 residential Internet users to high-data-rate IP over SONET/SDH core network connections. Typical application of these protocols is by a residential dial-up user of Internet service. To get access to the Internet, a user (or a modem) dials the telephone number of his ISP. The call is connected to a modem in the access server at the ISP's point of presence (PoP), that is, the point where access to Introduction to Telecommunications Network Engineering the ISP's service is provided. Now a point-to-point physical connection is established but to transmit independent IP packets necessary data link layer functions have to be implemented. IP packets are transmitted every now and then and in the meantime errors on the line could be interpreted as IP packets if no framing and error control is implemented. To solve this problem, PPP uses the data link layer protocol, called Link Control Protocol (LCP), which performs framing of IP packets and error control. LCP also defines a negotiation mechanism, which is used in the beginning and end of the data link layer connection. End sys- items may, for example, agree to use frame numbering, acknowledgments and retransmission for error recovery. They are needed in wireless connections but not typically used in PSTN or ISDN connections. Another problem is that the user's computer has no permanent IP address and before any communications an address has to be assigned. Typi- cally each ISP has a much smaller number of IP addresses than customers. For dynamic IP address assignment, a Network Control Protocol (NCP) is used after data link layer connection is established by LCP. NCP assigns one of the ISP's IP addresses for the customer at the beginning of the connection and releases it at the end [3]. 6.6.4 Internet Protocol The IP is the core protocol of the Internet. It provides a service for the trans- fer of data units, datagrams, between the host computer and the router as well as between routers. At the IP level, each datagram is handled as a sepa- rate transfer and not as part o