Basic Data Communication Concepts
Figure shows a model that constitutes the essential elements of data communication. Two nodes, or hosts, are connected by a communication channel. An interface connects each node with the channel. The channel carries signals that represent messages between the nodes. Protocols define the ground rules for the channel signals and for the messages.
Messages
The message is the primary purpose of the communication. It can take many forms. It may be data in the traditional sense of the word. It may also be a program or a file or a snippet of personal conversation or a request or status information or a stream of audio or video or some other agreed-upon purpose. We will assume that it is represented digitally, as a series of bits. Since data communication is predominantly serial, we usually describe the data as a byte stream. Regardless of form or content, the message is a communication between cooperating applications at each node. The meaning of the message is established by the protocols recognized by the cooperating applications.
As you may have noticed, one of the major limitations of the use of messages as a communication tool is that the message length may vary widely from application to application. Without some form of control, a streaming video download, for example, could tie up a communication channel indefinitely. This situation is obviously intolerable if there are other messages that need to share use of the channel.
Packets
To solve the related problems of channel availability and maximum utilization, there must be a way to break long messages into smaller units. These units are called packets. Packets can take turns using the channel, allowing sharing of the channel for different messages. Packets are used for most data communications. A packet consists of data of some kind encapsulated by information about the packet. A packet is equivalent to an envelope containing pages of data. Like envelopes, packets come in different shapes and sizes. A description of the packet, the designated receiver and source addresses, and information about the data enclosed is provided in a preamble or header, followed by the data. The amount of data depends on the type and length of the messages, the design of the packet, and the requirements of the channel. Some packets require a fixed amount of data, others allow a variable amount within some maximum limit. Some packet designs also include a trailer or footer at the end of the packet. The packet design used for a communication installation reflects the protocol suite in use.
The use of packets offers a number of important advantages in data communication:
- The use of packets simplifies operations and increases communication efficiency. It reduces communication overhead by making it possible to transmit a large block of data while requiring only a single block of overhead information to identify the destination and meaning of the enclosed data.
- It represents a reasonable unit for the routing of data. This factor is particularly important in wide area networks, where a packet of data may be passed through many different networks and communication channels before it reaches its destination.
- Packets offer an alternative to dedicating a channel for the entire length of a message. This increases utilization and availability of a channel by allowing packets from several sources to access and share a single channel.
- The use of packets presents a productive way to use a communication channel. A channel can be switched to route data packets to different destinations in such a way that each sender-receiver pair appears to have a channel to itself.
- The receiving computer is able to process a block of data all at once, instead of a character or a byte at a time. Further more, it is usually easier to organize the data, since there are fewer individual blocks of data to deal with.
It simplifies synchronization of the sending and receiving systems. Packets provide a clearly delineated burst of data, with an identifiable start and stop point.
General Channel Characteristics
The communication channel provides the path for the message between the two communicating nodes in the model. Although the model in Figure above represents the channel as a direct point-to-point connection between the nodes, this is not generally the case. In reality, the channel can take many different forms. In the simplest case, it might be a direct connection between nodes in a local area network. More typically, the communication channel is actually divided into segments, called links, with intermediate nodes between the links that forward packets from one link to the next. Data originates at one end point and passes through each link to reach the destination end point.
As an example, consider Figure 12.4. In this example, data (perhaps a Web request) originating from a home computer connects wirelessly through a router to a DSL modem. From there, the data passes through the DSL link to an Internet Service Provider, then through many additional connections to a computer somewhere on the Internet.
In other words, the communication channel between your Web browser and the Web server on the Internet may be divided into many links, each with its own characteristics. This is true in general of most communication channel connections. Conversely, there may be many nodes sharing the use of a single channel or channel link. Thus, a channel or channel link may be required to carry several messages from different sources and bound for different destinations simultaneously. The requirements for data communication must include the ability to share the channel elements among many different sender-receiver pairs and to direct messages to their correct nodes, wherever those nodes might be located.
One way to view the channel is to consider the connection between the end point sender-receiver pair as the communication channel for that pair.
