FUNI was developed by the ATM Forum in order to provide users
with the ability to connect between ATM networks and existing
frame-based equipment (e.g., routers, etc.). FUNI uses a T1/E1
interface and offers a relatively easy and cost-effective method for
users to take advantage of ATM infrastructure or an ATM backbone,
while not having to replace existing equipment with more expensive
ATM equipment.
The frame structure of FUNI is shown in the following
illustration:
|
FUNI PDU
|
|
|
Flag |
FUNI header
|
User SDU
|
FUNI FCS
|
Flag
|
|
2 |
1-n (n<=4096)
|
2 (up to 4)
|
1
|
FUNI frame
structure
The FUNI header is as follows:
|
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
|
Frame address
|
RSVD |
0 |
|
Frame address
|
CN |
RSVD |
CLP |
1
|
FUNI header
structure
RSVD
Reserved bits for interface management. These bits are
set to 0 unless the frame is used for management.
Frame
address
Octet 1, bits 6-3 are mapped
to the 4 LSBs of the VPI in the ATM cell header. The 4 MSBs of the
VPI are not coded in the address field. Octet 1, bits 8 and 7 and
Octet 2 bits 8-5 are mapped to the six LSBs of the VCI in the ATM
cell header.
FCS
16 bit frame check sequence.
CN
Congestion notification. If the PTI=01x in the last ATM
cell composing the FUNI frame, the CN is 1 for the FUNI frame,
otherwise it is 0.
CLP
The network equipment copies the CLP bit sent from the
user equipment into the CLP bit of all ATM cell headers constituting
the FUNI frame. The CLP bit from the network equipment to the user
equipment is always set to 0.
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