AAL 34

1
AAL 3/4
SSCS
SSCS-PDU
CPCS
CPCS-PDU
SAR
SAR-PDU
2
AAL 3/4
Non-Assured Mode (Unreliable)
Assured Mode (ARQ Protocols)
- Go_Back_N - Selective Repeat Request
Message Mode Entire AAL-PDU needed
Stream Mode Small AAL-PDUs allowed
3

• a) MESSAGE MODE
• AAL-SDU is passed across the AAL interface in
  exactly one AAL-SDU.
• This service provides transport of fixed size
  of variable length AAL-SDUs.
• 11 mapping, i.e., one SSCS-PDU consists of one
  AAL-SDU. SSCS
• accepts a block of information from a user and
  creates a SSCS-PDU.
• This includes a Header Trailer with protocol
  information and padding
• to make the PDU an integral multiple of 32
  bits.
• SAR accepts the SSCS-PDU from SSCS and segments
  it into N
• 44-octet SAR-PDUs (this last segment may
  contain some unused portion).

4

Data
SSCS-PDU Header (4 octets)
Message Mode
SSCS-PDU Trailer (4 octets)
Padding octets ( 0-3 octets )
SAR-PDU Header
H
SAR-PDU Trailer
Unused
AAL-SDU
AAL Interface
SSCS-PDU
SAR-PDUs
. . .
H
H

H
5

• Message mode is used for framed data transfer,
  e.g., high level protocols and applications would
  fit into this category, e.g., LAPD or Frame Relay
  would be in message mode.
• Advantage Detects errored SSCS-PDUs and discards
  them.
• Disadvantage Requires large buffer capacity.

6

• Streaming Mode
• The AAL service data unit is passed across the
  AAL interface in one or more AAL interface
  data units (AAL IDUs).
• The transfer of these AAL-IDUs across the AAL
  interface may occur separately in time and this
  service provides the transport of the variable
  length AAL-SDUs.
• It provides transport of variable length
  AAL-SDU.
• The AAL-SDU may be small as 1 octet and is always
  delivered as 1 unit because only this unit will
  be recognized by the application.

7
Streaming mode
Data
SSCS-PDU
Header (4 octets)
AAL SDUs
SSCS-PDU
Trailer (4 octets)
AAL Interface
Padding octets(0-3)
SSCS-PDU
Unused
SAR-PDU Header
H
H
H
H
SAR-PDU Trailer
SAR-PDUs
8

• Streaming mode is used for low speed continuous
  data with low delay requirements which may be as
  small as 1 octet.
• 1 block is transferred per cell. Data are
  presented to AAL in fixed size slots.
• Advantage Transfer delay of a message is low.
• A single SDU is passed to the AAL layer and
  transmitted in multiple SSCS-PDUs (pipelined or
  streamed mode).

9
AAL 3/4 Details
CPI Common Part Indicator (1 Octet)
Btag Beginning Tag (1 octet) BA Size Buffer
Size Allocation (2
octets) Length Length of CPCS-PDU
Payload (2 octets) AL Alignment (1
octet) Etag End Tag (1 octet) PAD Padding (0-3
octets) ST Segment Type (2 bits) SN Sequence
Number (4 bits) MID Multiplexing
Identification (10 bits) LI Length Indicator (6
bits) CRC Cyclic Redundancy Check
Code (10 bits)
0-65535 Bytes
Higher layer
AAL-SAP
H
T
CPCS-PDU Payload
Etag
CPI
Btag
BASize
PAD
AL
Length
Length
0-65535 Bytes
CPCS
SAR
T
H
T
H
44
44
SAR-PDU Payload
LI
CRC
SAR-PDU Payload
LI
CRC
ST
SN
MID
ST
SN
MID

48 octets
ATM-SAP
ATM Layer
Cell Header
Cell Payload
.
53 octets
10
The SAR sublayer is depicted in the Figure. The
SAR sublayer accepts variable length CS-PDUs from
the convergence sublayer and generates SAR-PDUs
with a payload of 44 octets, each containing a
segment of the CS-PDU. ST (Segment Type) The ST
identifies a SAR-PDU as containing a beginning of
message (BOM), a continuation of message (COM),
an end of message (EOM), or a single segment
message (SSM). All BOMs and COMs contain exactly
44 octets where EOM and SSM may have variable
lengths.
Segment Type Value
11
AAL 3/4 Segmentation
User Data
CPCS PDU
CPCS-H CPCS-PDU Payload CPCS-T
SAR PDU
SAR-H SAR-PDU Payload SAR-T
BOM
SAR PDU
SAR-H SAR-PDU Payload SAR-T
COM
SAR PDU
SAR-H SAR-PDU Payload SAR-T
EOM
ATM Cell
ATM-H ATM Cell Payload
12

• SN (Sequence Number)
• The SN allows the sequence of SAR-PDUs to be
  numbered modulo 16.
• SN is incremented by 1 relative to the SN of the
  previous SAR-PDU belonging to the same AAL
  connection (numbering modulo 16).
• These two fields enable the segments of the
  CS-PDU to be reassembled in the correct sequence
  and minimize the effect of errors on the
  reassembly process (counts for lost or
  misinserted cells, buffer overflows, and
  underflows bit errors).

