VOICE OVER IP CRASH COURSE - voiceip.com.ua

340Appendix Cinto smaller, more manageable pieces that are numbered by the TransportLayer before being passed onto the network for routing and delivery.This innovation offers several advantages. First, it eliminates the needfor the mechanical, switch-based hard drive, because the small packetscan now be handled blindingly fast by solid-state memory. Second,should a packet arrive with errors, it and it alone can be discarded andreplaced. In message-switched environments, an unrecoverable bit errorresulted in the inevitable retransmission of the entire message—not aparticularly elegant solution. Packet switching, then, offers a number ofdistinct advantages.As before, of course, there are also disadvantages. No longer is there(necessarily) a physically or logically dedicated path from the source tothe destination, which means that the ability to guarantee quality of service(QoS) on an end-to-end basis is severely restricted. There are waysaround this as you will see in the section that follows, but they are oftencostly and always complex. This is one of the reasons that IP telephonyis having a difficult time achieving widespread deployment. It works finein controlled, relatively small corporate environments where traffic patternscan be scrutinized and throttled as required to maintain QoS. Inthe public IP environment (read the Internet), however, no way exists toensure that degree of control. Will it work? Of course. Is it dependable?Absolutely not. A customer might be willing to call a friend with it, butthey would be less inclined to use the service for a business call—notbecause it’s bad, but because it’s not dependable or predictable. Until itis, the old circuit-switched telephone network will continue to enjoy itscentury (or two) in the sun. The day will certainly come, but for now itisn’t ready for prime time.Packet switching can be implemented in two very different ways. We’lldiscuss them now.Connection-Oriented NetworksWhen Meriwether Lewis and William Clark left St. Louis with the Corpsof Discovery in 1803 to travel up the Missouri and Columbia Rivers tothe Pacific Ocean, they had no idea how to get where they were going.They traveled with and relied on a massive collection of maps, instruments,transcripts of interviews with trappers and Native Americanguides, an awful lot of courage, and the knowledge of Sacagawea, the wifeof independent French-Canadian trader Toussaint Charbonneau, whoaccompanied them on their journey. As they made their way across thewilderness of the northwest, they marked trees every few hundred feet