Is the Internet different from telephone networks?

Yes and no. Most backbone and regional network traffic moves over leased phone lines, so at a low level the technology is the same. However, there is a fundamental distinction in how the lines are used by the Internet and the phone companies. The Internet provides connectionless packet-switched service whereas telephone service is circuit-switched. (We define these terms below.) The difference may sound arcane, but it has vastly important implications for pricing and the efficient use of network resources.

What is circuit-switching?

Phone networks use circuit switching: an end-to-end circuit must be set up before the call can begin. A fixed share of network resources is reserved for the call, and no other call can use those resources until the original connection is closed. This means that a long silence between two teenagers uses the same resources as an active negotiation between two fast-talking lawyers. One advantage of circuit-switching is that it enables performance guarantees such as guaranteed maximum delay, which is essential for real-time applications like voice conversations. It is also much easier to do detailed accounting for circuit-switched network usage.

How is packet-switching technology different from circuit-switching?

The Internet uses “packet-switching” technology. The term “packets” refers to the fact that the data stream from your computer is broken up into packets of about 200 bytes (on average), which are then sent out onto the network[3]. Each packet contains a “header” with information necessary for routing the packet from origination to destination. Thus each packet in a data stream is independent.

The main advantage of packet-switching is that it permits “statistical multiplexing” on the communications lines. That is, the packets from many different sources can share a line, allowing for very efficient use of the fixed capacity. With current technology, packets are generally accepted onto the network on a first-come, first-served basis. If the network becomes overloaded, packets are delayed or discarded (“dropped”).

How are packets routed to their destination?

The Internet technology is connectionless. This means that there is no end-to-end setup for a session; each packet is independently routed to its destination. When a packet is ready, the host computer sends it on to another computer, known as a router. The router examines the destination address in the header and passes the packet along to another router, chosen by a route-finding algorithm. A packet may go through 30 or more routers in its travels from one host computer to another. Because routes are dynamically updated, it is possible for different packets from a single session to take different routes to the destination.

Along the way packets may be broken up into smaller packets, or reassembled into bigger ones. When the packets reach their final destination, they are reassembled at the host computer. The instructions for doing this reassembly are part of the TCP/IP protocol.

Some packet-switching networks are “connection-oriented” (notably, X.25 networks, such as Tymnet and frame-relay networks). In such a network a connection is set up before transmission begins, just as in a circuit-switched network. A fixed route is defined, and information necessary to match packets to their session and defined route is stored in memory tables in the routers. Thus, connectionless networks economize on router memory and connection set-up time, while connection-oriented networks economize on routing calculations (which have to be redone for every packet in a connectionless network).

What is the physical technology of the Internet?

Most of the network hardware in the Internet consists of communications lines and switches or routers. In the regional and backbone networks, the lines are mostly leased telephone trunk lines, which are increasingly fiber optic. Routers are computers; indeed, the routers used on the NSFNET were modified commercial IBM RS6000 workstations, although custom-designed routers by other companies such as Cisco, Wellfleet, 3-Com and DEC probably have the majority share of the market.

What does “speed” mean?

“Faster” networks do not move electrons or photons at faster than the speed of light; a single bit travels at essentially the same speed in all networks. Rather, “faster” refers to sending more bits of information simultaneously in a single data stream (usually over a single communications line), thus delivering n bits faster. Phone modem users are familiar with recent speed increases from 300 bps (bits per second) to 2400, 9600 and now 19,200 bps. Leased-line network speeds have advanced from 56 Kbps (kilo, or 10^3 bps) to 1.5 Mbps (mega, or 10^6 bps, known as T-1 lines) in the late 80s, and then to 45 Mbps (T-3) in the early 90s. Lines of 155 Mbps are now available, though not yet widely used. The U.S. Congress had called for a 1 Gbps (giga, or 10^9 bps) backbone by 1996. This goal has been nearly achieved in testbeds, though it now looks like it will be at least a couple of more years before we see gigabit speeds in the public backbone.

