOM, COM, EOM, SSM) - SN: Sequence Number MID: Multiplex Identification - LI: Length Indicator BOM, COM, EOM: Beginning, Continuation, End Of Message SSM: Single Segment Message

AAL 3/4 CPCS-PDU								OVERHEAD
HEADER			SDU	PAD	TRAILER			8 octets (8+PAD)/L %
CPI	BTag	BAsize			AL	ETag	L
1 B	1 B	2 B	1-65535 B	0-3 B	1 B	1 B	2 B
Acronyms: CPI: Common Part Indicator - BTag: Beginning Tag - BAsize: Buffer Allocation Size AL: 32 bit Alignment - Etag: Ending Tag - L: PDU Length - B: Byte

ATM cell HEADER		SAR-PDU	SAR-OVERHEAD
	PT
5 Bytes		48 Bytes	0 Byte
PT: Payload Type

CPCS - PDU						OVERHEAD
SDU	PAD	TRAILER				8 Bytes (8+PAD)/L %
		UU	CPI	L	CRC
1-65535 B	0-47 B	1 B	1 B	2 B	4 B
PAD: Padding - UU: User Information - CPI: Common Part Indicator L: Length of the CPCS-PDU - CRC: Cyclic Redundancy Check - B: Byte