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 chapter describes
the ATM UNI cell header structure and the PDU structures for the
various ATM/SAR cell formats including: AAL0, AAL1, AAL2, AAL3/4 and
AAL5.
UNI Cell
The structure of the
UNI cell header is shown in the following
illustration.
|
4 |
8 bits
|
|
GFC
|
VPI
|
|
VPI
|
VCI
|
|
VCI
|
|
VCI
|
PT (3 bits)
|
CLP
|
|
HEC
|
|
Information
(48 bytes)
|
UNI 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.
PT
Payload type.
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 court.
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 SAR PDU
AAL2 was originally
intended for video transfer, but this use has since been replaced by
AAL5. For this reason, it is formally undefined. The structure of
the AAL2 SAR PDU is given in the following
illustration.
|
SN |
IT |
SAR PDU payload |
LI |
CRC |
AAL2 SAR
PDU
SN
Sequence number. Numbers the stream of SAR PDUs of a CPCS
PDU (modulo 16).
IT
Information type.
| BOM |
Beginning of
Message. |
| COM |
Continuation of
Message. |
| EOM |
End of
Message. |
SAR PDU
payload
Information field of
the SAR PDU.
LI
Length indicator.
CRC
Cyclic redundancy check which can correct up to two
erroneous bits.
AAL3/4 SAR PDU
Functions of AAL3/4 SAR
include identification of SAR SDUs; error indication and handling;
SAR SDU sequence continuity; and multiplexing and
demultiplexing.
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.
AAL3/4 CS PDU
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 structure of the
AAL3/4 CS PDU is shown in the following
illustration.
|
CPI |
Btag |
BAsize |
Information |
PAD |
AL |
Etag |
Length |
|
1 |
1 |
2 |
0-40 |
0-3 |
1 |
1 |
2 |
|
4-byte header
|
|
4-byte trailer |
AAL3/4 CS PDU
CPI
Common part indicator. Represents the units of the BAsize
and length field. A value of only zero indicates bytes.
Btag
Beginning tag. This field must have the same value as
Etag for the same CPCS-PDUs and a different value as the Btag/Etag
of preceding and successive CPCS-PDUs.
BAsize
Buffer allocation size. In
message mode this is equal to the length field. In streaming mode
this is equal to or greater than the length field.
Information
Variable information field of up to 40 bytes.
PAD
Up to 3 bytes of padding to achieve 32-bit alignment in
the information field.
AL
Alignment. A filling byte coded with zero.
Etag
End tag. Refer to Btag.
Length
Length of the Information
field. This value is used to indicate information loss or
gain.
AAL5 CS PDU
The structure of the
AAL5 CS PDU is as follows:
|
Information
|
PAD
|
UU
|
CPI
|
Length
|
CRC-32
|
|
1-48
|
0-47
|
1
|
1
|
2
|
4 bytes
|
|
|
8-byte trailer
|
AAL5 CS
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.
AAL5 SAR PDU
The AAL5 SAR PDU
consists of a 48-byte payload.
AAL5 SAR
PDU
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.
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