ATM NETWORKS

1
2. B-ISDN REFERENCE MODEL and PROTOCOL LAYERS

2
B-ISDN Protocol Reference Model
SNMP Simple Network Management
Protocol CMIP Common Management Information
Protocol

• Control Plane
• Supports Signaling
• Call Setup, Call Control, Connection Control
• User Plane
• Data Transfer, Flow Control, Error Recovery
• Management Plane
• Operation, Administration, Maintenance

3
Management Plane (Provides Control of ATM Switch)
Layer Management (Layered)
Plane Management (No Layered)

• Concerned with management of all the planes
• All management functions (Fault, Performance,
  Configuration, Operation, Security) which
  relates to the whole system are located in the
  Plane Management
• Provides coordination between all planes

• Use to manage each of the ATM layers with entity
  corresponding to each ATM layer
• OAM issues

4
Broadband Networking with SONET and ATM

• Flow Control
• Error Handling
• Message Segmentation

AdaptationLayer

• Segmentation Type
• Message Number
• Message ID

• 5 Byte Header
• 48 Byte Payload
• Handles cont. and bursty traffic

• SONET

USER
USER
5
Protocol Reference Model in the User Plane
Upper Layers
Abbreviations
AAL ATM Adaptation Layer SAR Segmentation
and Reassembly CS Convergence Sublayer PL
Physical Layer TC Transmission
Convergence PM Physical Medium
class A
class B
class C
class D
1
2
3
4

• Handling lost / misdelivered cells
• Timing recovery
• Interleaving

CellInformationField
CS
AAL

• Split frames / bit stream info cells
• Re-assemble frames / bit stream

SAR
Service Classes for AAL
Class
Type

• Cell routing
• Multiplexing / demultiplexing
• Generic flow control

CellHeader
Constant Bit Rate Variable Bit Rate Connection
Oriented Data Connectionless Data
A B C D
ATM

• Cell header verification and cell delineation
• Rate decoupling (insert idle cells)
• Transmission frame adaptation

TC
PL

• SEAL Simple and Efficient Adaptation Layer
• Type 5 AAL
• Acknowledged info transfer

• Bit timing
• Physical medium

PM
Remark See next page
6

• Remarks PMD ? Physical Medium Dependent
• TC ? Transmission
  Convergence
• Sublayer
• It separates transmission from the physical
  interface and allows ATM interfaces to be built
• on a large variety of physical interfaces

7
Physical Layer Functions

• a) Physical Medium (PM)
• PM sublayer provides the bit transmission
  capability including bit alignment
• Line coding and, if necessary, electrical/optical
  conversion is performed in this sublayer
• Optical fiber is used for the physical medium.
  Other media, coax cables are also possible
• Bit rates ? 155 Mbps or 622.080 Mbps.

8
PHYSICAL LAYER FUNCTIONS

• b) Bit Timing
• Generation and reception of waveforms which are
  suitable for the medium, the insertion, and
  extraction of bit timing information and the line
  coding if required
• CMI (Code Mark Inversion) (CCITT G.703) proposed
  for 155.520 Mbps interface.
• NRZ Nonreturn to Zero code proposed for optical
  interface.

9
LINE CODING

• Electrical Interface Coded Mark Inversion (CMI)
• For binary 0 ? always a positive transition at
  the midpoint of the binary unit time interval.
• For binary 1 ? always a constant signal level for
  the duration of the bit time. This level
  alternates between high and low for successive
  binary 1s.

0
0
0
0
0
0
1
1
1
1
1
Level A2
Level A1
10
LINE CODING

• Optical Interface Nonreturn to Zero (NRZ)
• For binary 0 ? Emission of light
• For binary 1 ? No emission of light
• Transition 0 ? 1 or 1 ? 0Otherwise no
  transition

