Cisco Systems 8600 Series manual Simple Gateway, Simple and Complex Gateway Formats

Functional Description

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 2-13.

Figure 2-13

Simple and Complex Gateway Formats

Simple gateway (AIT card) :

Frame Relay frame

HDR

HDR

Variable length

CRC

CRC

presented to FRP:

Built by FRP into

HDR

20 bytes

HDR

20 bytes

HDR

20 bytes

FastPackets:

ATM

HDR

48 Bytes

cells:

Complex gateway (AIT Card) :

Frame Relay frame

HDR

HDR

Variable length

CRC

CRC

presented to FRP:

Built by FRP into

HDR

20 bytes

HDR

20 bytes

HDR

20 bytes

FastPackets:

Back to Frame Relay

HDR

HDR

Variable length

CRC

CRC

frame in AIT:

AAL-5 ATM

cells

HDR

48 Bytes

HDR

generated

by AIT:

H8228

General Description 2-15