Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

cylindrical cable with a central conductor surrounded by an insulator that, in turn,is surrounded by a tubular grounded shield conductor. Coaxial cable and copperwiring carry electrical signals. Optical fiber cables use glass or plastic fibers to carrylight signals produced by lasers or light-emitting diodes. Optical fiber cables havethe advantage that they are not affected by electrical interference and therefore usuallyhave lower error rates than cables carrying electrical signals. There are also layersof software between the application program (application) with which the userinteracts and the hardware of the communications link.The data transfer rate of a computer network is usually described in megabitsper second (106 bps = 1 Mbps) or gigabits per second (10 9 bps = 1 Gbps). Theseunits should not be confused with megabytes per second (MBps) and gigabytes persecond (GBps), commonly used to specifY the data transfer rates of computer componentssuch as magnetic and optical disks. (Recall that a byte consists of eightbits.) A further complication is that kilobytes, megabytes, and gigabytes, when usedto describe the capacity of computer memory or a storage device, are defined inpowers of two: 1 kB = 2 10 bytes = 1,024 bytes, 1 MB =2 20 bytes = 1,048,576 bytes,and 1 GB = 2 30 bytes = 1,073,741,824 bytes (see Table 4.3 in Chapter 4). The maximaldata transfer rate is often called the bandwidth, a term originally used todescribe the data transfer rates of analog communications channels. An actual networkmay not achieve its full nominal bandwidth because of overhead or inefficienciesin its implementation. The term throughput is often used to describe themaximal data transfer rate that is actually achieved.In most networks, multiple computers share communication pathways. Mostnetwork protocols facilitate this sharing by dividing the information to be transmittedinto packets. Some protocols permit packets of variable size, whereas otherspermit only packets of a fixed size. Packets may be called frames, datagrams, or cells.Each packet contains information identifYing its destination. In most protocols, apacket also identifies the sender and contains information used to determinewhether the packet's contents were garbled during transfer. The computer at thefinal destination reassembles the information from the packets and may requestretransmission of lost packets or packets with errors.Large networks often employ switching devices to forward packets betweennetwork segments or even between entire networks. Each device on a network,whether a computer or switching device, is called a node, and the communicationspathways between them are called links. Each computer is connected to a networkby a network adapter, commonly called a network interface card, installed on theinput/output (I/O) bus of the computer (see Chapter 4). Each interface between anode and a network is identified by a unique number called a network address. Computersusually have only a single interface, but a switching device connecting two ormore networks may have an address on each network.On many small LANs today, all computers are directly connected-that is, allpackets reach every computer. However, on a larger network, every packet reachingevery computer would be likely to cause unacceptable network congestion. For thisreason, most networks larger than a small LAN, and even some small LANs, employpacket switching. The packets are sent over the network. Devices called bridges,switches, or routers (discussed later) store the packets, read the destination addresses,and send them on toward their destinations (a method called "store and forward"). Insome very large packet-switched networks, individual packets from one computer toanother may follow different paths through the network and can arrive out of order.Network protocols describe the methods by which communication occurs over anetwork. Both hardware and software must comply with established communica-