TRANSMISSION FREQUENCIES

Transmission media make possible the transmission of the electronic signals from one computer to another. These electronic signals express data values in the form of binary (on/off ) impulses, which are the basis for all computer information (represented as 1s and 0s). These signals are transmitted between the devices on the network, using some form of transmission media (such as cables or radio) until they reach the desired destination computer. All signals transmitted between computers consist of some form of electromagnetic (EM) waveform, ranging from radio frequencies through microwaves and infrared light. Different media are used to transmit the signals, depending on the frequency of the EM wave form. Figure illustrates the range of electromagnetic waveforms (known as the electromagnetic spectrum) and their associated fre-quencies.

The electromagnetic spectrum consists of several categories of wave forms, including radio frequency waves, microwave transmissions, and infrared light. The frequency of a wave is dependent upon the number of waves or oscillations that occur during a period of time. An example that all people can relate to is the difference between a high-pitched sound, such as a whistle, and a low-pitch sound such as a fog horn. A high- pitched sound has a very high frequency; in other words, numerous cycles of oscillation (or waves) occur each second. Whereas, a low frequency sound, such as the fog horn, is based on relatively few cycles or waves per second (see Figure ). Although sound is not an example of electromagnetic energy (it's mechanical energy), the principles are similar.

Radio frequency waves are often used for LAN signaling. Radio fre- quencies can be transmitted across electrical cables(twisted-pair or coaxial) or by radio broadcast. Microwave transmissions can be used for tightly focused transmis- sions between two points. Microwaves are used to communicate between earth stations and satellites, for example, and they are also used for line-of-sight transmissions on the earth's surface. In addition, microwaves can be used in low-power forms to broadcast signals from a transmitter to many receivers. Cellular phone networks are examples of systems that use low-power microwave signals to broadcast signals. Infrared light is ideal for many types of network communications.

Infrared light can be transmitted across relatively short distances and can be either beamed between two points or broadcast from one point to many receivers. Infrared and higher frequencies of light also can be transmitted through ?ber-optic cables. A typical television remote control uses infrared transmission. The next sections examine the major factors you should consider when evaluating what type of transmission media should be implemented.

Connection-Oriented and Connectionless

Compare the implications of using connection-oriented communications with connectionless communications.
The OSI Network layer determines the route a packet will take as it passes through a series of different LANs from the source PC to the destination PC. The complexity and versatility of Network layer addressing gives rise to two different communication modes for passing messages across the network, both of which are recognized under OSI:

  Connection-oriented mode. Error correction and ?ow control

are provided at internal nodes along the message path.

  Connectionless mode. Internal nodes along the message path do

not participate in error correction and flow control.  To understand the distinction between connection-oriented and connectionless communications, you must consider an important distinction between the OSI model's Data Link and Network layers. In theory, the Data Link layer facilitates the transmission of data across a single link between two nodes. The Network layer describes the process of routing a packet through a series of nodes to a destination on another link on the network. An example of this latter scenario is a message passing from a PC on one LAN segment through a series of routers to a PC on a distant part of the network. The internal nodes forwarding the packet also forward other packets between other end nodes. In connection-oriented mode, the chain of links between the source and destination nodes forms a kind of logical pathway connection. The nodes forwarding the data packet can track which packet is part of which connection. This enables the internal nodes to provide flow control as the data moves along the path. For example, if an internal node determines that a link is malfunctioning, the node can send a notification message backward, through the path to the source computer. Furthermore, because the internal node distinguishes among individual, concurrent connections in which it participates, this node can transmit (or forward) a "stop sending" message for one of its
connections without stopping all communications through the node. Another feature of connection-oriented communication is that internal nodes provide error correction at each link in the chain. Therefore, if a node detects an error, it asks the preceding node to retransmit. Connectionless mode does not provide these elaborate internal control mechanisms; instead, connectionless mode relegates all error  correcting and retransmitting processes to the source and destination nodes. The end nodes acknowledge the receipt of packets and retransmit if necessary, but internal nodes do not participate in flow control and error correction (other than simply forwarding messages between the end nodes).
 The advantage of connectionless mode is that connectionless communications can be processed more quickly and more simply because the internal nodes only forward data and thus don't have to track connections or provide retransmission or flow control. The differences between connection-oriented and connectionless modes of communication may be easier to understand by analogy Imagine talking to someone and then having her reafirm that she understood what you have told her after each sentence. Connectionless mode is like having a conversation with someone, but the speaker just carries on and assumes that the listener under- stands. Connection-oriented is slower, yet more reliable. Connectionless is faster, but has less capability to correct errors (misunderstandings in the conversation example) as they occur. Connectionless mode does have its share of disadvantages, however, including the following:

  Messages sometimes get lost due to an over?owing buffer or a

failed link along the pathway.

  If a message gets lost, the sender doesn't receive noti?cation.

  Retransmission for error correction takes longer because a

faulty transmission can't be corrected across an internal link.

