IP Packet Format
IP Packets Product Documentation contains the main data transmission protocol used in a variety of packet formats, packet example. By IP Packets Product Documentation, be able to enhance understanding of various protocol packets.
IP Packets Product Documentation contains the main data transmission protocol used in a variety of packet formats, packet example. By IP Packets Product Documentation, be able to enhance understanding of various protocol packets.
ATM Cell Header
In ATM switching, packets are called cells and have a fixed length of 53 bytes. As defined by ITU-T, ATM uses cells as basic units for information transmission, multiplexing, and switching. According to network interfaces, ATM cell headers are classified as UNI or NNI cell headers. The ATM cell header is 5 bytes long and carries the address and control information of the cell. By contrast, the payload is 48 bytes long and carries the specific information to be transmitted.
ATM UNI Cell Format
+--------------------+--------------------+
| GFC(4 bit) | VPI(4 bit) |
+--------------------+--------------------+
| VPI(4 bit) | VCI(4 bit) |
+--------------------+--------------------+
| VCI(8 bit) |
+--------------------+--------------------+
| VCI(4 bit) | PT(3 bit) | CLP(1 bit) |
+----------------+-----------+------------+
| HEC(8 bit) |
+--------------------+--------------------+
| |
| |
| 48-byte payload |
| |
| |
| |
+-----------------------------------------+
Field |
Length |
Description |
|---|---|---|
GFC |
4 bits |
Generic flow control. It applies only to UNIs and is used to control traffic or identify different access requests on a shared media network. |
VPI |
8 or 12 bits |
Virtual path identifier. For UNIs, it is 8 bits long and can be used to identify a maximum of 256 virtual paths (VPs). For NNIs, it is 12 bits long and can be used to identify a maximum of 4096 VPs. Some VPI/VCI values are reserved for special cells, which are briefly described in Table 1-19. |
VCI |
16 bits |
Virtual channel identifier. It can be used to identify a maximum of 65536 virtual channels (VCs). Some VPI/VCI values are reserved for special cells, which are briefly described in Table 1-19. |
CLP |
1 bit |
Cell loss priority, which is used for congestion control. Cells with the CLP value of 1 are preferentially dropped when traffic congestion occurs. |
PT |
3 bits |
Payload type, which is used to identify the information field. The following lists the PT values and corresponding meanings defined by ITU-T I.361:
When cells are used to carry user data:
|
HEC |
8 bits |
Header error control, which is used for error control in the cell header. It can correct 1-bit errors and detect multi-bit errors. HEC also performs cell delineation so that the location of the cell header can be identified based on the correlation between the HEC field and its previous four bytes. Because the VPI/VCI values vary according to links, the HEC needs to be recalculated for each link. |
ATM NNI Cell Format
+--------------------+--------------------+
| VPI(8 bit) |
+--------------------+--------------------+
| VPI(4 bit) | VCI(4 bit) |
+--------------------+--------------------+
| VCI(8 bit) |
+--------------------+--------------------+
| VCI(4 bit) | PT(3 bit) | CLP(1 bit) |
+----------------+-----------+------------+
| HEC(8 bit) |
+--------------------+--------------------+
| |
| |
| 48-byte payload |
| |
| |
| |
+-----------------------------------------+
Field |
Length |
Description |
|---|---|---|
GFC |
4 bits |
Generic flow control. It applies only to UNIs and is used to control traffic or identify different access requests on a shared media network. |
VPI |
8 or 12 bits |
Virtual path identifier. For UNIs, it is 8 bits long and can be used to identify a maximum of 256 virtual paths (VPs). For NNIs, it is 12 bits long and can be used to identify a maximum of 4096 VPs. Some VPI/VCI values are reserved for special cells, which are briefly described in Table 1-19. |
VCI |
16 bits |
Virtual channel identifier. It can be used to identify a maximum of 65536 virtual channels (VCs). Some VPI/VCI values are reserved for special cells, which are briefly described in Table 1-19. |
CLP |
1 bit |
Cell loss priority, which is used for congestion control. Cells with the CLP value of 1 are preferentially dropped when traffic congestion occurs. |
PT |
3 bits |
Payload type, which is used to identify the information field. The following lists the PT values and corresponding meanings defined by ITU-T I.361:
When cells are used to carry user data:
|
HEC |
8 bits |
Header error control, which is used for error control in the cell header. It can correct 1-bit errors and detect multi-bit errors. HEC also performs cell delineation so that the location of the cell header can be identified based on the correlation between the HEC field and its previous four bytes. Because the VPI/VCI values vary according to links, the HEC needs to be recalculated for each link. |
Special ATM Cells
Special Cell |
Description |
|---|---|
Idle cell |
The VPI, VCI, PTI, and CLP values are 0, 0, 0, and 1, respectively. This type of cell is used for rate adaptation. |
Unassigned cell |
The VPI, VCI, and CLP values are 0, 0, and 1, respectively. The PTI can be set to any value. |
OAM cell |
|
Signaling cell |
Signaling cells are divided into the following types:
|