0
0
0
0
0
0
1
1
1
1
1
Level A2
Level A1
11
ATM INTERFACES

• SONET/SDH 155 Mbps and 622 Mbps over OC-3
• (single mode fiber)
• Cell Based
• PDH Based (ATM cells mapped into PDH signals)
• (59 columns and 9 rows
• frame). Frame at 34.368 Mbps.
• FDDI based or 100 Mbps (same as in FDDI PMD uses
  multimode
• fiber and line coding of 4B/5B). (called TAXI
  interface).
• Early private UNI interfaces
• were based on TAXI interfaces.
• DS-3 (45 Mbps) Transfer of ATM cells on T3
  (DS-3) public carrier
• interface. It is cheaper than SONET links.
• STS-3 (155 Mbps) over Multimode fiber uses line
  coding of 8B/10B.
• STS-3 (155 Mbps) over Twisted Pair (using Taxi
  interface)
• uses line coding of 8B/10B.
• D1-T1 carriers (1.5 Mbps)

12
CELL BASED INTERFACE
This interface consists of a continuous stream
of cells where each cell contains 53 octets.
26
0
1
26
0
1
Physical layer OAM cell

• Synchronization achieved through HEC basis.
• Maximum spacing between successive physical
  layer cells is
• 26 ATM layer cells.
• After 26 consecutive ATM layer cells, a physical
  layer cell (idle
• cells or OAM cells) is enforced to adapt
  transfer capability to
• the interface rate.

13
Transmission Convergence Sublayer (TC)

• A. Transmission Frame Adaptation
• Adapts the cell flow according to the used
  payload structure of the transmission system in
  the sending direction.
• In the opposite direction, it extracts the cell
  flow out of the transmission frame.

14
B. Header Error Control (HEC)
Multiple-bit errror (Cell discarded)

• After initialization receiver is in the
  Correction Mode
• Single bit error detected ? corrected
• Multiple bit error detected ? cell discarded
• Receiver switches to Detection Mode
• In Detection Mode, each cell with a detected
  single-bit error is discarded.
• If a correct header is found, receiver switches
  to Correction Mode
• NOTE A noise burst of errors or other events
  that might cause a sequence of errors!!

Error detectedCell discarded
Correction Mode
Detection Mode
NoError
No error
Correction Single-bit error
15
Example
p Probability that a bit is in
error (1-p) Probability that a bit is NOT in
error p40 Probability that 40 bits are in
error (1-p)40 Probability that 40 bits are correct
16

• With what probability a cell is rejected when
• the HEC state machine is in the "Correction
  Mode"?

Correction Mode
Probability of a cell being rejected
Different Perspective When is a cell
accepted? Probability of having no errors in
cell header OR Probability of having a
single bit error in cell header
17

• With what probability a cell is rejected when
• the HEC state machine is in the "Detection
  Mode"?

Detection Mode
HEC will only accept ERROR-FREE cells.
Different Perspective What is the probability
that a cell header is correct?
18

1. Assume that the HEC state machine is in the
   Correction Mode. What is the probability that
   n successive cells will be rejected, where n gt
   1 ?

Correction Mode
Probability of n successive cells being accepted
(ngt1)
n1
Probability that 1 cell is accepted, i.e., the
entire header is error-free. What is that
probability? OR There is at most one bit error
in the header. What is that probability?
19
1
2
n2
Probability that the cell header (2) is correct
AND Previous case for cell 1 OR Probability that
the cell header (2) has at most 1 bit error
AND Probability that the cell header (1) is
correct (error free)
20
1
3
2
n3
Probability that the cell header (3) is correct
AND Previous case for cell n1 OR Probability
that the cell header (3) has at most 1 bit error
AND Probability that the cell header (2) is
correct ANDThe case for n1
21

1. Assume that the HEC state machine is in the
   Correction Mode. What is the probability p(n)
   that n successive cells will be accepted, where
   n gt 1 ?

