418 APPENDIX D: CONNE XTIONS H.323 GATEWAYLogical ConnectionsLocally, every device on an NBX LAN has the same physical access to thelocal network traffic as any other device. Consequently, addresses controlconnections because devices can only read information that is addressedto them. This makes addressing, and managing addresses, a key concernfor logical continuity.Logical continuity concerns extend throughout a network connectionbecause the identity of a frame (or packet) and its destination determinewhere it goes, how it is handled, and what happens to it.Because so many devices share the same physical media on the Internetand on the local network, there is always the possibility of incomplete ordegraded connections that arise from network congestion, deviceconfiguration, or addressing problems.Bridges, switches, routers, and firewalls can help to manage networkcongestion, conversions, and security. Configuration problems with ofany of these devices can cause connection difficulties.Bridges and switches are used to segregate areas of congestion within alocal network (switches are multiport bridges). Routers perform a similarfunction but at the Layer-3 level where they perform conversionsbetween LAN and WAN protocols. Firewalls, which are often built intorouters, 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 areconfigured, they can let packets pass or they can block them.Quality of Service Unlike switched network connections, Internet voice connections consistof a sequence of numbered data packets. Packet transfers across theInternet 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 qualitydeteriorates noticeably. The round-trip delay is typically no greater than400 ms. You can test this by using several “ping” commands.Voice conversations occur in “real-time,” so these packets need to bedelivered in a consistent manner and with the shortest delay. The goal isto deliver 32 regularly spaced packets to the recipient every second.
