. The classes of service include constant bit rate (CBR), variable bit rate, real time (VBR-rt), variable bit rate, non-real time (VBR-nrt), available bit rate, and unspecified bit rate. Each class of service has different performance and cost characteristics. PACKET SWITCHING One only need examine history over the past couple of hundred years to see that older communications models and methods bear resem- blance to some of the current crop of fast growing methods. For example it's not difficult to see the similarity between smoke signals and telegraph messages. If one takes the smoke signal model and imagines a sender sending a message to a receiver and the receiver repeating the message to another smoke sender and SO on, it's easy to see the resemblance to packet forwarding characteristics of the Inter- net protocol or email. If you have any interest at all in the Internet and have done any reading on the subject, you're aware that it's based on packet switch- ing. Packet switching depends on some basic functional elements including transmission links, and a switching engine called a router. 4 NETWORK TECHNOLOGY AND METHODOLOGY There are a lot of similarities between cell-based switching and packet-based switching, and there are some differences. Packet switching is simply making decisions about where to send the packet at hand. Packets are like cells in the sense that they must be opened, intelligence found about where they are headed and where they have been, and then switched and/or routed. The instructions are just inside the packet with a few other tidbits of information. One of the fundamental differences between packet switching and cell switching is at the heart of most of the ambiguity and hand- wringing that occurs when considering the routing-switching-laye 2-layer 3 solution. It's really quite simple. Cells, PDH streams, and PPP (HDLC) are layer 2 functions. What separates these techniques from packet techniques is a time base. Packets, at least IP datagrams that make up user datagram protocol and T