First cell is rejected
What is the probability that a cell is rejected?
? Case a)
Different Perspective
? Probability that all header bits of a cell are
correct ? Probability that one single bit error
in a cell header
22
Remaining n-1 successive cells
Now, HEC is in Detection Mode
What is the probability that (n-1) successive
cells are rejected, i.e., there will be errors
in the headers for the remaining (n-1) cells
23
EFFECT OF ERROR IN CELL HEADER
Incoming Cell
Error in Header?
No
Valid cell (intended service)
Yes
Apparently valid cell With errored
header (unintended service)
Error detected
No
Yes
Current mode?
Detection
Discarded Cell
Correction
Error incorrectable?
Yes
No
Correction attempt
Unsuccessful
Successful
24
HEC Generation Algorithm (I.432)

• Every ATM cell transmitter calculates the HEC
  value across the first 4 octets of the cell
  header and inserts the result in the fifth octet
  (HEC field) of the cell header.
• The HEC value is defined as the remainder of
  the division (modulo 2) by the generator
  polynomial x8x2x1 of the product x8 multiplied
  by the content of the header excluding the HEC
  field to which the fixed pattern 01010101 will be
  added modulo 2.
• The receiver must subtract first the coset value
  of the 8 HEC bits before calculating the syndrome
  of the header.
• Device always preset to 0s.
• Key Word CRC (Cyclic Redundancy Check
  Algorithm)

25
ATM CELL STRUCTURE
8 7 6 5 4 3 2 1
Octet
1 2 3 4 5 53
HEADER (5 octets)
PAYLOAD (48 octets)
8 7 6 5 4 3 2 1
GFC
VPI
1 2 3 4 5 53
VCI
VPI
VCI
PT
VCI
PR
HEC
PAYLOAD (48 octets)
26
HEC GENERATION ALGORITHM

• The HEC field contains the 8-bit FCS (Frame
  Check Sequence) obtained by dividing the first 4
  octets (32 bits) of the cell header multiplied by
  28 by the CRC code (generator polynomial)
• (x8x2x1)

27
HEC Generation Algorithm (I.432)

• This HEC code can
• Correct single bit errors
• Detect multiple bit errors
• REMARK If a code corrects t errors, it can
  detect (2t 1) errors!!!!!
• i.e., Here ? (up to 3 bits)

Purpose

• Protects the header control information
• Helps to find a valid cell (cell delineation and
  boundaries)

28
CELL DELINEATION
(This process allows identification of cell
boundaries)
Correct HEC
Bit-by-Bit
Cell-by-Cell
HUNT
PRESYNC
Incorrect HEC
? consecutive incorrect HEC
? consecutive correct HEC
SYNCH
29
Cell Delineation (cont.)

• In Hunt State ? a cell delineation algorithm is
  performed bit-by-bit to determine if the HEC
  coding law is observed (i.e., match between
  received HEC and calculated HEC).
• Once a match is achieved, it is assumed that one
  header has been found and the method enters the
  PRESYNCH state.
• The HEC algorithm is performed cell-by-cell. If ?
  consecutive correct HECs are found, SYNCH state
  is entered if not the system goes back to HUNT
  state.
• SYNCH is only left (to HUNT) state if ?
  consecutive incorrect HECs are identified.

30
Cell Delineation (cont.)

• ? and ? are design parameters that influence the
  performance of cell delineation process.(?7 and
  ?6).
• Greater values of ? result in longer delays in
  recognizing a misalignment but in a greater
  robustness against false alignment.
• Greater values of ? result in longer delays in
  establishing synchronization but in greater
  robustness against false delineation.

31
Cell Delineation (cont.)

• Remarks
• A 155.520 Mbps ATM system will be in SYNCH state
  for more than 5349 years even when the bit error
  probability is BER10-4.
• This method may fail if the header HEC occurs in
  the info field (maliciously or accidentally) ?
  Cell Payload Scrambling.
• To overcome ? the info field contents scrambled
  using a self-synchronizing scrambler with
  polynomial X43 1. Header itself is not
  scrambled.

32
The probability of 7 consecutive incorrect HEC
withBER10-4 A The probability that 7
consecutive cells are in error.1- (1-10-4)40
7 1.61610-17 A 1/A ?
The number of cells sent in order to have a 7
consecutive error cells (Unit? Cells)How often
does event A occur in terms of
ATM cells.

