A Look at Modems
One of the most common methods for connecting to a TCP/IP network such as the Internet is through a phone line. Telephone access is now an everyday feature of home and traveling computers. Dial-up access is also an option on many office networks, where dial-up service can offer inexpensive Internet access or provide a link for a worker who travels or has an office at home. In most cases this dial-up access is accomplished using a modem.
A modem provides network access through a phone line. The term is short for MOdulate/DEModulate. Engineers created modems because the industry saw the enormous benefit of providing a way for computers to communicate over the world's most accessible transmission medium: the global telephone system. Telephone lines have grown more sophisticated in recent years. Some lines are now capable of transmitting digitized data; other lines are not. In any case, even digital telephone systems are not designed to automatically handle a network protocol like TCP/IP. The purpose of a modem is to transform the digital protocol transmissions from a computer into an analog signal that can pass through the interface with the phone system and to transform incoming analog signals from the phone line into a digital signal that the receiving computer understands.
As you learned in Hour 3, "The Network Access Layer," local networks such as ethernet and token ring employ elaborate access strategies for enabling the computers to share the network medium. By contrast, the two computers at either end of a phone line do not have to compete for the transmission medium with other computers—they have to share it only with each other. This type of connection is called a point-to-point connection (see Figure 8.1).
A point-to-point connection is simpler than a LAN-based configuration because it doesn't have to provide a means for multiple computers to share the transmission medium. At the same time, a connection through a phone line has some limitations. One of the biggest limitations is that transmission rates over a phone connection are much slower than rates over a LAN-based network such as ethernet.
This reduced transmission speed lends itself to a protocol that minimizes the data overhead of the protocol itself—less is better. As you'll learn in this hour, as modems have become faster, modem protocols have taken on additional responsibilities.
Another challenge of dial-up protocols is the great diversity of hardware and software configurations they must support. On a local network, a system administrator oversees and controls the configuration of each computer, and the protocol system depends on a high degree of uniformity among the communicating devices. A dial-up connection, on the other hand, can occur from almost anywhere in the world. Dial-up protocols must contend with a wider and more varied range of possibilities regarding the hardware and software of the communicating machines.
Early modem protocols were merely a method for passing information across the phone line, and in that situation, the logical addressing and internetwork error control of TCP/IP were not necessary or even desirable. Later, with the arrival of local networks and the Internet, engineers began to think about using a dial-up connection as a means of providing network access. The first implementations of this remote network access concept were an extension of earlier modem protocols. In these first host dial-up schemes, the computer attached to the network assumed all responsibility for preparing the data for the network. Either explicitly or implicitly, the remote computer acted more like a terminal (see Figure 8.2), directing the networked host to perform networking tasks and sending and receiving data across the modem line through an entirely separate process.
However, these early host dial-up schemes had some limitations. They reflected an earlier, centralized model of computing that placed huge demands on the computer providing the network access. (Imagine the configuration in Figure 8.2 with several computers simultaneously connected to the dial-up server.) They also made inefficient use of the processing power of the remote computer.
As TCP/IP and other routable protocols began to emerge, designers began to imagine another solution in which the remote computer would take more responsibility for networking tasks, and the dial-up server would act more like a router. This solution (shown in Figure 8.3) was more consistent with the newer, less centralized paradigm of computer networks and also closer to the true nature of TCP/IP. In this arrangement, the remote computer operates its own protocol stack, with the modem protocol(s) acting at the Network Access layer. The dial-up server accepts the data and routes it to the greater network.
Dial-up protocols, therefore, began to work directly with TCP/IP and became an integral part of the stack. This hour covers the two most common TCP/IP modem protocols:
PPP has replaced SLIP as the method of choice for dial-up Internet connections. I include SLIP in this discussion primarily for its historical importance, and for its importance in the development of PPP and other later protocols. The remainder of this hour takes a closer look at SLIP and PPP.
By the Way
Both SLIP and PPP are built on lower-level serial communication protocols that see to the details of actually modulating and demodulating the signal. These serial communication protocols provide what would be considered OSI Physical layer functions.