ATM

1
Asynchronous Transfer Mode (ATM)
2
An Overview of ATM

• A technology for multiplexing fixed-length cells
  from a variety of sources to a variety of remote
  locations.
• Capable of moving data at a wide range of speeds,
  but aimed at very high speed (100-1000 Mb/s).
• Capable of handling data from a variety of media
  (e.g. voice, video, and data) using a single
  interface.
• ATM is a connection-oriented protocol.
• Connections can be switched or permanent.
• Signalling procedures are used to set up switched
  calls.
• Certain quality of service (QoS) is guaranteed
  for each connection. QoS parameters may include
  as cell loss rate, max./avg. cell delay, delay
  jitter, cell error rate, cell misinsertion rate,
  etc.
• QOS is negotiated at connection setup.

3
An Overview of ATM

• ATM operates on a best effort basis cells with
  errors, or that encounter congestion, are
  silently dropped.
• Two types of connections point-to-point and
  multipoint.
• Service Carried in Fixed Length Cells (53
  octets).
• 5 Octets Header
• 48 Octets Payload

4
ATM Networking
IS
IS
IS
ES
ES

• UNI User-Network Interface
• NNI Network-Network Interface

5
ATMCell Format
?? 48 Octets
GFC VPI VCI PTI CLP HEC
4 8 16 3 1
8 ??
(a) UNI Cell format
?? 48 Octets
VPI VCI PTI CLP HEC
12 16 3 1
8 ??
(b) NNI Cell format

• GFC Generic Flow Control (4 bits). Used by the
  flow
• control mechanism at the UNI.
• VPI Virtual Path Identifier (8 bits). Used for
  directing
• cells within the ATM network.
• VCI Virtual Channel Identifier (16 bits).
• PTI Payload Type Identifier (3 bits).
  Identifies the type
• of data being carried by the
  cell.
• CLP Cell Loss Priority (1 bit). 1 low
  priority.
• HEC Header Error Correction (8 bits). Generated
  and
• inserted by the physical layer.
  For first 4 octets.
• Correct single-bit errors and
  detect some
• multiple-bit errors.

bit 1 AAL indication bit 2 EFCI (upstream
congestion) bit 3 data or OAM cells
6
Pre-assigned VPI/VCI Values

• Unassigned Cell Indication (VPI 0, VCI 0)
• Meta signalling (VCI1)
• Meta signalling is the bootstrap procedure used
  to establish and release a signalling VC. Not
  used for PVC setup.
• General broadcasting signalling (VCI2)
• OAM F4 flow indication -- segment and end-to-end
  (VCI3 and VCI4)
• F4 VP level OAM
• F5 VC level OAM, segment or end-to-end (PT100
  or 101)
• Point-to-Point Signalling (VCI5)
• Carriage of Interim Local Management Interface
  (ILMI) messages (VPI0, VCI16)

7
Preassigned, Reserved VPI/VCI Values
8
Cell Multiplexing and Cell Switching Examples
100
200
100
100
200
100
100
0
0
UTP
UTP
300
300
1
1
SMF
SMF
200
400
200
2
2
MMF
MMF
150
150
150
3
3
UTP
UTP
ATM Switch
9
ATM Protocol Stack
Management plane
Control plane
User plane
Higher Layers
Higher Layers
ATM Adaptation Layer
Virtual Channel Functions
Plane management
ATM Layer
Virtual Path Functions
Layer management
Physical Layer (PMD)
10
ATM Layer Service

• Transparent transfer of 48-octet data unit
• Deliver data in sequence on a connection
• Two levels of multiplexing (VC, VP)
• Three types of connections
• Point-to-point
• Point-to-Multipoint
• Multipoint-to-Multipoint (??)
• Transport is best-effort
• Network QoS negotiation
• Traffic control and congestion control

11
ATM Layer Functions

• Cell multiplexing and switching
• Cell rate decoupling
• Cell discrimination based on pre-defined VPI/VCI
• Quality of Service (QoS)
• Payload type characterization
• Generic flow control
• Loss priority indication and Selective cell
  discarding
• Traffic shaping

12
Cell Rate Decoupling and Cell Discrimination

• Cell Rate Decoupling
• ATM sending entity adds unassigned cells to the
  assigned cell stream in order to adjust to the
  cell rate acquired by the payload capacity of the
  physical layer (R).
• The receiving ATM entity shall extract and
  discard the unassigned and invalid cells from the
  flow of cells coming from the physical layer (R).
• Cell Discrimination
• Meta signalling
• General broadcast signalling
• Point-ot-point Signalling
• Segment OAM F4 flow cell
• End-to-end OAM F4 flow cell
• ILMI message
• User data