33
53 8 / 155.52 Mbps C (538) of
bits/cell Link Speed of bits/sec
C is how long does it
take to send one ATM cell through the 155 Mbps
link. k 1 / A C 6.187106 53 8
/ 155.52 Mbps 1.68681011 k ? in terms of
seconds k / (365246060) ? approx. 5349 years..
34
Cell Rate Decoupling (Speed Matching)

• Adapts cell stream into Transmission Bit Rate
  (Insertion / Discarding idle cells in particular
  for SONET Interface). SONET uses synchronous cell
  time slots!
• Note Cell Based Interface ? No need for this
  function.

35
Cell Rate Decoupling (cont.) (Speed Matching)
ATM Transmitter
ATM Receiver
Insert Idle or Unassigned cells
Remove the Idle or Unassigned cells
Transmitter multiplexes multiple streams
queueing them if an ATM cell is not immediately
available. If the queue is empty, when the time
arrives to fill the next synchronous cell time
slot, then the Transmission Convergence Sublayer
inserts an Idle cell (or the ATM layer inserts
an Unassigned cell.)
36
ATM Layer Functions

• Cell Multiplexing/Demultiplexing
• Cell VPI/VCI Translation
• Cell Header Generation/Extraction
• GFC Function

37
ATM Layer Functions

• i) CELL MULTIPLEXING/DEMULTIPLEXING
• In the transmit direction, cells from individual
  VPs
• and VCs are multiplexed into one resulting
  stream.
• At the receiving side ? the cell demultiplexing
  function
• splits the arriving cell stream into the
  individual
• cell flows appropriate to the VP or VC.

38
ATM Layer Functions
ii) CELL VPI/VCI TRANSLATION - At ATM
switching nodes, the VPI and VCI translation
must be performed. - Within VP switch,
the value of the VPI field of each
incoming cell is translated into a new VPI value
for the outgoing cell. - At a VC
switch, the values of the VPI as well as the
VCI are translated into new values.
39
ATM Layer Functions
iii) CELL HEADER GENERATION/EXTRACTION -
This function is applied at the termination
points of the ATM layer. -
Transmit Side After receiving the cell
information from the AAL, the cell
header generation adds the appropriate ATM
cell header except for the HEC values. HEC is
done at Physical Layer. VPI/VCI values
could be obtained by a translation from
the SAP identifier. - Receive Side The cell
header extraction function removes the
cell header. Only the cell information is passed
to the AAL. - This function could also
translate a VPI/VCI value into a SAP
identifier.
40
ATM Layer Functions
iv) GFC FUNCTIONS - Supports the control of
the ATM traffic flow in a UNI. It can be
used to alleviate short overload
conditions. - Control of cell flows toward
the network but not flow control from the
network. - No effect within the network.
41
Virtual Path and Virtual Circuit Concept

• ATM cells flow along entities known as VIRTUAL
  CHANNELS. A VC is identified by its virtual
  circuit identifier (VCI).
• VC ? set up between 2 end-users (like VC in X.25
  gt Indiv. Log connection).
• VP ? Bundle of VCs ? having the same end
  points (Group logical connection reserved trunk
  of connections).
• All cells in a given VC follow the same route
  across the network and are delivered in the order
  they were transmitted.

• VCs are transported within Virtual Paths (VPs). A
  VP is identified by its virtual path identifier
  (VPI). VPs are used for aggregating VCs together
  or for providing an unstructured data pipe.

42
Virtual Path and Virtual Circuit Concept

• Optical links will be capable of transporting
  hundreds of Mbps where VCs fill kbps. Thus,
  a large number of simultaneous channels have to
  be supported in a transmission link. Typically
  10K simultaneous channels are considered (thus,
  VCI field up to 16bits).
• Since ATM is connection oriented, each
  connection is characterized by a VCI which is
  assigned at Call-Set-Up.
• When connection is released, VCI values on the
  involved links will be released or can be reused
  by other components.