Since the channel may be made up of multiple links, the interfaces at each end of the connection may differ from each other and the characteristics of the end-to-end channel may differ from, and depend upon, those of the individual links. For example, the computer initiating a message might be connected to a network using a telephone modem, which transmits messages one byte at a time using audio tones as a signaling method. The receiving computer might be connected to the network using Ethernet, which expects messages formatted as digital packets consisting of many bytes of data, together with additional bytes that define the specific characteristics of the particular packet. Again, there are protocols and standards that define the makeup of the packets. The network must be capable of converting the message from one format to another at the intermediate nodes when required. The points where conversion is required for the previous example are noted in Figure 12.4.
Our primary concerns for an end-to-end connection are the interface characteristics of the end points and the rate of speed with which data can be moved successfully through the channel, usually measured in bits per second and known as the bit rate or bandwidth of the overall channel. ‘‘Successfully’’ in this case means that any noise or errors incurred during the passage through the channel can be removed and that the message can be accurately recovered at the receiving end.
MEDIUM A communication channel medium can be either guided or unguided. Radio waves transmitted from an antenna are unguided. They may be received by any radio receiver tuned to the corresponding radio frequency within the range and directionality of the transmitting antenna. Unguided media include cellular phone, broadcast radio, microwave, wireless networking, infrared light, and satellite technologies. Laser signals that are not confined to an optical cable are also generally considered unguided, although the field of view is extremely narrow. Note in particular that unguided communication channels are inherently insecure, since they can be intercepted easily by anyone within the field of view of the channel. Wireless networking is particularly vulnerable to interception because the transmitting antenna is generally omnidirectional.
Guided media limit communications to a specific path constrained to a cable of some sort. Guided media can be either electrical or optical and include various forms of wire and fiber optic cables.
DATA TRANSMISSION DIRECTIONALITY Channels can also be characterized by the direction in which the messages can flow. A channel that carries messages in only one direction is known as a simplex channel. Television broadcasting stations use a simplex channel. Programs are sent from a transmitting antenna to television receivers, but the receivers do not respond with messages or data back to the broadcasting station. A channel may carry messages in both directions, but only one direction at a time. This channel is known as a half-duplex channel. If the computer at point B wants to send a message to point A, it must wait until point A has stopped transmitting to do so. Most walkie-talkies are half-duplex communication devices. Channels which carry signals simultaneously in both directions are called full-duplex channels. Traditional telephone lines are full-duplex channels. Both parties can speak simultaneously, and each can hear the other. Some channels are made up of separate lines for each direction. Some practitioners characterize these as full duplex; others refer to these as dual-simplex channels. The PCI-Express bus specification calls them lanes, a term that is likely to catch on within the network community.
NUMBER OF CONNECTIONS Like buses, a communication channel can be point-to-point or multipoint, although the choice is often predetermined by the nature of the medium. Wireless networking, for example, is, of necessity, multipoint, because there is no realistic technological way to limit the number of radio signals in a given space. Conversely, fiber optics are usually point-to-point because of the difficulty of tapping into a fiber optic cable. Note that even a point-to-point channel can be shared by packets arriving at its input node from different sources.
Some channel characteristics are determined innately by the medium. For example, unguided messaging must be carried by an analog signal: radio transmission is based intrinsically on sine waves, which are analog. Signaling is achieved by varying certain properties of the radio wave at the transmitter and detecting the variations at the receiver. This process is called modulation and demodulation. (A modem works on the same principle.) The signals in guided media may be either analog or digital, although digital is usually preferred because of its better immunity to noise and the ease with which the medium can be shared by multiple messages.Audio and video are analog in nature, but are converted to digital and processed digitally in the computer.
Today, the most common end-node interface to a channel is a local area network connection, usually either wired or wireless Ethernet. Nonetheless, there are other possible interfaces to consider: Bluetooth, WiMax, DSL or cable link, various forms of cell phone technology, older types of network connections, and, to a more limited extent, telephone modem. Each technology has its own requirements.Regardless of the characteristics of the end-to-end communication channel and of its links, we must re-emphasize the fact that the message must ultimately arrive at its destination node in a form expected and recognized by the application receiving it.
Packet Routing
In the previous section, you saw that the typical communication channel is made up of a series of intermediate nodes, connected together by links. Packets are passed along the links from node to node. We next consider how the path is selected.
