HP 200 Series Services And Applications page 255

Using Synchronous Pass-Through to Consolidate Synchronous Traffic
The PAD (packet assembler/dissembler) at site C is connected to the X.25
switch at site B using the sync pass-through ports labeled
Traffic from terminal 1 to host B follows the path:
terminal 1 to PAD
PAD to site C router
site C router to site B router
site B router to X.25 packet switch
X.25 packet switch to host B
Traffic from terminal 2 to host A follows the path:
terminal 2 to PAD
PAD to the site C router
site C router to the site B router
site B router to the attached X.25 packet switch
X.25 packet switch back to site B router
site B router to site A router
site A router to attached X.25 packet switch
X.25 packet switch to host A
Note that traffic from terminal 2 to host A does not use the router path from
site C to site A. The PAD is connected to the X.25 switch at site B using sync
pass-through.
Sync Pass-Through Encapsulation
The router constructs LAN packets by encapsulating the synchronous
frames received on a sync pass-through port, as shown in figure 3. The local
and remote station addresses assigned by the user are used as source and
destination addresses. A special type field, recognized only by HP and
Wellfleet routers, is added to mark the frame as a sync pass-through packet.
The original CRC is removed.
These packets are then transmitted on the appropriate interface by the
bridging service. Since the bridging service is used to convey sync pass-
through traffic, bridging must be enabled on all router interfaces that may be
required to convey sync pass-through traffic. Additional encapsulating
protocol is added, depending on the type of circuit on which the sync
pass-through packet is transmitted. (Figure 3 shows the encapsulation used
on synchronous point-to-point circuits.)
Sync Pass-Through Encapsulation
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