Computer networks permit the transfer of information between computers, allowing computers to share devices, such as printers, laser multiformat cameras, and infor-mation storage devices, and enabling services such as electronic transmission of mes-sages (e-mail), transfer of computer files, and use of distant computers. Networks, based upon the distances they span, may be described as local area networks (LANs) or wide area networks (WANs). A LAN connects computers within a department, a building such as medical center, and perhaps neighboring buildings, whereas a WAN connects computers at large distances from each other. LANs and WANs evolved separately and a computer can be connected to a WAN without being part of a LAN.
However, most WANs today consist of multiple LANs connected by medium or long distance communication links. The largest WAN is the Internet.
A server is a computer on a network that provides a service to other computers on the network. A computer with a large array of magnetic disks that provides data stor-age for other computers is called a file server. There are also print servers, application servers, database servers, e-mail servers, web servers, etc. A computer, typically a workstation, on a network that makes use of a server is called a client.
Two common terms used to describe client-server relationships are thick client and thin client. “Thick client” describes the situation in which the client computer
provides most information processing and the function of the server is mainly to store information, whereas “thin client” describes the situation in which most information processing is provided by the server and the client mainly serves to display the informa-tion. An example would be the production of volume rendered images from a set of CT images. In the thin client relationship, the volume rendered images would be produced by the server and sent to a workstation for display, whereas in a thick client relation-ship, the images would be produced by software and/or a graphics processor installed on the workstation. The thin client relationship can allow the use of less capable and less expensive workstations and enable specialized software and hardware, such as a graphics processor, on a single server to be used by several or many workstations.
Networks have both hardware and software components. A physical connec-tion must exist between computers so that they can exchange informaconnec-tion. Common physical connections include coaxial cable, copper wiring, optical fiber cables, and radio, including microwave, communication systems. A coaxial cable is a cylindri-cal cable with a central conductor surrounded by an insulator that, in turn, is sur-rounded by a tubular gsur-rounded shield conductor. Coaxial cable and copper wiring carry electrical signals. Optical fiber cables use glass or plastic fibers to carry near-infrared radiation signals produced by lasers or light emitting diodes. Optical fiber cables have several advantages over cables or wiring carrying electrical signals, par-ticularly when the cables must span long distances—optical fiber is not affected by electrical interference and thus usually has lower error rates; it can carry signals greater distances before attenuation, distortion, and noise require the use of repeaters to read and retransmit the signals; and it permits much greater information transmis-sion rates. There are also layers of software between the application program with which the user interacts and the hardware of the communications link.
Network protocols are standards for communication over a network. A protocol provides a specific set of services. Both hardware and software must comply with established protocols to achieve successful transfer of information. Failure to con-form to a common protocol would be analogous to a person speaking only English attempting to converse with a person speaking only Arabic. Several hardware and software protocols will be described later in this chapter.
In most networks, multiple computers share communication pathways. Most network protocols facilitate this sharing by dividing the information to be transmit-ted into packets. Packets may be called frames, datagrams, or cells. Some protocols permit packets of variable size, whereas others permit only packets of a fixed size.
Each packet has a header containing information identifying its destination. In most protocols, a packet also identifies the sender and contains information used to deter-mine if the packet’s contents were garbled during transfer. The final destination com-puter reassembles the information from the packets and may request retransmission of lost packets or packets with errors.
Large networks usually employ switching devices to forward packets between network segments or even between entire networks. Each device on a network, whether a computer or switching device, is called a node and the communications pathways between them are called links. Each computer is connected to a network by a network adapter, also called a network interface, installed on the I/O bus of the computer or incorporated on the motherboard. Each interface between a node and a network is identified by a unique number called a network address. A computer usually has only a single interface, but a switching device connecting two or more networks may have an address on each network.