43
VIRTUAL PATH / VIRTUAL CIRCUIT CONCEPT
VP
TRANSMISSION PATH
VC
Virtual Path
Text
VCI 1 (text)
Voice
VCI 2 (voice)
Video
VCI 3 (video)
ATM Network Interface
44
VIRTUAL PATH/VIRTUAL CIRCUIT CONCEPT

• Each VP has a different VPI value and each VC
  within a VP has a different value.
• Two VCs belonging to different VPs at the same
  interface may have identical VCI values.
• VPI is changed at points where a VP link is
  terminated.
• VCI is changed at points where a VC link is
  terminated.

45
Goal ? Multimedia Communication

• Video Voice ? Time Sensitive (Delay bounds)
• Data ? Loss Sensitive (Loss bounds)
• Allows the network to add or remove
• components during the connection
• e.g. Video Telephony ? Start with voice (only
  single VC)
• ? Add
  video later (on another VC)
• ? Add
  data (on another VC)
• ?
  Signaling (on another VC)

46
EXAMPLE

• Three VP connections exist from A to B. They are
  seen by A as corresponding to the values p, q, r
  of the VPI field, and by B as corresponding to
  the values p2, q2, r2. Whenever A wants to send
  some information to B on the VP connection seen
  as p, it writes the value p in the VPI field of
  the cell.
• The VP switches T1, T2 and T3 swap the VPI
  labels according to the lookup tables. The VCI
  field is not changed by the VP switches, so it
  can be used by A to multiplex several VC
  connections on any one of the three VP
  connections. Therefore, at the VC level, A has at
  its disposal three direct links to B.

A
B
A
B
VC Level
VP Level
p
p2
p
p2
p1
T1
T2
q
q2
q
q2
T3
r
r
r2
r2
47
SWITCHING OF VCs and VPs

• Routing functions for VPs are performed at a VP
  switch.
• This routing involves translation of the VPI
  values of the incoming VP links to the VPI values
  of the outgoing VP links. VCI values remain
  unchanged.
• VC switches terminate both VC links and
  necessarily VP links.
• VPI and VCI translation is performed.

VP Switching
48
VP and VC SWITCHING
VCI 23
VCI 24
49
MORE ABOUT VCs and VPs

• A VP Connection
• Contains multiple VC connections.
• VC connections may be built up of multiple VP
  connections.
• Use of VPI simplifies routing table lookup.

Virtual Channel Connection
Virtual Channel View
50
VCs and VPs (Cont.)

• The inter-networking of the VP and VC switches
  is illustrated in Figure.
• There exist VP connections (x and y) between A
  and T T and B.
• Assume now that A wants to setup a VC connection
  to B using those two VP connections.
• The network has to provide a VCI value, say a1,
  for the A to T link, and a VCI value, say a2, for
  the T to B link.
• The VC connection from A to B is thus made of
  two VC links only.
• At switching points D1 through D4, only the VPI
  field is swapped.
• At the switching point T, both VPI and VCI
  fields are swapped.
• The situation is thus similar to that where A
  and B would be access nodes in a circuit switched
  network, T would be a transit node, and D1
  through D4 would be cross-connects.

51
Example for VCIs and VPIs

• A VP is established between Subscriber A and
  Subscriber C transporting 2 individual
  connections, each with a separate VCI.
• Remark The VCI values used (1,2,3 and 3,4 in the
  example) are NOT translated in the switches,
  which are only switching on the VPI field.

52
Namings

• VC
• Virtual Channel ? Virtual Circuit
• VC Link
• A point where a VCI value is assigned to another
  where that value is translated or terminated.
• VC Identifier
• A value which identifies a particular VC link
  for a given VP Connection.
• VCC (Virtual Channel Connection)
• A concatenation of VC links that extends between
  2 points. (cell sequence integrity preserved)

53

• VP
• Bundle of VCs.
• VP Link
• A group of VC links, identified by a common
  value of VPI, between a point where a VPI value
  is assigned and the point where that value is
  translated as terminated.
• VP Identifier
• Identifies a particular VP Link.
• VPC (Connection)
• A concatenation of VP Links.

