General Description 2-15
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
HDR HDR Variable length CRC CRC
HDR HDR
Frame Relay frame
presented to FRP:
Built by FRP into
FastPackets:
AAL-5 ATM
cells
generated
by AIT:
Back to Frame Relay
frame in AIT:
Variable lengthHDR
HDR 20 bytes HDR 20 bytes HDR 20 bytes
HDR CRC CRC
48 Bytes
Complex gateway (AIT Card) :
Simple gateway (AIT card) :
HDR HDR CRC
HDR HDR HDR
HDR
ATM
cells:
Variable length
20 bytes 20 bytes 20 bytes
48 Bytes
Frame Relay frame
presented to FRP:
Built by FRP into
FastPackets:
H8228
CRC
