Figure C-8 Simple And Complex Gateway Formats - Cisco BPX-BXM-155-8DX Installation And Configuration Manual

Appendix C ATM: An Overview
Simple Gateway
With the Simple Gateway protocol, the AIT card in the IPX switch (or BTM in the IGX switch) loads
24-byte FastPacket cells into ATM cells in ways that are consistent with each application. (Each of the
two FastPacket cells loaded into the ATM Cell is loaded in its entirety, including the FastPacket header.)
For example, two FastPackets can be loaded into one ATM cell provided they both have the same
destination. This adaptation is performed by the IPX switch AIT card or the IGX switch BTM card.
The AIT (or BTM) is configured to wait a given interval for a second FastPacket to combine in one
ATM cell for each FastPacket type. The cell is transmitted half full if the wait interval expires. High
priority and non-time stamped packets are given a short wait interval. High priority FastPackets will
not wait for a second FastPacket. The ATM trunk interface will always wait for Frame Relay data
(bursty data) to send two packets. NPC traffic will always have two FastPackets in an ATM cell.
Complex Gateway, Frame Relay to ATM Network Interworking
Starting with Release 8.1, with the Complex Gateway capability, the FRSM card in the MGX 8220, the
AIT card in the IPX switch (or BTM card in the IGX switch) streams the Frame Relay data into ATM
cells, cell after cell, until the frame has been completely transmitted. Since only the data from the
FastPacket is loaded, the Complex Gateway is an efficient mechanism. Also, discard eligibility
information carried by the Frame Relay bit is mapped to the ATM cell CLP bit, and vice versa. See
Chapter 13 for further information on Frame Relay to ATM interworking. A comparison of the simple
gateway and complex gateway formats is shown in Figure C-8.
Figure C-8
Simple gateway (AIT card) :
Frame Relay frame
presented to FRP:
Built by FRP into
FastPackets:
ATM
cells:
Complex gateway (AIT Card) :
Frame Relay frame
presented to FRP:
Built by FRP into
FastPackets:
Back to Frame Relay
frame in AIT:
AAL-5 ATM
cells
generated
by AIT:
Release 9.3.0, Part Number 78-10674-01 Rev. D0, July 2001
Simple and Complex Gateway Formats
HDR HDR
HDR 20 bytes
HDR
HDR HDR
HDR 20 bytes
HDR HDR
HDR
Variable length
HDR 20 bytes
48 Bytes
Variable length
HDR 20 bytes
Variable length
48 Bytes
Cisco BPX 8600 Series Installation and Configuration
CRC CRC
HDR 20 bytes
CRC CRC
HDR 20 bytes
CRC CRC
HDR
C-9