54
PVC and SVC

• Permanent Virtual Circuits (PVC)
• Established by a network operator in which
  appropriate VPI/VCI values are programmed for a
  given source and destination (for long time).
• VPs ? 0, , 256 (manually configured)
• PVCs are established by provisioning usually
  last a long time (months/years).
• Switched Virtual Circuits (SVC)
• Established automatically through a signalling
  protocol (Q.2931B) and lasts for short time
  (minutes/hours).
• VCs ? 0, , 65535 (automatically configured)

55
SOFT PVC

• Part of the connection is permanent and part of
  it is switched.
• Hybrid of PVC and SVC!!!

56

• VCC ? 0 - 31
• 0, 5 ? Call set up (Signalling)
• 0, 16 ? Network Management
• (Integrated Local Management Interface ILMI)
• 32 - 65535 ? User Data
• 0, 17 ? For LAN Emulation Configuration Server
  (LECS)
• 0, 18 ? For Private NNI (PNNI)
• 0, 19 or 0, 20 ? Reserved for future use.

57
Advantages of VP/VC Concept

• Simplified Network Architecture Network
  transport functions can be separated into those
  related to an individual logical connection (VC)
  and those related to a group of logical
  connections (VP).
• Increased Network Performance and Reliability
  The network deals with fewer, aggregated
  entities.
• Reduced Processing and Short Connection Setup
  Time Much of the work is done when the VP is set
  up. The addition of new VCs to an existing VP
  involves minimal processing.
• Enhanced Network Services The VP is used
  internal to the network but is also visible to
  the end user. Thus, the user may define closed
  user groups or closed networks of VC bundles.

58
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

59
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 timing
• Class 4 Connectionless Data Service
• SMDS, Ethernet, Internet, Data Traffic,
• No constraints.

60
Traffic Classes
61
General Structure of AAL
Service Data Unit (SDU) crosses the SAP PDU
is data unit between peer layers
62
General Data Unit Naming Convention
63
Structure of AAL with SSCS and CPCS
AAL-SAP
AAL-PDU Primitives
SSCS
Service Specific Convergence Sublayer (SSCS)
SSCS-PDU
CS
Primitives
AAL Common Part (CP)
AAL
Common Part Convergence Sublayer (CPCS)
CPCS
CPCS-PDU Primitives
Segmentation And Reassembly (SAR)
SAR
SAR-PDU Primitives
ATM-SAP
64
AAL Type 1

• AAL 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)

65
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.

66
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

67
AAL Type 1 (Cont.)

• 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

SAR-PDU Header CS-PDU Header CS-PDU Trailer
PCI
68
AAL Type 1 (Cont.)
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)
69
AAL Type 1 (cont.)

• Receivers 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.

70
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.

71
AAL 1
STACK
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

• Forward Error Correction
• No Retransmission

Segmentation/Re-assembly Sublayer -
reads matrix columns(47bytes) - effect
interleaving
(124,128) Reed-Solomon Code Polynomial
undefined Corrects 2 errored bytes per row
Corrects 4 erasure bytes (knows position)
Uses interleaving
ATM
72
FEC in AAL1
R-S Code with 4 byte FEC
Reading
Cell 1 Byte 1
Cell 2 Byte 1
Cell 124 Byte 1
Cell 1 Byte 2
Cell 2 Byte 2
Cell 124 Byte 2
Cell 1 Byte 47
Cell 124 Byte 47
Cell 2 Byte 47
Reed-Solomon Code recovers up to 4 lost cells in
a block of 128.
73
AAL 1
User Data Bit Stream
Higher Layers
AAL-SAP
CS
CPCS-PDU Payload
AAL

H
H
H
SAR
1B
47B
48 Bytes
ATM-SAP
ATM Layer

Cell Payload
H
Cell Payload
H
Cell Payload
H
5B
53 Bytes
74
SAR-PDU of AAL 1
1 Octet
47 Octets
Cell Header
SN
SNP
SAR-PDU Payload
4 bits
4 bits
SAR-PDU Header
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 mis-inserted cells (Error
Detection Correction)
75
SAR-PDU Header of AAL 1
1 bit
3 bits
3 bits
1 bit
Sequence Count
Even Parity
CSI
CRC
SN Field
SNP Field
76
CSI Field

• Sequence Count 0, .., 7
• CSI bit used to transfer TIMING or DATA
  STRUCTURE information.
• 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.

