NETWORKS CAME INTO EXISTENCE AS SOON as there was two of something: two cells, two animals and, obviously, two computers. While the overwhelming popularity of the Internet leads people to think of networks only in a computer context, a network exists anytime there is communication between two or more parties. The differences between various networks are matters of implementation, as the intent is the same: communication. Whether it is two people talking or two computers sharing information, a network exists. The implementations are defined by such aspects as medium and protocol. The network medium is the substance used to transmit the information; the protocol is the common system that defines how the information is transmitted.
In this chapter, we discuss the types of networks, the methods for connecting networks, how network data is moved from network to network, and the protocols used on today’s popular networks. Network design, network administration, and routing algorithms are topics suitable for an entire book of their own, so out of necessity we’ll present only overviews here. With that in mind, let’s begin.Circuits vs. Packets
In general, there are two basic types of network communications: circuit-switched and packet-switched. Circuit-switched networks are networks that use a dedicated link between two nodes, or points. Probably the most familiar example of a circuit-switched network is the legacy telephone system. If you wished to make a call from New York to Los Angeles, a circuit would be created between point A (New York) and point B (Los Angeles). This circuit would be dedicated—that is, there would be no other devices or nodes transmitting information on that network and the resources needed to make the call possible, such as copper wiring, modulators, and more would be used for your call and your call only. The only nodes transmitting would be the two parties on each end.
One advantage of a circuit-switched network is the guaranteed capacity. Because the connection is dedicated, the two parties are guaranteed a certain amount of transmission capacity is available, even though that amount has an upper limit. A big disadvantage of circuit-switched networks, however, is cost. Dedicating resources to facilitate a single call across thousands of miles is a costly proposition, especially since the cost is incurred whether or not anything is transmitted. For example, consider making the same call to Los Angeles and getting an answering machine instead of the person you were trying to reach. On a circuit-switched network, the resources are committed to the network connection and the costs are incurred even though the only thing transmitted is a message of unavailability.
A packet-switched network uses a different approach from a circuit-switched network. Commonly used to connect computers, a packet-switched network takes the information communicated on the network and breaks it into a series of packets, or pieces. These packets are then transmitted on a common network. Each packet consists of identification information as well as its share of the larger piece of information. The identification information on each packet allows a node on the network to determine whether the information is destined for it or the packet should be passed along to the next node in the chain. Once the packet arrives at its destination, the receiver uses the identification portion of the packet to reassemble the pieces and create the complete version of the original information. For example, consider copying a file from one computer in your office to another. On a packet-switched network, the file would be split into a number of packets. Each packet would have specific identification information as well as a portion of the file. The packets would be sent out onto the network, and once they arrived at their destination, they would be reassembled into the original file.
Unlike circuit-switched networks, the big advantage of packet-switched networks is the ability to share resources. On a packet-switched network, many nodes can exist on the network, and all nodes can use the same network resources as all of the others, sharing in the cost. The disadvantage of packet-switched networks, however, is the inability to guarantee capacity. As more and more nodes sharing the resources try to communicate, the portion of the resources available to each node decreases.
Despite their disadvantages, packet-switched networks have become the de facto standard whenever the term “network” is used. Recent developments in networking technologies have decreased the price point for capacity significantly, making a network where many nodes or machines can share the same resources cost-effective. For the purposes of discussion in this book, the word “network” will mean a packet-switched network.Internetworking
A number of different technologies exist for creating networks between computers. The terms can be confusing and in many cases can mean different things depending on the context in which they’re used. The most common network technology is the concept of a local area network, or LAN. A LAN consists of a number of computers connected together on a network such that each can communicate with any of the others. A LAN typically takes the form of two or more computers joined together via a hub or switch, though in its simplest form two computers connected directly to each other can be called a LAN as well. When using a hub or switch, the ability to add computers to the network becomes trivial, requiring only the addition of another cable connecting the new node to the hub or switch. That’s the beauty of a packet-switched network, for if the network were circuit-switched, we would have to connect every node on the network to every other node, and then figure out a way for each node to determine which connection to use at any given time.
LANs are great, and in many cases they can be all that’s needed to solve a particular problem. However, the advantages of a network really become apparent when you start to connect one network to another. This is called internetworking, and it forms the basis for one of the largest known networks: the Internet. Consider the following diagrams. Figure 1-1 shows a typical LAN.