The maximal data transfer rate of a link or a connection is called the bandwidth, a term originally used to describe the data transfer capacities of analog communications
channels. An actual network may not achieve its full nominal bandwidth because of overhead or inefficiencies in its implementation. The term throughput is commonly used to describe the maximal data transfer rate that is actually achieved. Bandwidth and throughput are usually described in units of megabits per second (106 bps 5 1 Mbps) or gigabits per second (109 bps 5 1 Gbps). These units should not be confused with megabytes per second (MBps) and gigabytes per second (GBps), commonly used to specify the data transfer rates of computer components such as magnetic and optical disks. (Recall that a byte consists of eight bits. Megabytes and gigabytes are defined in Table 5-3.) The latency is the time delay of a transmission between two nodes. In a packet-switched network, it is the time required for a small packet to be transferred. It is determined by factors such as the total lengths of the links between the two nodes, the speeds of the signals, and the delays caused by any intermediate repeaters and packet switching devices.
On some small LANs, all computers are directly connected, that is, all packets reach every computer. However, on a larger network, every packet reaching every computer would be likely to cause unacceptable network congestion. For this reason, most networks larger than a small LAN, and even many small LANs, employ packet switching. The packets are sent over the network. Devices such as switches and routers (to be discussed shortly) store the packets, read the destination addresses, and send them on toward their destinations, a method called “store and forward.” In some very large packet-switched networks, individual packets from one computer to another may follow different paths through the network and may arrive out-of-order.
Networks are commonly designed in layers, each layer following a specific pro-tocol. Figure 5-20 shows a model of a network consisting of five layers. Each layer in the stack provides a service to the layer above.
The top layer in the stack is the Application Layer (Layer 5 in Fig. 5-20). Appli-cation programs, commonly called appliAppli-cations, function at this layer. AppliAppli-cations are programs that perform useful tasks and are distinguished from systems software, such as an operating system. On a workstation, applications include the programs, such as an e-mail program, word processing program, web browser, or a program for displaying medical images, with which the user directly interacts. On a server, an application is a program providing a service to other computers on the network.
The purpose of a computer network is to allow applications on different computers to exchange information.
Network communications begin at the Application Layer on a computer. The appli-cation passes the information to be transmitted to the next lower layer in the stack.
Layer5
Application ■■FIGURE 5-20 TCP/IP Network Pro-tocol Stack. Another network model, the Open Systems Interconnection (OSI) model, is also commonly used to model networks. The OSI model has seven layers. The bottom four layers of the OSI model match the bottom four layers shown in this figure. The OSI model has two addi-tional layers between the transport layer and the application layer.
The information is passed from layer to layer, with each layer adding information, such as addresses and error-detection information, until it reaches the Physical Layer (Layer 1 in Fig. 5-20). The Physical Layer sends the information to the destination computer, where it is passed up the layer stack to the application layer of the desti-nation computer. As the information is passed up the layer stack on the destidesti-nation computer, each layer removes the information appended by the corresponding layer on the sending computer until the information sent by the application on the sending computer is delivered to the intended application on the receiving computer.
The lower network layers (Layers 1 and 2 in Fig. 5-20) are responsible for trans-mission of packets from one node to another over a LAN or point-to-point link, and enable computers with dissimilar hardware and operating systems to be physically connected. The lowest layer, the Physical Layer, transmits physical signals over a communication channel (e.g., the copper wiring, optical fiber cable, or radio link connecting nodes). The protocol followed by this layer describes the signals (e.g., voltages, near-infrared signals, or radiowaves) sent between the nodes. Layer 2, the Data Link Layer, encapsulates the information received from the layer above into packets for transmission across the LAN or point-to-point link, and transfers them to Layer 1 for transmission. The protocol followed by Layer 2 describes the format of packets sent across a LAN or point-to-point link. It also describes functions such as media access control (determining when a node may transmit a packet on a LAN) and error checking of packets received over a LAN or point-to-point link. It is usually implemented in hardware.
Between the lower layers in the protocol stack and the Application Layer are intermediate layers that mediate between applications and the network interface.
These layers are usually implemented in software and incorporated in a computer’s operating system. Many intermediate level protocols are available, their complex-ity depending upon the scope and complexcomplex-ity of the networks they are designed to serve. Some functions of these intermediate layers will be discussed later in this chapter when the methods for linking multiple LANs are described.