Quality Of Service - 3Com NBX 100 Administrator's Manual

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PPENDIX ONNE TIONS ATEWAY
Logical Connections
Locally, every device on an NBX LAN has the same physical access to the
local network traffic as any other device. Consequently, addresses control
connections because devices can only read information that is addressed
to them. This makes addressing, and managing addresses, a key concern
for logical continuity.
Logical continuity concerns extend throughout a network connection
because the identity of a frame (or packet) and its destination determine
where it goes, how it is handled, and what happens to it.
Because so many devices share the same physical media on the Internet
and on the local network, there is always the possibility of incomplete or
degraded connections that arise from network congestion, device
configuration, or addressing problems.
Bridges, switches, routers, and firewalls can help to manage network
congestion, conversions, and security. Configuration problems with of
any of these devices can cause connection difficulties.
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.