13
Virtual Channels, Virtual Paths, and the Physical
Channel
????
????
????
????
????
100
100
100
100
200
200
??
100
100
200
200
200/300
300/10
200
100/100
300/40
200
300
300
10
10
300
300
20
20
30
30
40
40

• ?? (VPI/VCI)
• (100,100),(100,200),(200,100),
• (200,200),(200,300),(300,10),
• (300,20),(300,30),(300,40)

14
Physical Link, Virtual Path, and Virtual Channel
virtual channel connection
virtual channel link
VC link
ATM Layer
virtual path connection
VP link
VP link
transmission path
Phy. Layer
Digital Section
Regenerator Section
Crossing point
Endpoint
15
Virtual Channels

• The virtual Channel (VC) is the fundamental unit
  of transport in a B-ISDN. Each ATM cell contains
  an explicit label in its header to identify the
  virtual channel.
• a Virtual Channel Identifier (VCI)
• a Virtual Path Identifier (VPI)
• A virtual channel (VC) is a communication channel
  that provides for the transport of ATM cells
  between two or more endpoints for the purpose of
  user-user, user-network, network-network
  information transfer. The points at which the ATM
  cell information payload is passed to a higher
  layer signify the endpoints of a VC.
• A Virtual Channel Identifier (VCI) identifiers a
  particular VC within a particular VP over a UNI
  or NNI.

16
Virtual Paths

• A Virtual Path (VP) is a group of Virtual
  Channels that are carried on the same physical
  facility and at a given reference point in the VP
  share the same Virtual Path Identifier (VPI)
  value.
• The VP boundaries are delimited by Virtual Path
  Terminators (VPT).
• AT VPTs, both VPI and VCI are processed.
• Between VPTs associated with the same VP, only
  the VPI values are processed (and translated) at
  ATM network elements.
• The VCI values are processed only at VPTs, and
  are not translated at intermediate ATM network
  elements.

17
Why Virtual Paths and Virtual Channel ?

• Assume the identification field contains 8 bits.
  All used
• as VCI. Then the size of the mapping table
  is 256.

New
New
VCI OP
VCI OP
VCI
VCI
0
0
(2) 1 (3)
(1) 2 (2)
(127) 2 (35)
(35) 1 (255)
1
1
(208) 2 (254)
(254) 1 (38)
255
255
(1)
(3)
(2)
(127)
(255)
(35)
(208)
(38)
0
(254)
0
A
B
1
1
255
255
18
Why Virtual Paths and Virtual Channels ?

• Assume the identification field contains 8 bits.
  VPI takes 3 bits
• and VCI takes 5 bits. Then the size of the
  mapping table is 8.

(1/1)
(0/1)
(0/31)
(1/31)
(7/25)
(7/25)
0
0
(7/1)
1
0
(0/31)
1
1
31
(1/25)
7
0
7
1
31
0
1
7
19
Virtual Channels Examples
Port (a,1)
Port (c,2)
New
VPI/VCI OP
VPI/VCI
????
(200,10) 4 (300,10)
????
??
VPI/VCI
(100,10)
??
VPI/VCI
1 (100,10)
D
A
a
1
4
c
2 (100,20)
1 (300,10)
2
2
(100,20)
(300,10)
????
(200,10)
(200,10)
(200,20)
??
VPI/VCI
????
(100,20)
(300,10)
(300,20)
1 (300,10)
??
4
VPI/VCI
1
B
C
b
2
1
2
d
3
1 (100,20)
????
(100,10)
??
VPI/VCI
Port (b,1)
Port (d,1)
New
New
1 (100,10)
VPI/VCI OP
VPI/VCI OP
VPI/VCI
VPI/VCI
Port (b,2)
(200,10) 3 (300,20)
(300,20) 2 (100,20)
New
(200,20) 2 (300,10)
VPI/VCI OP
VPI/VCI
(100,10) 4 (200,10)
20
Virtual Path/Virtual Channel (VP/VC) Switches
VC1
VP3
VP1
A
C
VP Switch
VC2
SW1
SW4
VP/VC Switch (VP Terminator, VPT)
SW3
VP4
VP2
VC4
SW5
SW2
B
D
VC3
21
ATM Adaptation Layers (AAL)

