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ATM
relies on cell-switching technology. ATM cells have a fixed length
of 53 bytes which allows for very fast switching. ATM creates
pathways between end nodes called virtual circuits which are
identified by the VPI/VCI values.
This page describes the
ATM UNI cell header structure and the PDU structures for the various
ATM/SAR formats including: AAL0, AAL1, AAL2, AAL3/4 and
AAL5.
UNI/NNI Cell
The UNI or NNI cell
header comprises the first 5 bytes of the ATM cell. The remaining 48
bytes comprise the payload of the cell whose format depends on the
AAL type of the cell. The structure of the UNI and NNI cell headers
are given here:
4 |
8 bits
|
GFC
|
VPI
|
VPI
|
VCI
|
VCI
|
VCI
|
PT (3 bits)
|
CLP
|
HEC
|
Information
(48 bytes)
|
UNI Cell
header
4 |
8 bits
|
VPI
|
VPI
|
VCI
|
VCI
|
VCI
|
PTI (3 bits)
|
CLP
|
HEC
|
Information
(48 bytes)
|
NNI Cell
header
GFC Generic flow control (000=uncontrolled
access).
VPI Virtual
path identifier.
VCI Virtual
channel identifier. Together, the VPI and VCI comprise the VPCI.
These fields represent the routing information within the ATM
cell.
PTI Payload
type identifier.
CLP Cell loss
priority.
HEC Header
error control.
AAL0
AAL0 cells are
sometimes referred to as raw cells. The payload consists of 48 bytes
and has no special meaning.
AAL1 PDU
The structure of the
AAL1 PDU is given in the following illustration.
SN
|
SNP
|
SAR PDU
|
CSI
|
SC
|
CRC
|
Parity
|
Payload
|
1 bit
|
3 bits
|
3 bits
|
1 bit
|
47 bytes
|
AAL1 PDU
SN Sequence number. Numbers the stream of SAR PDUs of a CPCS
PDU (modulo 16).
CSI Convergence sublayer indicator. Used for residual time
stamp for clocking.
SC Sequence
count.
SNP Sequence
number protection.
CRC Cyclic
redundancy check calculated over the SAR header.
Parity Parity
calculated over the CRC.
SAR PDU payload 47-byte user information field.
AAL2
Click here for more information on AAL2 decoding and
analysis.
AAL2 provides
bandwidth-efficient transmission of low-rate, short and variable
packets in delay sensitive applications. It supports VBR and CBR.
AAL2 also provides for variable payload within cells and across
cells.
CID
|
LI
|
UUI |
HEC |
SAR PDU payload
|
8 bits |
6 bits |
5 bits |
5 bits |
1-45/64 bytes |
AAL2 CPS
packet
CID Channel identifier.
LI Length
indicator.
UUI User-to-user indicator.
HEC Header
error control.
SAR PDU
payload Information field of
the SAR PDU.
The structure of the AAL2 SAR PDU is given in
the following illustration.
OSF
|
SN
|
P |
PDU payload
|
PAD |
8 bits |
6 bits |
5 bits |
1-45/64 bytes |
0-47 bytes |
AAL2 SAR
PDU
OSF Offset
field. Identifies the location of the start of the next CPS packet
within the CPS-PDU.
SN Sequence
number. Protects data integrity.
P Parity.
Protects the start field from errors.
PDU Payload Information field of the SAR PDU.
PAD Padding.
AAL3/4
AAL
3/4 consists of message and streaming modes. It provides for
point-to-point and point-to-multipoint (ATM layer) connections. The
CS is divided into two parts: service specific and common part. This
is illustrated in the following diagram:
Higher Layers
|
ATM Adaptation
Layer AAL3/4 |
Convergence
Sublayer
CS
|
Service
Specific SS CS |
Common Part CP CS
|
Segmentation
and Reassembly Sublayer SAR |
ATM Layer
|
AAL3/4 Packet
AAL3/4
packets are used to carry computer data, mainly SMDS
traffic.
The
functions of the AAL3/4 CPCS include connectionless network layer
(Class D), meaning no need for an SSCS; and frame relaying
telecommunication service in Class C. The CPCS PDU is composed of
the following fields:
Header
|
Info
|
Trailer
|
CPI
|
Btag
|
Basize
|
User info
|
Pad
|
0
|
Etag
|
Length
|
1
|
1
|
2
|
0-65535
|
0-3
|
1
|
1
|
2 bytes
|
AAL3/4 CPCS
PDU
CPI Message type. Set to zero when the BAsize and Length
fields are encoded in bytes.
