HP 200 Series Services And Applications page 43

Branch Office Routing
Packet-by-Packet Compression
Packet-by-packet compression (HP PPC) is an innovative HP data-link
compression technology that improves throughput and can eliminate the
need to add costly incremental bandwidth. Unlike other compression
solutions, HP PPC was developed for use with multiprotocol data over all
link types including leased lines, packet switching networks, and circuit
switching networks. Because other compression solutions require a reliable
link protocol such as LAP-B, they are not suitable for use on links, such as
frame relay, that use an unreliable link-level protocol. HP PPC does not
require special protocols and will thus be available for use on any link type.
With HP PPC, compression is achieved using very small dictionaries that
conserve memory. This helps make compression achievable (cost-effective),
even on routers with many synchronous ports.
Additionally, HP PPC protects against data expansion. If the resulting
compressed packet is larger than the uncompressed packet, the packet is
sent uncompressed. A typical example is when the packet being transmitted
is already compressed.
Like other compression schemes based on the Lempel-Ziv algorithm, the
actual compression ratio obtained depends on the type of data being trans-
mitted. Generally, the compression ratio obtained over a long period of time
averages about 1.8 to 1.
HP PPC is implemented in software and is currently supported over leased
lines at link speeds up to 64 Kbit/s. Compression will be available on dial-up
circuits and frame relay circuits in 1994, and on X.25 in 1995.
Prioritization/Bandwidth Reservation
One of the characteristics of extended LAN networks is that traffic is
inherently bursty. This means that varying amounts of data are transmitted
at irregular time intervals. This can have adverse effects on certain types of
applications–especially those that rely on bridging of connection-oriented
protocols such as IBM's SDLC. Problems such as connection timeouts and
unpredictable response times may occur when connectionoriented data is
queued waiting for the transmission of other data.
HP's traffic prioritization is a feature that a network designer can use to
ensure that delay-sensitive data is given priority over data that is less sensi-
tive to delay. For example, SDLC traffic that is bridged from a terminal
controller to a front-end processor can be given priority over other routed
traffic.
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