You can see there are a number of computers, or nodes, connected to a common point. In networking parlance, this is known as a star configuration. This type of LAN can be found just about anywhere, from your home to your office, and it’s responsible for a significant portion of communication activity every day. But what happens if you want to connect one LAN to another?
As shown in Figure 1-2, connecting two LANs together forms yet another network, this one consisting of two smaller networks connected together so that information can be shared not only between nodes on a particular LAN, but also between nodes on separate LANs via the larger network.
Figure 1-2. Two connected networks
Because the network is packet-switched, you can keep connecting networks together forever or until the total number of nodes on the network creates too much traffic and clogs the network. Past a certain point, however, more involved network technologies beyond the scope of this book are used to limit the traffic problems on interconnected networks and improve network efficiency. By using routers, network addressing schemes, and long-haul transmission technologies such as dense wavelength division multiplexing (DWDM) and long-haul network protocols such as asynchronous transfer mode (ATM), it becomes feasible to connect an unlimited number of LANs to each other and allow nodes on these LANs to communicate with nodes on remote networks as if they were on the same local network, limiting packet traffic problems and making network interconnection independent of the supporting long-distance systems and hardware. The key concept in linking networks together is that each local network takes advantage of its packet-switched nature to allow communication with any number of other networks without requiring a dedicated connection to each of those other networks.Ethernets
Regardless of whether we’re talking about one network or hundreds of networks connected together, the most popular type of packet-switched network is the Ethernet. Developed 30 years ago by Xerox PARC and later standardized by Xerox, Intel, and Digital Equipment Corporation, Ethernets originally consisted of a single cable connecting the nodes on a network. As the Internet exploded, client-server computing became the norm, and more and more computers were linked together, a simpler, cheaper technology known as twisted pair gained acceptance. Using copper conductors much like traditional phone system wiring, twisted pair cabling made it even cheaper and easier to connect computers together in a LAN. A big advantage to twisted pair cabling is that, unlike early Ethernet cabling, a node can be added or removed from the network without causing transmission problems for the other nodes on the network.
A more recent innovation is the concept of broadband. Typically used in connection with Internet access via cable TV systems, broadband works by multiplexing multiple network signals on one cable by assigning each network signal a unique frequency. The receivers at each node of the network are tuned to the correct frequency and receive communications on that frequency while ignoring communications on all the others.
A number of alternatives to Ethernet for local area networking exist. Some of these include IBM’s Token Ring, ARCNet, and DECNet. You might encounter one of these technologies, as Linux supports all of them, but in general the most common is Ethernet.Ethernet Frames
On your packet-switched Ethernet, each packet of data can be considered a frame. An Ethernet frame has a specific structure, though the length of the frame or packet is variable, with the minimum length being 64 bytes and the maximum length being 1518 bytes, although proprietary implementations can extend the upper limit to 4096 bytes or higher. A recent Ethernet specification called Jumbo Frames even allows frame sizes as high as 9000 bytes, and newer technologies such as version 6 of the Internet Protocol (discussed later) allow frames as large as 4GB. In practice, though, Ethernet frames use the traditional size in order to maintain compatibility between different architectures.
Because the network is packet-based, each frame must contain a source address and destination address. In addition to the addresses, a typical frame contains a preamble, a type indicator, the data payload, and acyclic redundancy checksum (CRC). The preamble is 64 bits long and typically consists of alternating 0s and 1s to help network nodes synchronize transmissions. The type indicator is 16 bits long, and the CRC is 32 bits. The remaining bits in the packet consist of the actual packet data being sent (see Figure 1-3).
Figure 1-3. An Ethernet frame
The type field is used to identify the type of data being carried by the packet. Because Ethernet frames have this type indicator, they are known as self-identifying. The receiving node can use the type field to determine the data contained in the packet and take appropriate action. This allows the use of multiple protocols on the same node and the same network segment. If you wanted to create your own protocol, you could use a frame type that did not conflict with any others being used, and your network nodes could communicate freely without interrupting any of the existing communications.
The CRC field is used by the receiving node to verify that the packet of data has been received intact. The sender computes the CRC value and adds it to the packet before sending the packet. On the receiving end, the receiver recalculates the CRC value for the packet and compares it to the value sent by the sender to confirm the packet was received intact.
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