Btag Beginning
tag. This is an identifier for the packet. It is repeated as the
Etag.
BAsize Buffer
allocation size. Size (in bytes) that the receiver has to allocate
to capture all the data.
User info The
actual information that is being sent by the user.
PAD Padding
field which is used to achieve 32-bit alignment of the length of the
packet.
0 All-zero.
Etag End tag.
Must be the same as Btag.
Length Must be the same as
BASize.
IP
frames encapsulated over ATM
The structure of the
AAL3/4 SAR PDU is illustrated below:
ST
|
SN
|
MID
|
Information
|
LI
|
CRC
|
2
|
4
|
10
|
352
|
6
|
10 bits
|
|
|
|
2-byte header
|
44 bytes
|
2-byte trailer
|
48
bytes |
AAL3/4 SAR PDU
ST Segment type. Values may be as
follows:
Segment
type |
Value |
Meaning |
BOM |
10 |
Beginning of
message |
COM |
00 |
Continuation of
message |
EOM
|
01 |
End of
message |
SSM
|
11 |
Single
segment message |
SN Sequence number. Numbers the stream of SAR PDUs of a CPCS
PDU (modulo 16).
MID Multiplexing identification. This is used for
multiplexing several AAL3/4 connections over one ATM
link.
Information This field has a fixed length of 44 bytes and contains
parts of CPCS PDU.
LI Length indication. Contains the
length of the SAR SDU in bytes, as follows:
Segment
type |
LI |
BOM,
COM |
44 |
EOM |
4,
..., 44 |
EOM
|
(Abort)63 |
SSM |
9,
..., 44 |
CRC Cyclic redundancy check.
AAL5
The type 5 adaptation
layer is a simplified version of AAL3/4. It also consists of message
and streaming modes, with the CS divided into the service specific
and common part. AAL5 provides point-to-point and
point-to-multipoint (ATM layer) connections.
AAL5 is used to carry
computer data such as TCP/IP. It is the most popular AAL and is
sometimes referred to as SEAL (simple and easy adaptation layer).
The structure of the AAL5 CPCS PDU is composed of the following
fields:
Info
|
Trailer
|
User info
|
Pad
|
Control
|
Length
|
CRC
|
0-65535
|
0-47
|
2
|
2
|
4 bytes
|
AAL5 CPCS
PDU
User
info The actual information
that is sent by the user. Note that the information comes before any
length indication (as opposed to AAL3/4 where the amount of memory
required is known in advance).
Pad Padding bytes to make the entire packet (including
control and CRC) fit into a 48-byte boundary.
Control Reserved
bytes which are set to all zeros.
Length Length
of the user information without the Pad.
CRC CRC-32. Used
to allow identification of corrupted transmission.
The structure of the
AAL5 SAR PDU is as follows:
Information
|
PAD
|
UU
|
CPI
|
Length
|
CRC-32
|
1-48
|
0-47
|
1
|
1
|
2
|
4 bytes
|
|
|
8-byte trailer
|
AAL5 SAR
PDU
Information Variable length field containing the CS
information.
PAD Padding
used to cell align the trailer which may be between 0 and 47 bytes
long.
UU CPCS
user-to-user indication to transfer one byte of user
information.
CPI Common
part indicator is a filling byte (of value 0). This field is to be
used in the future for layer management message
indication.
Length Length of the Information field.
CRC-32 Cyclic redundancy check computed from the Information
field, PAD, UU, CPI and Length fields. It is a 32-generator
polynomial.
F4/F5 OAM PDU
The structure of the F4
and F5 OAM cell payload is given in the following
illustration.
