Hypertec ISDN 10T Router - Manual (Page 20)

Concepts and Principles of Operation
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Hypertec ISDN 10T Router to negotiate for either PAP or CHAP authentication protocol whichever
the remote end prefers.
Multilink PPP
The PPP Multilink Protocol (RFC 1717) is a standards based extension of the PPP (Point-to-Point
Protocol) standard. It allows you to combine channels into a 'Multilink bundle' so that data can be sent
at higher rates. Multilink PPP can implement a feature known as 'packet fragmentation' where larger
individual packets are chopped into smaller fragments. These fragments are then distributed among
two channels in use. The receiver at the other end of the links collects the fragments, reassembles them
and delivers them in the original order.
Multilink PPP allows packet fragmentation over two B channels to reduce latency and speed up
transmission. The actual implementation of fragmentation however demands significant CPU
overhead. This overhead prevents most ISDN SOHO router vendors from doing the packet
fragmentation, instead packets are distributed between two B channel in a round-robin fashion.
P1 P2 P2 P1
P1
P2
R1 R2
Figure 0-4, Packet transmission without fragmentation, round-robin
When packets are sent with fragmentation, the larger packets are chopped into two fragments of equal
size and distributed from Router 1 to Router 2 over the two channels in use.
P1 P2 P2 P1
R1 R2
F1 F1
F2 F2
Packet 2 Packet 1
Figure 0-5, Packet transmission with fragmentation, chop-in-half
Not all ISDN routers or access servers accept packet fragmentation. To guarantee compatibility, the
Hypertec Router sends packets in round-robin over the Multilink PPP. Hypertec ISDN 10T Router can be
configured to accept both round-robin and fragmented packets. There is a possibility of packet out-of-
sequence when sending and receiving packet in round-robin fashion, Hypertec ISDN 10T Router will re-
order the packets when compression is enabled. This will minimize the history reset request caused by the
packet out-of-sequence. The diagram above illustrates how packet fragmentation changes the way packets
are transmitted between two routes. When packets are send in round-robin, packets are sent whole across
the B-channels.