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This page tells you some things that you always wanted to know about ATM Adaption Layers (AAL). Cellware can provide this (and more) in a handy pocket guide. Ask for your own copy.

Adaption layer 1 (AAL1)
Adaption layer 2 (AAL2)
Adaption layer 3/4 (AAL3/4)
Adaption layer 5 (AAL5)
Real capacity of ATM to transport data
Abbreviations explained


This AAL was designed by the ITU-T for continuous bit-rate (CBR) data such as voice and video. The simplest application is circuit emulation. AAL1 includes specifications for segmenting a continuous signal for transport in individual ATM cells

It also must transmit timing information through the network.

Particularly voice traffic is very sensitive to small variations in the timing. Some video CODECs are even more touchy and rely on plesiochronous timing stability to work at all.

Adaptive clock recovery and the SRTS process (Synchronous Residual Timestamp) are specified by the ITU-T in Recommendation I.363.1. The jitter and wander requirements for G.703 interfaces are specified in G.823.

Traffic shaping

No active shaping is required for AAL1 as it is automatically "well shaped" to produce cells with a very regular spacing.

P   = octet offset of data block over 2 cells (= 0111 111 if not required)
CSI = Convergence Sublayer Indication
CRC = Cyclic Redundancy Check
SN  = Sequence Number
(non-P;   CSI = 0
P format; CSI = 1 only if SN = 0,2,4 or 6)
SNP = Sequence Number Protection
Py  = Parity (even, 1 bit)
SAR = Segmentation And Reassembly
PDU = Protocol Data Unit

see ITU-T standard I.363

See discussion about AAL1.

Cellware AAL1 board (Manual - 50K text + pictures)


CRC = Cyclic Redundancy Check
IT  = Information Type
LI  = Length Indication
PDU = Protocol Data Unit
SAR = Segmentation And Reassembly
SN  = Sequence Number

see ITU-T standard I.363


AAL3/4 and AAL5 both deal with the transport of traditional data packets (PDUs or protocol dat units). AAL3/4 was originally specified by the ITU-T and has been specified by the SMDS Interest Group (SIG) for the transport of SMDS packets. Cellware's TA-LAN uses AAL3/4 for just this purpose.

The ATM Forum decided that AAL3/4 was too complex and had an excessive overhead. AAL5 was then specified in a much simpler form with less data protection but less overhead. AAL5 now dominates the packet scene.

Traffic shaping

Once a packet is available, ready to be cut up, we could send the resulting cells out at the full speed of the ATM line. This burst can cause an overload. This is avoided by agreeing a maximum bandwidth in the traffic contract with the ATM switch . When a packet is ready to go, gaps are deliberately introduced between cells. In this way bursts and thus cell loss are avoided.

AL      = 32 bit Alignment (using 0)
Btag    = Beginning Tag field per CPCS-PDU must be identical to Etag
BA-size = Buffer Allocation size
BOM     = Beginning Of Message
COM     = Continuation Of Message
CPI     = Common Part Indicator
(0 = octet as unit for BA-size, all other values for further study)
CPCS    = Common Part Convergence Sublayer
CPCS-H  = CPCS-Header
CPCS-T  = CPCS-Trailer
CRC     = Cyclic Redundancy Check
EOM     = End Of Message
Etag    = End Tag field - see Btag
IT      = Information Type
Length  = Length of CS-SDU
LI      = Length Indication (max 44 octets)
MID     = Multiplex Identification
PAD     = Padding (0)
PDU     = Protocol Data Unit
SAR     = Segmentation And Reassembly
SDU     = Service Data Unit
SN      = Sequence Number
SSM     = Single Segment Message
ST      = Segment Type

see ITU-T standard I.363

see Cellware's AAL3/4,5 board (Datasheet)


BOM      = Beginning Of Message	(PTI=0)
COM      = Continuation Of Message	(PTI=0)
CPCS     = Common Part Convergence Sublayer
CPCS-UU  = CPCS User-to-User indication
CPI      = Common Part Indicator
CRC      = Cyclic Redundancy Check
EOM      = End Of Message	(PTI=1)
Length   = Length of CPCS-SDU (Length=0 ->Abort function)
PAD      = Padding (0) to 47 octets
PDU      = Protocol Data Unit
Reserved = 32 bit alignment of CPCS-PDU trailer
PTI      = Payload Type Indication
SDU      = Service Data Unit

see ITU-T standard I.363

see Cellware's AAL3/4,5 board (Datasheet)

Constant bit-rate AAL5

Note that in a further attempt to simplify matters, there is a big push for "constant bit-rate AAL5". This was driven by the ATM Forum and its application in transferring voice and video over an ATM powered Internet is clear. It is also well suited to the transfer of DVB data. (DVB - digital video broadcasting)

see Cellware's AAL5-CBR board (Datasheet)

How much capacity is really available for payload, when using ATM?

                              SDH/SONET        E3
                             -----------   ----------  	
Bit-rate on the line         155 520 000   34 368 000	
Available for ATM cells      149 760 000   33 920 000  A
Available for cell payload   135 631 698   30 720 000  B=A*48/53
AAL1 payload (using SDT)     131 393 208   29 760 000  C=B*46.5/48
AAL3/4 payload (SSM)#        101 723 774   23 040 000  D=B*36/48
AAL5 payload (SSM)#          113 026 415   25 600 000  E=B*40/48
(# - worst case)			

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