77
P Non-P Formats
AAL-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)
78
Structure Pointer Field
SAR-PDU Header
User Data
P-Format Sequence Counter 0,2,4,6
Reserved Bit
Offset Field
7 Bits
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.
1 Octet Pointer field to indicate the offset into
the current payload of the first octet of a nDSO
payload.
SN even ? uses
Value of n may be as large as 92 in the P-format
since pointer is repeated every other cell when
supporting AAL 1.
79
AAL1
STD Mode (Structured Data
Transfer)
Unstructured Data Transfer
n x DSO (64kbps) Service (supports an octet
structured n DSO Service)
DS1/E1 (1.544Mbps)
DS3/E3 (45Mbps)
including timing SRTS Method
CSI bit (in even SN values) for SDT to convey
information about internal byte alignment
structure of the user data bit stream.
(4-bit RTS included in CSI Bit !!) One sent in
(1,3,5,7)
80
Structured Data Transfer

• Kind of fractional DS1/E1 service where the
  user
• only requires an n64kbps (DS0) connection
  where
• n can be small as 1
• and as high as 24 for DS1 (T1) and 30 for E1.
• An n64 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-- gt P format.

81
EXAMPLE STRUCTURED DATA TRANSFER
192
1
1
1
1
1
1
1
1
192
192
192
192
192
192
192
DS1 Signal
193 175 18 193 165
28 193 147 46 193
137
47376
46368
46368
47376
CS-PDUs
p
SN0 CSI1 P-Format
SN2 CSI1 P-Format
SN1 CSI0 Non-P-Format
SN3 CSI0 Non-P-Format
0-1 93 Octets
2-393 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.
82
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.

83
DS1 CIRCUIT EMULATION USING AAL 1
EXAMPLE UNSTRUCTURED DATA TRANSFER
ATM Cells
octets
DS1 Signal
SRTS CS
SAR-PDUs
bits
RTS
1
192
1
1
192
192
Time
1
192
1
192
1
192
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.
84
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.

85
Handling of Lost and Misinserted Cells in AAL1

• 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.

86
TIMING (CLOCK) RECOVERY TECHNIQUES IN AAL 1

• Adaptive Clocking in AAL 1
• (No Network clock is available).
• Synchronous Residual Time Stamp Approach (SRTS)
• (Global Network Clock is available)

87
Adaptive Clocking in AAL 1
Common network reference clock is not available!!!
Used for Transfer Delay Variable
1. Adaptive Clocking (Receiver)
Cells
PLAYOUT BUFFER
Receiver reads info. with a local clock.
Receiver writes received info field in this
buffer.
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.
(Content) Filling level of the buffer is used to
control the frequency of the local clock.
PLL (Phase Lock Loop) Provides 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 gt PLL
slowed down Overflowgt PLL speeded up
88
Synchronous Residual Time Stamp (SRTS) Approach
BASIC IDEA Convey a measure of the frequency
difference between the reference clock and
source clock. Network reference clock is
available, source clock is not syncronized!
NETWORK
Sender
Receiver
Common Network Clock
Local Clock
Local Clock
Odd of segments
CSI field Sequence field
TIMESTAMP
Difference between the local and network clocks.
Difference between 2 clocks
Transport this info. in odd numbered Cells (CSI
Field) to destination
89
(Assumed)

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

Source Transmitter

• 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.

90
Tolerance
Source clock N cycles T seconds
Source Frequency (fs)
t
M
nom
M
M
M
min
nom
max
Derived Network Frequency (fnx)
t
y
y
Residual value M
4
2
91
Sample Hold
1
C
fs
t
N
4 Bit SRTS encoded in CSI bit for SAR-PDUs with
Sequence Numbers 1,3,5,7
fnx
1
4 Bit Counter
fn
X
Network Reference clock frequency fn is divided
by x such that 1 lt fnx/fs lt 2
92

• 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 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!!!!