• AAL Reference Structure
• AAL Type 1
• AAL Type 2
• AAL Type 3/4
• AAL Type 5
• SAAL

Management plane
Control plane
User plane
Higher Layers
Higher Layers
ATM Adaptation Layer
Plane management
Virtual Channel Functions
ATM Layer
Virtual Path Functions
Layer management
Physical Layer (PMD)
22
AAL Reference Structure
SAP
Service Specific CS (SCCS)
(may be Null)
Convergence Sublayer (CS)
Primitives
AAL
Common Part CS (CPCS)
Primitives
SAR
SAR(Common)
SAP
23
AAL
Layer MGMT
Service Specific Layers
message
AAL
48 byte payloads
...
ATM
add 5 byte header
...
Transmission
Convergence
Sublayer
PMD
24
AAL Functions
Functions Parameters Error
Detection CRC, length, correlation tags Framing
of user data units Payload type/segment type Cell
sequence indication Cell sequence count
field Multiplexing Message ID
(MID) Error Correction FEC, retransmission Flow
Control Credit window Timing
Recovery Time stamp
25
ATM Adaptation Layers (AAL)

• In order to carry data units longer than 48
  octets in ATM cells, an adaptation layer is
  needed.
• The ATM adaptation layer (AAL) provides for
  segmentation and reassembly of higher-layer data
  units and for detection of errors in
  transmission.
• Since the ATM layer simply carries cells without
  concern for their contents, a number of different
  AALs can be used across a single ATM interface.
• The AAL maps the user, control, or management
  protocol data units into the information field of
  the ATM cell and vice versa.
• To reflect the spectrum of applications, four
  service classes have been defined by CCITT.

26
CCITT Services Classifications
Class A Class B Class C Class D
Attribute
Packetized
Connection
Circuit
Datagram
Oriented
Emulation
voice/video
Data
Timing between source and destination
Required Not required
Bit Rate Constant
Variable
Connection mode
Connection oriented Connectionless

• Examples
• Class A (CBR) 64kbps digital voice
• Class B (rt-VBR, nrt-VBR) Variable bit rate
  encoded video
• Class C (UBR,ABR) Frame Relay over ATM, File
  Transfer (Telnet, FTP, TCP),...
• Class D (ABR) SMDS over ATM, IP over ATM,...
• Class X Raw Cell Service (e.g., proprietary AAL)

27
AAL Service Classification
Class A Class B Class C
Class D
Signalling
Circuit
Packetized
Connection
Datagram
(Q.93B)
Oriented
Emulation
voice/video
Data
Attribute
AAL 4
AAL 3
AAL1 AAL2
SAAL
AAL 5
Timing between source and destination
Required Not required
Bit Rate Constant
Variable
Connection Mode
Connection oriented Connectionless
28
AAL Types

• Five AALs have been accepted for consideration by
  the CCITT.
• AAL 1 is meant for constant-bit-rate services
  (voice).
• AAL 2 is meant for variable-bit-rate services
  with a required timing relationship between
  source and destination (audio and video).
• AAL 3 was originally meant for connection-oriented
  variable -bit-rate services without a required
  timing relationship it has now been merged with
  AAL 4.
• AAL 3/4 and 5 are meant for connectionless
  services (e.g. connectionless data).
• Only AALs 3/4 and 5 are of interest for IP
  networking.

29
AAL 1 (Constant Bit Rate -CBR) Functions

• Emulation of DS1 and DS3 Circuits
• Distribution with forward error correction
• Handle cell delay for constant bit rate
• Transfer timing information between source and
  destination
• Transfer structure information (structure
  pointer)
• Provide indication of unrecoverable lost or
  errored information

SAR PDU
Header SN SNP 47 Octets
Payload
Seq
CRC
EP
CSI
Count
1 3 3
1
30
AAL1 Supports Circuit Emulation

• Synchronous Residual Time Stamp (SRTS)
• DS1, DS3
• Require accurate frequency clock
• 4-bit Residual Time Stamp (RTS) for clock
  aligning
• RTS is generated once every 8 cell times, carried
  in CSI bit of odd cells
• Structured Data Transfer (SDT)
• nxDS0 (64kbps)
• 1-octect pointer carried in payload once every
  two cells (even cells) indicates the offset into
  the current payload of the first octect of an
  nxDS0 payload
• The octect contains 1 reserved bit and 7-bit
  offset field which points to start of up to 93
  octect structure (474693)