13

• MID (Multiplexing Identification)
• The MID is used to identify a CPCS connection
  on a single ATM-layer connection.
• This allows for more than one CPCS connection
  for a single ATM-layer connection.
• The SAR sublayer, therefore, provides the means
  for the transfer of multiple, variable-length
  CS-PDUs concurrently, over a single ATM layer
  connection between AAL entities.
• Different AAL connections on a single ATM layer
  connection where AAL connections must have
  identical QoS requirements.
• Multiplexiing/Demultiplexing is performed on an
  end-to-end basis. AAL 3/4 multiplex different
  streams of AAL/SDUs across a single Virtual
  Connection.
• For CO, each logical connection between AAL
  users is assigned a unique MID value.
• Thus, the cell traffic from up to 210 different
  AAL connections can be multiplexed and
  interleaved over a single ATM connection.
• For CL service, MID field can be used to
  communicate a unique identifier associated with
  each CL user and again traffic from multiple AAL
  users can be multiplexed.

14
3 sessions Multiplexed onto VC2

• From a single host to forward along the same VC
  and be separated at the destination.
• All sessions having the same QoS ? MID finds
  which cell belongs to which session. MID ?
  desirable ? Carriers charge for each
  connection set up and for each second for an open
  connection.
• If a pair of hosts have several sessions open
  simultaneously giving each one its own VC ?
  expensive.
• If 1 VC can handle the job (enough BW use)

15
AAL 3/4 Multiplexing Example A data
communication terminal has 2 inputs with a
98-octet packets arriving simultaneously destined
for a single ATM output port using the AAL 3/4
protocol. Two parallel instances of the CPCS
sublayer encapsulate the packets the packets with
a header and trailer. These are passed to 2
parallel SAR processes that request the CPCS-PDU
or two different MIDs resulting in a BOM, COM,
and EOM segment for each input packet. Since all
these occurs in parallel, the ATM cells are
interleaved on output.
16
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17

• LI (Length Indicator)
• The LI contains the number of octets (binary
  coded)
• from the CS-PDU which are included in the
• SAR-PDU payload.
• Maximum value is 44. It aids in the detection
• of reassembly errors such as loss or gain of
  cells.

18

• CRC ( Cyclic Redundancy Check )
• The CRC is a 10-bit sequence used to detect
  bit errors across
• the whole SAR-PDU.
• This includes the CS-PDU segment and hence the
  user data.
• Remainder of the division (modulo 2) by the
  generator polynomial
• of the product of x10 and the content of the
  SAR-PDU, including
• the SAR-PDU header, SAR-PDU payload and LI
  field of SAR-PDU.
• The polynomial is G(x) x10x9x5x4x1.
• Result of CRC calculation is placed with the
  LSB right justified in
• the CRC-field (CRC-10 to detect errors).

19

• CPI ( Common Part Indicator )
• The CPI is used to interpret subsequent
  fields for the CPCS
• functions in the Header/Trailer.
• CPI of 0 indicates that the BAsize field
  contains an estimate
• of incoming CPCS-PDU and LI exact size.

20

• BTag ( Beginning Tag )
• Sender inserts same value in BTag and ETag for
  a given
• CPCS-PDU and changes the value for each
  successive CPCS-PDU.
• Receiver checks the values for each successive
  CPCS-PDU.
• It also checks the value of BTag in the
  CPCS-Header with the
• value of ETag in trailer.
• BTag and ETag are set to the same value to help
  error detection.

21

• BASize ( Buffer Allocation Size )
• The BAsize indicates the receiver the maximum
• buffering requirements to receive the
  CPCS-PDU.
• BAsize is binary encoded as number of counting
  units.
• Size of counting units is identified by the
  CPI field.
• BAsize field estimates the incoming CPCS-PDU
  size in bytes.
• Length field contains the exact size of
  CPCS-PDU in bytes.
• In Message Mode, BAsize value is encoded equal
  to the
• CPCS-PDU payload length.
• In Streaming Mode, BAsize value is encoded
  equal to or greater
• than the CPCS-PDU length.