93
AAL 2

• For low bit rate communications, e.g., for
  compressed voice traffic.
• Main Idea multiplex many users within a single
  ATM VCC, where each
• users 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
94
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!!!
95
STRUCTURE OF AAL TYPE 2
AAL SAP
Service Specific Convergence Sublayer (SSCS)
AAL-SDU
SSCS-PDU Trailer
User Packet
SSCS-PDU Header
A A L
SSCS-PDU
CPS-SDU
Common Part Sublayer (CPS)
CPS-Packet Header
CS-Packet Payload
CPS Packet
Start Field
CPS-PDU Header
CPS- Packet
CPS-Packet
PAD
CPS-PDU (48 octets)
ATM SAP
A T M
ATM Layer
ATM Header
ATM Cell Payload
ATM Cell
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

96
CPS-PACKET FORMAT
CPS-INFO
CID PPT
LI
UUI
HEC

CPS-Packet Payload (Variable length)
CPS-Packet Header (3 octets)
CPS-Packet (48 octets default 64 octets optimal)
CID Channel Identifier (8 bits) Values

• 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)

97
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.

98
A
B
C
D
A
B
C
D
AAL2
AAL2
ATM
ATM
ATM Network
PHY
PHY
AAL2 can multiplex several data streams
99
AAL-SAP
SSCS
SSCS
CIDZ
CIDY
SSCS
CIDX
CSP
ATM-SAP
Functional model of AAL2 (sender side)
100
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 PPT3, the CPS packet is serving
• an AAL network management function
  associated with
• the management of the channel identified
  in the
• CID field.

101
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 gt 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.
102
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

103
CPS-PACKET FORMAT (Ctd)
HEC Header Error Control (5 bits)
5 bit CRC Generator Polynomial x5x21
(excluding CPS packet payload and error
correction). Detectable 1 and 2 bit errors.
104
CPS-PDU FORMAT
Start Field (STF) indicates the position of the
first packet
SN
P
CPS-Packet
CPS-Packet
PAD
OSF
CPS-PDU Payload (47 octets)
CPS-PDU Header
CPS-PDU (48 octets)
OSF Offset Field (6 bits) 6 bit pointer gt
Position Indication of first CPS-packet
(starting point of the next CPS packet header
within the cell) Values 0-40
First CPS packet boundary

(0Next to OSF) 47-63 No
CPS packet boundary SN Sequence Number (1 bit)
mod 2 (value 1 or 0) P Parity (1 bit) Odd
parity for STF PAD Padding (0-47 octets)
105
Packets are streamed into successive payloads
CPS Packet
Cell Period
Pointer in OSF points to find start of a CPS
packet in cell
ATM Header
First Packet
Padding All 0s
ATM 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.

106
EXAMPLE
User 1
User 2
User 3
User 4
User 5

Rt VBR Sources
16
16
16
16
16

CPS Packets
16
16
16
16
16
3
3
3
3
3
CPS PDUs SAR

1
1
19
19
19
9
10
18
STF
STF
ATM Layer

ATM Header
ATM Header
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.
107
PACKET CAN STRADDLE CELLS!!
ATM Cell 1
ATM Cell 2
H
H
Packet 1
Packet 2
ket 3
Packet 4
Pac-
Packe-
108
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 CID1,
• 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.

109

• Open Questions
• Timing mechanisms???
• Error correction schemes?
• FEC but with QoS considerations!!

110
AAL 3/4
SSCS
SSCS-PDU
CPCS
CPCS-PDU
SAR
SAR-PDU
111
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
112

• 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).

113

Data
Message Mode
SSCS-PDU Header (4 octets)
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
114

• 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.

115

• 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.

116
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
117

• 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).

118
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
ST
SN
MID
SN
MID

48 octets
ATM-SAP
ATM Layer
Cell Header
Cell Payload
.
53 octets
119
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.
ST ST Field
BOM COM EOM SSM 10 00 01 11
Segment Type Value
120
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
121

• 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).

122
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.
123

• Multiplexing/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.

124
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)

125
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.
126
(No Transcript)
127

• 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.

128
CRC ( Cyclic Redundancy Check ) The CRC is a
10-bit se