Quality Of Service - 3Com 3C10402B Administrator's Manual

476 A D: C X H.323 G
PPENDIX ONNE TIONS ATEWAY
Bridges and switches are used to segregate areas of congestion within a
local network (switches are multiport bridges). Routers perform a similar
function but at the Layer-3 level where they perform conversions
between LAN and WAN protocols. Firewalls, which are often built into
routers, protect intranets from unauthorized internet users.
All of these devices can filter packets based on source address,
destination address or packet type. Depending on how the devices are
configured, they can let packets pass or they can block them.

Quality of Service

Unlike switched network connections, Internet voice connections consist
of a sequence of numbered data packets. Packet transfers across the
Internet are subject to delays or loss or both. If these delays are great
(larger than 200 ms), or if the packet loss is excessive, voice quality
deteriorates noticeably. The round-trip delay is typically no greater than
400 ms. You can test this by using several "ping" commands.
Voice conversations occur in "real-time," so these packets need to be
delivered in a consistent manner and with the shortest delay. The goal is
to deliver 32 regularly spaced packets to the recipient every second.
The frequency response, dynamic range, and noise of a voice
conversation depend on the voice representation. If all data packets reach
their destination, the system provides voice of a specified quality.
The H.323 standard accommodates alternative voice compression
standards that allow users to trade some voice quality for bandwidth by
selecting a different compression standard (G.711 or G.723).
Consequently, packet loss and delay are crucial to the Quality of Service.
Packet Loss
Packet loss can occur for reasons discussed in Bandwidth, Congestion,
and Connections, next.
Bandwidth
Bandwidth is the capacity to carry information. By using H.323, the same
bandwidth that supports one uncompressed G.711 voice connection can,
instead, support several compressed G.723 conversations with little
noticeable difference in quality.