22

• PAD
• Between end of CPCS-PDU payload and 32-bit
• aligned CPCS-PDU trailer, there will be 0-3
• unused octets for padding? makes the CPCS-PDU
  
• an integral multiple of 32 bits to make end
  system
• processing more effificient.
• These are used as filler octets and do not
  convey
• any information.
• It may be set to zero and its value is ignored
  at
• the receiving end.

23

• AL (Alignment )
• The AL is used to achieve 32-bit alignment in
  the CPCS-PDU trailer.
• AL field complements the CPCS-PDU trailer to 32
  bits.
• This unused octet is strictly used as a filler
  octet and does not
• convey any information, i.e., it simply
  makes the trailer a
• full of 32 bits to simplify the receiver
  design.
• AL field should be set to 0.
• ETag ( End Tag )
• The ETag is used to associate the CPCS-PDU
  trailer with the
• CPCS-PDU header the transmitter will insert
  the same value
• into the BTag and ETag fields.

24

• Reassembly Process
• Normally, BOM-COMs-EOM..etc.
• The first BOM causes the AAL to note the MID
  and SN fields,
• and then look for following COMs which
  contain the same MID
• and have correctly incremented the SN fields.
  
• Payload is extracted from each SAR-PDU to
  from the CPCS-PDU.
• Finally, when EOM arrives, in sequence and
  matching MID value,
• then the CPCS-PDU is complete.
• Final error checking ? Matching ETag BTag
  and ensuring
• the Length field matches that the received
  data.

25

• Remark
• If a BOM occurs with the MID of a current
  CPCS-PDU
• being reassembled, COM EOM SAR-PDUs
  arriving with
• a MID value not corresponding to a current
  CPCS-PDU are ignored.
• Those arriving with an out-of-sequence, SN field
  indicates an
• error occurred so reassembly aborted.

26
Example 1

• Suppose a sequence of SAR-PDU is transmitted
  through AAL 3/4.
• 1. Suppose BOM SAR-PDU is lost on the way. What
  happens at the receiving end?
• CS-PDU will be discarded.

Discard
Detect
Btag missing Length will also can do it !
27

• 2. One of the COM SAR-PDU is lost. What happens
  at the receiving end?
• CS-PDU will be discarded. (same as above
  violation of sequence number)
• Note SAR layer cannot detect the problem with
  CS. Since it has LI field (that complete data is
  not received), ETAG and BTAG fields.

SN ? Violated Buf ? CS ? SAR will not detect the
problem SN ? will be missing Length ? will also
detect.
28

• 3. Special Case Suppose COM successive EOM
  BOM are lost assuming SN is matched. What happens
  at the receiving end?
• 2 PDUs get concatenated into the same CS-PDU
• On the CPCS layer, Btag and Etag will be
  different for 2 PDUs (Error occurred). Hence,
  everything will be discarded.

Btag Etag will be different! ? Discard !!
29

• 4. 16 consecutive COM SAR-PDUs are lost. What
  happens at the receiving end?
• When EOM SAR-PDU is received, the CS-PDU will be
  discarded because it is shorter than BAsize
  indication (Buffer Allocation size) field.
• SAR does not recognize that SAR_PDUs were lost
  because it uses mod 16 SN, and hence after 16
  data units, the SN is repeated. However, when EOM
  is delivered to the CPCS, the CPCS will check the
  length field in the trailer of CPCS-PDU that it
  has assembled and will detect the assembled data
  is shorter than the length field. CPCS will
  discard it.

SN modulo 16 BA size will detect it! ? SAR will
not compare it!
30

• 5. Multiple 16 consecutive COM SAR-PDUs are lost.
  What happens at the receiving end?
• Any sequence of lost COM SAR-PDU that is
  multiple of 16 result same as before because mod
  16 SN.

31
Example 2

• 1. EOM-SAR-PDU of the first block sequence is
  lost.
• ?
• The partial CS-PDU of the first block will be
  discarded when another BOM SAR-PDU is received.
• (SAR will send an ABORT signal to CPCS to
  terminate the Re-assembly)
• So that the CPCS can release the re-assembly
  buffer.