Current T-3 45 Mbps lines can move data at a speed of 1,400 pages of text per second; a 20-volume encyclopedia can be sent coast to coast in half a minute. However, it is important to remember that this is the speed on the superhighway—the access roads via the regional networks still mostly use the much slower T-1 connections.

Why do data networks use packet-switching?

Economics can explain most of the preference for packet-switching over circuit-switching in the Internet and other public networks. Circuit networks use lots of lines in order to economize on switching and routing. That is, once a call is set up, a line is dedicated to its use regardless of its rate of data flow, and no further routing calculations are needed. This network design makes sense when lines are cheap relative to switches.

The costs of both communications lines and computers have been declining exponentially for decades. However, since about 1970, switches (computers) have become relatively cheaper than lines. At that point packet switching became economic: lines are shared by multiple connections at the cost of many more routing calculations by the switches. This preference for using many relatively cheap routers to manage few expensive lines is evident in the topology of the backbone networks. For example, in the NSFNET any packet coming on to the backbone had to pass through two routers at its entry point and again at its exit point. A packet entering at Cleveland and exiting at New York traversed four routers but only one leased T-3 communications line.

What are ATM and cell-switching technologies?

The international telephone community has committed to a future network design that combines elements of both circuit and packet switching to enable the provision of integrated services. The ITU (formerly CCITT, an international standards body for telecommunications) has adopted a “cell-switching” technology called ATM (asynchronous transfer mode) for future high-speed networks. Cell switching closely resembles packet switching in that it breaks a data stream into packets which are then placed on lines that are shared by several streams. One major difference is that cells have a fixed size while packets can have different sizes. This makes it possible in principle to offer bounded delay guarantees (since a cell will not get stuck for a surprisingly long time behind an unusually large packet).

An ATM network also resembles a circuit-switched network in that it provides connection-oriented service. Each connection has a set-up phase, during which a “virtual circuit” is created. The fact that the circuit is virtual, not physical, provides two major advantages. First, it is not necessary to reserve network resources for a given connection; the economic efficiencies of statistical multiplexing can be realized. Second, once a virtual circuit path is established switching time is minimized, which allows much higher network throughput. Initial ATM networks are already being operated at 155 Mbps, while the non-ATM Internet backbones operate at no more than 45 Mbps. The path to 1000 Mbps (gigabit) networks seems much clearer for ATM than for traditional packet switching.

What changes are likely in network technology?

At present there are many overlapping information networks (e.g., telephone, telegraph, data, cable TV), and new networks are emerging rapidly (paging, personal communications services, etc.). Each of the current information networks was engineered to provide a particular type of service and the added value provided by each different type was sufficient to overcome the fixed costs of building overlapping physical networks.

However, given the high fixed costs of providing a network, the economic incentive to develop an “integrated services'' network is strong. Furthermore, now that all information can be easily digitized separate networks for separate types of traffic are no longer necessary. Convergence toward a unified, integrated services network is a basic feature in most visions of the much publicized “information superhighway” (e.g., [National Academy of Sciences1994]). The migration to integrated services networks will have important implications for market structure and competition.

When will the “information superhighway” arrive?

The federal High Performance Computing Act of 1991 aimed for a gigabit per second (Gbps) national backbone by 1995. Five federally-funded testbed networks are currently demonstrating various gigabit approaches. To get a feel for how fast a gigabit per second is, note that most small colleges or universities today have 56 Kbps Internet connections. At 56 Kbps it takes about five hours to transmit one gigabit!

Efforts to develop integrated services networks also have exploded. Several cable companies have already started offering Internet connections to their customers[4]. ATT, MCI and all of the “Baby Bell” operating companies are involved in mergers and joint ventures with cable TV and other specialized network providers to deliver new integrated services such as video-on-demand. ATM-based networks, although initially developed for phone systems, ironically have been first implemented for data networks within corporations and by some regional and backbone providers.