Figure 12.5 illustrates a simplified version of an end-to-end channel with some of its intermediate nodes. In some cases, the movement of data from node to node is obvious: there is only a single path. In many cases, however, there may be several choices. Figure 12.5 shows two possible channel paths out of many between end nodes A and B. Overall, in a large interconnection of networks, a so-called internet (with a small i), there may be thousands of possible paths connecting end nodes A and B.
There are two basic techniques for selecting the path through a channel: circuit switching and packet switching. A third technique, virtual circuit switching, is an important alternative to ordinary packet switching that also operates on packets.
Traditional telephony uses circuit switching. Circuit switching dedicates a path for the exclusive use of the sender-receiver pair for the entire length of time of the connection. The previous discussion of POTS in Section 12.2 was an example of circuit switching. The telephone circuits are dedicated to the individual lines for the length of the phone call. Circuit switching is inefficient and is rarely used today, even for telephony.
A virtual circuit is a channel path that is set up when a connection is established for communication between two end nodes, and maintained until the connection is closed. Data is sent through the channel in packets; each packet follows the same channel links. However, the links and intermediate nodes are shared with other connections, making the use of the channel more effective. Figure 12.6 shows the use of two virtual circuits, one connecting end nodes A and B, another connecting end nodes C and D. These two circuits share intermediate nodes k, n, and p, as well as the path between n and p. The use of virtual circuits simplifies the routing of packets and also assures that packets will arrive in the correct order, since all packets follow the same path. However, congestion at an intermediate mode or through an intermediate channel segment that is used by several different virtual circuits can affect the overall performance of the network.
Some network protocols use virtual circuit technology as the basis for packet flow. ATM (asynchronous transfer method, not the bank machine!) is one example. ATM uses very small packets (53 bytes) and careful path selection to control traffic. The fact that packets always arrive in correct order makes ATM effective for streaming data, such as video. The use of extremely small packets minimizes time delay through the ATM network, assuring that video will traverse the network in a timely and consistent fashion.
Ordinary packet switching, usually called datagram switching, assumes that each packet is routed from node to node independently, based on such criteria as the shortest path to the packet’s destination and traffic conditions, At each intermediate node, the next link is determined by the node’s switch or router at the time the packet arrives. TCP/IP uses datagram switching exclusively for all of its routing decisions.
Now consider the most common network scenario, illustrated in Figure 12.7. (network of networks). In this scenario, each end node is linked to an intermediate node that is part of a network, most commonly a local area network.The intermediate links connect nodes belonging to various networks together. A component at each intermediate node routes the packet to the next appropriate node. It also converts the data format of the packet to the format required for the next link, if necessary. The component may be a computer programmed to do routing, but it’s more likely to be a router or a gateway. Routers and gateways are specialized devices used to interconnect networks and pass packets from one network to the other. Depending on the network protocols in use, either ordinary packet switching or virtual circuit switching will be used to guide the decisions made at each router or gateway as the packet is forwarded from node to node through the system. This same explanation also describes the functioning of the Internet (with a capital I).
As you’ve just seen, routers and gateways are used to set the path that each packet takes to move through the channel. A simplified diagram of a router is shown in Figure 12.8. The router consists of one or more input ports, one or more output ports, a switch mechanism, and a processor with memory. The input ports and output ports are connected to links.
Routing protocols are sent to the router processor and stored, using packets with control information. The basic operation of a router is simple. When a packet arrives at an input port, the processor makes a decision on where the packet is to be directed and sets the switch to direct the packet to the correct output port. Routers are used wherever the incoming networks and outgoing networks operate on the same set of network protocols, although the physical characteristics of the links might be different. For example, a router could be used to switch packets between wireless and wired Ethernet networks.
Gateways operate similarly, but are intended for use when two dissimilar networks are connected together. The router operation is the same; the major difference is that the gateway is able to convert the packet headers that arrive at the input ports to meet the requirements of the different types of networks at the output ports. Traditionally, gateways have been thought of as complex routing devices that converted (in both directions) between TCP/IP networks and the older network protocols that were common on large mainframe systems. Since most modern mainframes also operate predominantly using the TCP/IP protocols, the use of this type of gateway is now relatively rare. Gateways are sometimes used to interconnect TCP/IP networks with Frame Relay network links that are supplied by some vendors for connection to computers beyond the local area. Similarly, although we rarely think about DSL and cable modems as routing equipment, it is worth noting that they do fit the technical definition of a gateway.
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