32
2. EOM-SAR-PDU of the first block BOM-SAR-PDU
of the second block are both lost ? i) Sequence
numbers or ii) E-Tag of the CPCS trailer of the
second block or iii) Length field of the second
message will catch the errors. SN of 2 subsequent
messages are randomly related (AAL is free to
pick any between 0 and 15 range for initial SN
of the first SAR-PDU of a message). Suppose first
message ends with a sequence like , 6, 7 and the
next message starts with 6, 7 , if EOM (SN6) of
first message is lost BOM (SN7) of second
message is lost, sequence will appear correctly,
,6 ,7 So in this case if SN does not help in
the SAR, the E-Tag will help. If they agree, then
the length field in the CPCS-PDU of second
message will catch it.
33
AAL 5

• The new AAL was introduced in the study process
  of CCITT at the end of 1991.
• Its description was published in the 1994 CCITT
  recommendations.
• Designed for the same class of service as AAL
  3/4, it has the advantage of being simpler and
  requiring less overhead.
• Unlike AAL 3/4, it allows all 48 octets of the
  cell information field to be used for the
  transport of CS-PDU segments, the only SAR
  protocol information being provided by a bit in
  the ATM cell header, as explained below.

34
AAL 5

• This means that there is neither multiplexing nor
  error control at the SAR sublayer.
• However, there is a CRC field (CRC-32) in the CS
  sublayer.
• There are also similarities with AAL 3/4.
• The two modes of service defined,
• message and streaming mode
• are the same as in AAL3/4.

35

• The Convergence Sublayer of AAL 5 has been
  subdivided into a CPCS part and a SSCS part.
• CPCS
• --supports streaming mode and message mode
• SSCS uses the same SSCS as AAL 3/4 and provides
  
• assured or non-assured data delivery.

36

• The protocol control information field of the
  SAR sublayer uses the ATM-layer-user-to-ATM-layer-
  user parameter (AAU) contained in the ATM header
  to indicate that a SAR-PDU contains the end of
  a CS-PDU.
• When the bit is set to 1, it indicates the end
  of the CPCS-PDU when the bit is set to 0 it
  indicates the continuation or the beginning of a
  CS-PDU.
• This is necessary to enable the SAR to copy
  with reassembly of the CS-PDU in the presence of
  errors.
• If no indication of the end of the CS-PDU was
  provided, the loss of a cell, and hence the loss
  of a segment of the CS-PDU, would mean that all
  subsequent reassembly operation would be
  incorrect.
• By indicating the end of the CS_PDU, the loss
  of a single cell would limit the error to one
  CS-PDU, unless the lost cell contained the end
  indication in which case the error would be
  restricted to 2 CS-PDUs.

37
ATM CELL STRUCTURE
8 7 6 5 4 3 2 1
1 2 3 4 5 53
Octet

• Octets are sent in increasing order
• ? 1,2,3
• Within an octet the bits are sent
• in decreasing order ? 8,7,6,5,4 ...

HEADER (5 octets)
PAYLOAD (48 octets)
User Network Interface (UNI) Cell Structure
Network Network Interface (NNI) Cell Structure
8 7 6 5 4 3 2 1
8 7 6 5 4 3 2 1
1 2 3 4 5 53
1 2 3 4 5 53
GFC
VPI
VPI
VCI
VPI
VPI
VCI
GFC Generic Flow Control VPI Virtual Path
Identifier VCI Virtual Channel Identifier PT
Payload Type PR Priority HEC Header Error
Control
VCI
VCI
VCI
VCI
PT
PT
PR
PR
HEC
HEC
PAYLOAD (48 octets)
PAYLOAD (48 octets)
38

• PAYLOAD TYPE (PT)
• First Bit ? 0 ? User
  Information
• First Bit ? 1 ? Network
  Management or Maintenance Function
• Second Bit ? Whether CONGESTION has been
  experienced or not.
• Third Bit ? known as AAU
  (ATM-User-to-ATM-User) used in AAL5 to convey
  information between end users.

39
Remark

• Lack of LI field ? No way for SAR to distinguish
  between CPCS-PDU octets and filler in the lost
  SAR-PDU. There exists no way for SAR entity to
  find the CPCS-PDU trailer in the last SAR-PDU.
• To avoid these situations - CPCS-PDU payload be
  added out so that the last bit of the CPCS
  trailer occurs as the last bit of the final
  SAR-PDU.
• No Sequence Number ? Receiver must assure that
  all SAR-PDUs arrive in proper order for
  reassembly. CRC should guarantee that.

40

• Lack of MID It is not possible to interleave
  cells from different CPCS-PDUs. (Each successive
  SAR-PDU carries a portion of the current CPCS-PDU
  or the first block of the next CPCS-PDU).
• 32-bit CRC for AAL 5 ?
• G(x)x32x26x23x22x16x12x11x10x8x7x5x4
  x2x1
• Probability of undetected cell misordering is
  2-32.

