System information

The ATM Cell
Book Title
21-436
Figure 21-7 VCCs Transported Within VPs
The ATM Cell
The ATM cell is the fixed-length transmission unit defined by the ATM standard. An ATM cell
contains two major types of information: the payload and the header. The payload is the information
to be transferred through an ATM network. It can include data, voice, image, or video. The header
is the information used to route the cell through the network and to ensure that the cell is forwarded
to its destination.
Every ATM cell is 53 bytes long. The first 5 bytes contain header information, and the remaining 48
bytes contain the payload (see Figure 21-8).
Figure 21-8 An ATM Cell
The 5-byte header (see Figure 21-9) contains several different fields (see Table 21-2). The 48 bytes
following the header (the payload) contain user data.
Figure 21-9 The User-Network Interface ATM Cell Header Format
VP1
VP2
VPn
Physical
link in public
or private network
VP=Virtual path
VC=Virtual channel
VP1
VP2
VPn
VC1
VC2
VCN
VC1
VC2
VCN
VC1
VC2
VCN
VC1
VC2
VCN
VC1
VC2
VCN
VC1
VC2
VCN
Byte
1
User Data
Cell header
Byte
6
Byte
53
Payload
GFCByte 1
Byte 2
Byte 3
Byte 4
5-byte
header
Byte 5
Byte 6
Bit 8 Bit 1
Byte 53
VPI
VCI PT CLP
VPI VCI
VCI
HEC
Payload
Payload
s4
s4
s4
8
8
8
8
8
1
1
1
1
1
48-byte
payload
2