476 APPENDIX D: CONNE XTIONS H.323 GATEWAYBridges 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.The frequency response, dynamic range, and noise of a voiceconversation depend on the voice representation. If all data packets reachtheir destination, the system provides voice of a specified quality.The H.323 standard accommodates alternative voice compressionstandards that allow users to trade some voice quality for bandwidth byselecting a different compression standard (G.711 or G.723).Consequently, packet loss and delay are crucial to the Quality of Service.Packet LossPacket loss can occur for reasons discussed in Bandwidth, Congestion,and Connections, next.BandwidthBandwidth is the capacity to carry information. By using H.323, the samebandwidth that supports one uncompressed G.711 voice connection can,instead, support several compressed G.723 conversations with littlenoticeable difference in quality.