41
AAL 5
PAD Padding (0 to 47 octets) Length Length of
CPCS-SDU (2 octets) CPCS-UU CPCS user-to-user
indication (1 octet) CRC Cycle Redundancy Check
(4 octets) CPI Common Part Indicator (1 octet)
42
SAR PDU Format for AAL5
PT (Payload Type) The PT belongs to the ATM
header and it conveys the value of the
ATM-layer-user-to-ATM-layer-user indication.

43
AAL 5 Example

• Two 98-byte packets arrive simultaneously.
• Two parallel instances of this CPCS
  sublayer.
• Add a trailer to each packet.
• Note that the entire packet does not
  have to
• be received before it can begin the SAR
  
• function as in AAL 3/4 to insert the
  correct
• buffer allocation size.
• The packets are segmented by 2 parallel SAR
  processes.
• Here these cells are destined for the
  same VPI/VCI and
• hence only one can be sent at a time

44
(No Transcript)
45
Example

• 1. Single bit error in 1 of the SAR-PDUs occurs.
• CS-PDU will be discarded when lost SAR-PDU is
  received due to CRC failing. In trailer ? (CRC -
  checking CS layer ? AAU1 after getting EOM-SAR
• 2. Suppose one of the cells with AAU0 is lost.
• ? Find SAR-PDU (AAU1) will cause the CPCS check
  the LENGTH FIELD of the CPCS-PDU trailer.
  Trailer is always in the last cell (AAU1), and
  CRC will ALSO be checked.
• 3. One of the cells AAU1 is lost?
• ? Error either by CRC or by mismatch of the
  length field in CPCS-PDU trailer OF THE NEXT
  ARRIVING CELL with AAU1.

46
When trailer for next message (the one that is
lost with AAU1 cell) is received. This will
result in a loss of both corrupted and the next
message. REMARK The LEN field of CS-PDU is
limited to 2 bytes. So a max. of 64K bytes can be
sent before the end of message error can be
triggered off. If the total size of both CS-PDUs
is greater than 64K, receiver will detect the
error.
47
Sources of Cell Losses a) Errors on the
transmission media b) Discarding cells for
congestion control c) Processing errors in
switching nodes and end-points Effect of Cell
Loss on Reassembly Cell Loss AAL 3/4 may pass
partially reassembled CPCS-PDUs to the user along
with an error indicator. AAL 5 Can only pass up
an error indicator
48

• AAL 3/4 Reassembly
• Receiver SAR/CPCS rejects all COM EOM cells
  passed to it. BOM is required. If BOM lost, the
  entire CPCS-PDU is discarded.
• Incorrect SN progression between SAR-PDU reveals
  the loss of a COM.
• If a multiple of 16 consecutive cells is lost,
  then the SN wraps around, but the loss of data is
  detected by the CPCS-PDU being undersized.
• To detect EOM loss, two methods exist

49
METHOD 1 If the BOM of the next CPCS-PDU on the
same MID arrives before the EOM for the current
CPCS-PDU, then the partially reassembled CPCS-PDU
must be released by the SAR/CPCS. Entire
partially reassembled CPCS-PDU received to that
point is considered valid passed to the AAL
user along with an error indication. This is
only the case when EOM is lost or where a cell
burst knocks out some COMs followed by the EOM.
A cell loss burst that knocks out the EOM the
following BOM slips past the SN checks, will be
detected when B-tag E-tag fields fail to match.
50
REMARK The length indicator may fail to pick up
this error, if the cell burst loses as many cells
as are added by concatenating the 2 CPCS-PDU
fragments. In this case only the first 44 bytes
of the first CPCS-PDU may be legitimately
retrieved. For the second one ---gt bad luck ....
51
METHOD 2 Attach a timer to each CPCS-PDU under
reconstruction signal an error when it is not
reassembled within a certain time frame. AAL 5
Encapsulation Seq-Checking DO NOT EXIST as in
AAL 3/4. Reassembly Errors are detected only
when CPCS-PDU trailer arrives. Impossible to know
how much has been received already is correct.
Single-bit errors in SAR-PDU are not picked up
until the CPCS-PDU CRC is calculated ---gt if
incorrect the entire CPCS-PDU is discarded.
52

• Lost of cells with AAU0, detected by an
  incorrect CRC when the trailer arrives. If CRC
  fails to flag the error, the length field
  mismatch ensures the CPCS-PDU is discarded.
• Loss of Cells with AAU1 detected in 3 ways
• SAR-PDU of the following CPCS-PDU may be
  appended to the first, resulting in a CRC error
  (or length mismatch).
• AAL may enforce second CPCS-PDU can flag an
  error and cause the assembled data to be
  discarded.
• A timer attached to CPCS-PDU reassembly. If it
  expires, assembled is discarded.