31
AAL 2 (VBR) Protocol Data Unit (PDU)
ATM PDU
SAR PDU
Header SN IT 47 Octets
Payload LI CRC

• SN Sequence number
• IT Information TypeBOM,COM,EOM,SSM
• Length Indicator

32
AAL 3/4

• The variable bit rate (VBR) adaptation layer,
  defined in CCITT recommendation I.363, is defined
  for services (e.g. data) that require bursty
  bandwidth.
• Comprises two sublayers
• the convergence sublayer (CS)
• the segmentation and reassembly sublayer (SAR)

33
AAL 3/4 CS and SAR PDU Structures
CS-PDU Header 4 Octets
CS-PDU Trailer 4 Octets
PAD 0-3 Octets
CS-PDU User Information lt 65,535 Octets
Common Part Indicator
CS User Infor. Length
Alignment
ETag
BTag BASize
1 1 2

1 1 2

• CPI00000000
• Btag/Etag Beginning/Ending Tag -- 256 increment
  counters
• BAsize receiving side maximum buffering
  requirement (gt CPCS-PDU)
• Pad make CPCS-PDU on 32-bit boundary
• AL(Alignment) make trailer 32-bit aligned
• Length CPCS-PDU size

34
AAL 3/4 SAR Sublayer
SAR-PDU Header 2 Octets
SAR-PDU Trailer 2 Octets
Segmentation Unit ,SAR-PDU Payload 44
Octets
p
SAR Type
SAR SN
Length
SAR-PDU CRC
MID
2 4 1 9

6 10

• ST COM(00),BOM(10),EOM(01),SSM(11)
• SN Modulo 16 sequence counter
• P(Priority) 1- Priority CS-PDU, 0- Normal CS-PDU
• MID (Multiplexing ID) -- Multiplexing multiple
  CPCS connections on a
• 
  single ATM connection
• LI Length lt44
• CRC on Cell Header, SAR-PDU payload and LI

35
AAL 5 PDU Structure

• The Simple and Efficient Adaptation Layer (SEAL),
  attempts to reduce the complexity and overhead of
  AAL 3/4.
• It eliminates most of the protocol overhead of
  AAL 3/4.
• AAL 5 comprises a convergence sublayer and a SAR
  sublayer, although the SAR is essentially null.

CS-PDU Trailer 8 Octets
PAD 0-47 Octets
CS-PDU User Information lt 65,535 Octets
Protocol Control
CRC
Length
2 2 4
36
AAL 5 Segmentation and Reassembly
User SDU
CS-PDU Trailer 8 Octets
PAD 0-47 Octets
CPCS-PDU Payload (CPCS-SDU) lt 65,535 Octets
48 octets
48 octets
...
More F
More T
More T
Data T
User Data
User Data
ATM cell
ATM cell
ATM cell

• Control
• CPCS-UU -- CPCS User-to-User Indication
• (1 octet). Transparently transfer CPCS user
• to user information
• CPI -- Common Part Indicator (1 octet). Align
• trailer to 64 bits. Possible identification
  of
• layer management message.

CRC-32
Control
Length
2 2 4
37
AAL 5

• When a network node has a user datagram to
  transmit, it first converts it to a CS-PDU by
  adding the pad (if necessary) and trailer.
• Then it breaks the CS-PDU into 48-octet SAR-PDUs
  and transmits each in an ATM cell on the same
  virtual channel.
• Since there is no AAL 5 SAR header, an
  end-of-frame indication in the ATM cell header is
  required SDU type of 1 (binary value 0X1) in the
  PTI field.
• The receiver simply concatenates cells as they
  are received, watching for the end-of-frame
  indication.
• The higher layer is responsible for ignoring PDUs
  with CRC errors.
• Some applications may discard PDUs with errors
  others may choose to use them.

38
SAAL Structure
Primitives
Service Specific Coordination Function (SSCF)
Service Specific Convergence
Sublayer
Service Specific Connection Oriented
Peer-to-Peer Protocol (SSCOP)
Peer-to-Peer Message
Peer-to-Peer Message
Common Part AAL Peer-to-Peer Protocol
(CP-AAL)
Common Part
Primitives
39
SAAL

• Reside between Q.93B and ATM Layer.
• SAAL is used to provide reliable transport of
  Q.93B messages between peer Q.93B entities.
• SAAL CP-AAL uses AAL5 Common Part Protocol.
• SSCOP can be used for any reliable service.
• SSCF maps primitives from MTP 3 to the required
  SSCOP signals and vice versa, and
• flow control
• maintains link status
• reports to layer management when a link is
  released