OAM type
|
Function type
|
Function
specific |
Reserved
|
CRC-10
|
|
4
|
4
|
360
|
6
|
10
|
bits |
48 bytes
|
|
F4/F5 OAM
PDU
CRC-10 Cyclic redundancy check: G(x) =
x10+x9+x5+x4+x+1
OAM type / Function type The possible values for OAM type and function type are
listed below:
OAM type
|
Value
|
Function
type |
Value
|
Fault Management
|
0001 |
Alarm Indication
Signal (AIS) |
0000 |
|
|
Far End Receive
Failure (FERF) |
0001 |
|
|
OAM Cell Loopback
|
1000 |
|
|
Continuity Check
|
0100 |
Performance
Management |
0010 |
Forward Monitoring
|
0000 |
|
|
Backward Reporting
|
0001 |
|
|
Monitoring and
Reporting |
0010 |
Activation/
Deactivation |
1000 |
Performance
Monitoring |
0000 |
|
|
Continuity Check
|
0001
|
OAM F4
cells operate at the VP level. They use the save VPI as the user
cells, however, they use two different reserved VCIs, as
follows:
VCI=3 |
Segment OAM F4
cells. |
VCI=4 |
End-end OAM F4
cells. |
OAM F5
cells operate at the VC level. They use the save VPI and VCI as the
user cells. To distinguish between data and OAM cells, the PTI field
is used as follows:
PTI=100 (4)
|
Segment OAM F5 cells
processed by the next segment. |
PTI=101 (5)
|
End-to-end OAM F5 cells which
are only processed by end stations terminating an ATM
link. |
RM Cells
There are two types of
Rate Management (RM) cells: RM-VPC, which manages the VP level and
RM-VCC, which manages the VC level.
The format of RM-VPC
cells is shown in the following illustration:
ATM Header:
VCI=6 and PTI=110 (5 bytes) |
RM protocol
identifier (1 byte) |
Message type (1
byte) |
ER (2 bytes)
|
CCR (2 bytes)
|
MCR (2 bytes)
|
QL (4 bytes)
|
SN (4 bytes)
|
Reserved (30
bytes) |
Reserved (6
bits) + CRC-10 (10 bits)
|
RM-VPC cell
format
RM protocol identifier
Always 1 for ABR
services.
Message type
This field is comprised of several bit
fields:
Bit |
Name |
Description |
8 |
DIR |
Direction of the RM cells.
0=forward, 1=backward. |
7 |
BN |
BECN. 0=source is generated;
1=network is generated. |
6 |
CI |
Congestion Indication. 0=no
congestion, 1=congestion. |
5 |
NI |
No increase. 1=do not
increase the ACR. |
4 |
RA |
Not
used. |
ER Explicit rate.
CCR Current cell
rate.
MCR Minimum cell
rate.
QL Not
used.
SN Not
used.
RM-VCC
cells are exactly the same as RM-VPC cells, except that the VCI is
not specified. The cell is identified solely by the PTI bits.
Reserved VPI/VCI
Values
A number of VPI/VCI
values are reserved for various protocols or functions, e.g., 0,5 is
used for signalling messages. Following is a list of all reserved
VPI/VCI values and their designated meanings:
VPI
|
VCI
|
Description |
0 |
0 |
Idle cells. Must
also have GFC set to zero. Idle cells are added by the
transmitter to generate information for non-used cells. They
are removed by the receiver together with bad cells.
|
0 |
1 |
Meta signalling
(default). Meta-signalling is used to define the subchannel
for signalling (default value: 0,5). |
Non-zero |
1 |
Meta signalling.
|
0 |
2 |
General broadcast
signalling (default). Can be used to broadcast signalling
information which is independent of a specific service. Not
used in practice. |
Non-zero |
2 |
General broadcast
signalling. |
0 |
5 |
Point-to-point
signalling (default). Generally used to set-up and release
switched virtual circuits (SVCs). |
Non-zero |
5 |
Point-to-point
signalling. |
|
3 |
Segment OAM F4 flow
cell. OAM cells are used for continuity checks as well as to
notify and acknowledge failures. |
|
4 |
End-to-end OAM F4
flow cell. |
|
6 |
RM-VPC cells for
rate management. |
0 |
15 |
SPANS. The Simple
Protocol for ATM Network Signalling is a simple signalling
protocol, developed by FORE systems and used by FORE and other
manufacturers working in cooperation with FORE, for use in ATM
networks. Refer to Chapter 4 for more information.
|
0 |
16 |
ILMI. The Interim
Local Management Interface is used to manage and compare
databases across an ATM link. This is used for signalling
address registration, RMON applications, SNMP, etc. Refer to
ILMI in this book for more information. |
0 |
18 |
PNNI signalling.
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