MODMER / Interfészek / ATM interfész

Az ATM interface - Frame formats

UNI cell header
NNI cell header
IDLE cell
OAM cell
AAL1_PDU
AAL2_PDU
AAL3/4_PDU
AAL5_PDU

Cell types:

information cells
signalling cells
OAM cells
unassigned cells
cells used for cell rate decoupling

    UNI cell header: (ITU-T I.361)

    <--------direction of propagation
  bit1                                             bit424
    +-----+-----+-----+----+-+-----+------------------+
    |     |     |     |    |C|     |                  |
    | GFC | VPI | VCI | PT |L| HEC | INFO (48 octets) |
    |     |     |     |    |P|     |                  |
    +--4--+--8--+-16--+-3--+1+--8--+------------------+
    | bits  bits               bits|
    |<------- 5 octets ----------->|

    NNI cell header:

    <--------direction of propagation
  bit1                                             bit424
    +-----------+-----+----+-+-----+------------------+
    |           |     |    |C|     |                  |
    | VPI       | VCI | PT |L| HEC | INFO (48 octets) |
    |           |     |    |P|     |                  |
    +-12--------+-16--+-3--+1+--8--+------------------+
    | bits  bits               bits|
    |<------- 5 octets ----------->|

The transmission of the cells is continuous, there is no gap between them. If there is no cell to be transmitted sender insert physical layer cells (idle, OAM).

The header in details:

GFC - Generic Flow Control (000=Uncontrolled Access)
VPI - Virtual Path Identifier (VP - virtual path) - (trunk identifier) Routing information
VCI - Virtual Channel/Circuit Identifier (VC - virtual channel) - Routing information

PT - Payload Type

          Pre assigned cells - cells used by the physical layer
            PTI
            000 - Idle cell
            100 - Physical,layer OAM cell
            PPP - reserved for use
          Payload
            000 - User data cell SDU type 0
            001 - User data cell SDU type 1
            010 - User data cell SDU type 0
            011 - User data cell SDU type 1
            100 - OAM
            101 - OAM
            110 - resource management
            111 - reserved for future

CLP - Cell Loss Priority (1=subject to discard)
HEC - Header Error Control (I.432) CRC from the first 4 bytes of the header (polinom: x^8+x^2+x+1)

IDLE cell:

GFC 0
VPI 0
VCI 0
PT  0
CLP 1
HEC 01010010

UNASSIGNED cell:

GFC
VPI 0
VCI 0
PT  any
CLP 0
HEC

Frame types of the AAL :

AAL1_PDU

     AAL1_SAR_PDU
     +--------------+-------+------+------------+
     | CSI  | SC    | CRC   |Parity| Payload    |
     +-1bit-+-3bits-+-3bits-+-1bit-+--47 octets-+

AAL2_PDU

     AAL2_SAR_PDU
     +------+------+---------+---+------------+
     | SN   | IT   | Payload |LI | CRC        |
     +------+------+---------+---+------------+

AAL3/4_PDU

     CPAAL3/4_CS_PDU: - user data
     +-------+-------+---------+-----------------+----------+-------+------+-------+
     | CPI   | Btag  | BAsize  | User info       | PAD      | AL    | Etag | Length|
     +-1 oct-+-1 oct-+-2 oct---+-0...65535 oct---+-0..3 oct-+-1 oct-+-1oct-+-2 oct-+

     AAL3/4_SAR_PDU - (ATM_DATA request)
     +----------------------------------------------------------+
     | ST     | SN     | MID     | Payload   | LI     | CRC     |
     +-2 bits-+-4 bits-+-10 bits-+-44 octets-+-6 bits-+-10 bits-+

AAL5_PDU

Sources used this type of framing to transmit packets with variable length on the ATM network.



The user side frame of AAL5: CPAAL5_PDU - (user data)
first                                                            last
bit                                                               bit
+-------------------+-----------------+---------+---------+---------+
| PDU   User info,  |      PAD        | Control | Length  | CRC32   |
|       Payload     |                 |         |         |         |
+- 0 ... 65535 Oct -+-- 0 ... 47 Oct -+- 2 Oct -+- 2 Oct -+- 4 Oct -+
                                               trailer

The ATM side fram of AAL5 : AAL5_SAR_PDU - (ATM_DATA request)
+------------+
| Payload    |
+--48 octets-+

The frame is assembled by the AAL5. Parts of the frame:

PDU - user data intended to transmit
PAD - padding: the length of the PDU+PAD+trailer must be an integer multiple of 48 octets.
trailer - the "header" of the AAL5 frame (8 octets)
- Control - unused
  - UU - User-to-User indication - 1 octet
  - CPI - Common Part Indicator - 1 octet
- Length - length of PDU without PAD
- CRC32 - control sum