 is now broken into smaller, more manageable pieces that are numbered by the Trans- port Layer before being passed into the network for routing and delivery. This innovation offers several advantages. First, it eliminates the need for the mechanical, switch-based hard drive, because the small packets can now be handled blindingly fast by solid-state memory. Second, should a packet arrive with errors, it and it alone can be discarded and replaced. In message-switched environments, an unrecoverable bit error resulted in the inevitable retransmission of the entire message-not a particularly elegant solution. Packet switching, then, offers a number of distinct advantages. As before, of course, there are also disadvantages. There is no longer (necessarily) a physically or logically dedicated path from the source to the destination, which means that the ability to guarantee quality of ser- vice on an end-to-end basis is severely restricted. There are ways around this, as you will see in the section that follows, but they are often costly and always complex. This is one of the reasons that IP telephony had a difficult time achieving widespread deployment. It worked fine in con- trolled, relatively small corporate environments where traffic patterns could be scrutinized and throttled as required to maintain quality of ser- vice (QoS). In the public IP environment, however (read the Internet), there was no way to ensure that degree of control. Today, however, such concerns have been largely eliminated; we will discuss VoIP in greater detail in a later chapter. Packet switching can be implemented in two very different ways. We'll discuss them now. Connection-Oriented Networks Capt. Lewis is brave, prudent, habituated to the woods, & familiar with Indian manners & character. He is not regularly educated, but he possesses a great mass of accurate observation on all the subjects of nature which present themselves here, & will therefore readily select those only Downloaded from Digital Engineering Library @ McGraw-Hill (www.di