It is important to remember that the OSI model is not a set of rules for communication; the OSI model is a framework in which models of communication are explained. As such, individual imple-mentations of connectionless protocols can attenuate some of the preceding disadvantages. It is also important to remember that connection-oriented mode, although it places much more emphasis on monitoring errors and controlling traf?c, doesn't always work either. Ultimately, the choice of connection-oriented or connectionless communications mode depends on interoperability with other systems, the premium for speed, and the cost of components.

How Peer OSI Layers Communicate

Communication between OSI layers is both vertical within the OSI layers, and also horizontal between peer layers in another computer (see Figure). This is important to understand, because it affects how data is passed within a computer, as well as between two computers. When information is passed within the OSI model on a computer, each protocol layer adds its own information to the message being sent. This information takes the form of a header added to the beginning of the original message. The sending of a message always goes down the OSI stack, and hence headers are added from the top to the bottom (see Figure). When the message is received by the destination computer, eachlayer removes the header from its peer layer. Thus at each layer headers are removed (stripped ) by the receiving computer after the information in the header has been utilized. Stripped headers are removed in the reverse order in which they were added. That is, the last header added by the sending computer, is the ?rst one stripped off and read by the receiving computer. In summary, the information between the layers is passed along vertically. The information between computers is essentially horizontal, though, because each layer in one computer talks to its respective layer in the other computer.

THE OSI REFERENCE MODEL

Having a model in mind helps you understand how the pieces of the networking puzzle  together. The most commonly used model is the Open Systems Interconnection (OSI) reference model. The OSI model, first released in 1984 by the International Standards Organization (ISO), provides a useful structure for de?ning and describing the various processes underlying networking communications. The OSI model is a blueprint for vendors to follow when developing protocol implementations. The OSI model organizes communication protocols into seven levels. Each level addresses a narrow portion of the communication process. Figure 2.1 illustrates the levels of the OSI model.

Standards Organizations and the ISO

The development and implementation of de jure standards is regulated by standards organizations. For example, the CCITT (this is a French acronym that translates to the International Consultative Committee for Telegraphy and Telephony) and the Institute of Electrical and Electronic Engineers (IEEE), among other organizations, are responsible for several prominent network standards that support the International Standards Organization's objective of network interoperability. The International Standards Organization (ISO)-whose name is derived from the Greek pre?x iso, meaning "same"-is located in Geneva, Switzerland. ISO develops and publishes standards and coordinates the activities of all national standardization bodies. In  1977, the ISO initiated efforts to design a communication standard based on the open systems architecture theory from which computer networks would be designed. This model came to be known as the Open Systems Interconnection (OSI) model. This model has become an accepted framework for analyzing and developing networking components and functionality

STANDARDS

The network industry uses two types of standards: de facto standards and de jure standards. To understand the concept of open systems architecture, you must be familiar with the concepts of de facto and de jure standards. De facto standards arise through widespread commercial and educational use. These standards often are proprietary and usually remain unpublished and unavailable to outside vendors. Unpublished and unavailable standards are known as closed system standards. Published and accessible standards, on the other hand, are known as open system standards. Through the growing acceptance of the concept of interoperability, many closed, proprietary systems (such as IBM's Systems Network Architecture) have started to migrate toward open system standards. Certainly, de facto standards are not always closed system standards. Some examples of proprietary open system standards include Novell's NetWare network operating system and Microsoft's Windows. The second type of standards, de jure standards, are nonproprietary, which means that no single company creates them or owns the rights to them. De jure standards are developed with the intent of enhancing connectivity and interoperability by making speci?cations public so that independent manufacturers can build to such specifications.TCP/IP, "Transport Protocols," is an example of a nonproprietary de jure standard. Several permanent committees comprised of industry representatives develop de jure standards. Some examples of these committees are

the IEEE (Institute of Electrical and Electronic Engineers) and the IRTF (Internet Engineering Task Force). Although these committees are supported by manufacturer subscriptions, and in some cases government representatives, they are intended to represent the interests of the entire community and thus remain independent of any one

manufacturer's interests. Subscribing to de jure standards reduces the risk and cost of developing hardware and software for manufacturers. After a standard has been ?nalized, a component manufacturer sub- scribing to it can develop products with some confidence that the products will operate with components from other companies that also subscribe to the same standards.

An example of a de jure standard is the set of rules that guide how web pages are transferred between computers or how ?les are trans- ferred between systems. These de jure standards are created by the IRTF to facilitate communication between different systems. One problem of de jure standards, though, is the possibility of a vendor choosing to follow only part of a given standard. The frequent result is a product that claims to conform to the standard, but that in reality fails to operate with other products in the way one might believe or expect.

Directory Services

Directory services, also known as the x.500 standard, provide location information for different entities on the network. Their main function is to act as an information booth, directing resource requests on the network to the location of the resource. When a client is requesting to use a printer or to ?nd a server or even a speci?c application, the directory service tells the client where the resource is on the network and whether the resource is available (see Figure ). This is a service that more and more networking systems are moving towards. As networking systems have developed, they have begun to include this feature. This is similar to a large company having an information desk, whereas a small company probably would not. Examples of computer systems that use directory services include Novell NetWare 4.11, Banyan VINES, Microsoft Exchange Server, and the soon-to-be-released Windows NT 5.0.