OSI Reference Model

The OSI Reference Model

In 1983, the International Standards Organization (ISO) created the OSI, or X.200, model. It is a multilayered model for facilitating the transfer of information on a network. The OSI model is made up of seven layers, with each layer providing a distinct network service. By segmenting the tasks that each layer performs, it is possible to change one of the layers with little or no impact on the others. For example, you can now change your network configuration without having to change your application or your presentation layer. The basic OSI model is depicted in Figure 2.1.

The OSI model was specifically made for connecting open systems. These systems are designed to be open for communication with almost any other system. The model was made to break down each functional layer so that overall design complexity could be lessened. The model was constructed with several precepts in mind: 1) Each layer performs a separate function; 2) The model and its levels should be internationally portable; and 3) The number of layers should be architecturally needed, but not unwieldy.

Each layer of the model has a distinct function and purpose:

·         Application layer--Provides a means for the user to access information on the network through an application. This layer is the main interface for the user to interact with the application and therefore the network. Examples include file transfer (FTP), DNS, the virtual terminal (Telnet), and electronic mail (SMTP).

·         Presentation layer--Manages the presentation of the information in an ordered and meaningful manner. This layer's primary function is the syntax and semantics of the data transmission. It converts local host computer data representations into a standard network format for transmission on the network. On the receiving side, it changes the network format into the appropriate host computer's format so that data can be utilized independent of the host computer. ASCII and EBCDIC conversions, cryptography, and the like are handled here.

·         Session layer--Coordinates dialogue/session/connection between devices over the network. This layer manages communications between connected sessions. Examples of this layer are token management (the session layer manages who has the token) and network time synchronization.

·         Transport layer--Responsible for the reliable transmission of data and service specification between hosts. The major responsibility of this layer is data integrity--that data transmitted between hosts is reliable and timely. Upper layer datagram’s are broken down into network-sized datagram’s if needed and then implemented using the appropriate transmission control. The transport layer creates one or more than one network connection, depending on conditions. This layer also handles what type of connection will be created. Two major transport protocols are the TCP (Transmission Control Protocol) and the UDP (User Datagram Protocol). IP (Internet Protocol) is a good example of a network layer interface.

·         Network layer--Responsible for the routing of data (packets) to a system on the network; handles the addressing and delivery of data. This layer provides for congestion control, accounting information for the network, routing, addressing, and several other functions.

·         Data link layer--Provides for the reliable delivery of data across a physical network. This layer guarantees that the information has been delivered, but not that it has been routed or accepted. This layer deals with issues such as flow regulation, error detection and control, and frames. This layer has the important task of creating and managing what frames are sent out on the network. The network data frame, or packet, is made up of checksum, source address, destination address, and the data itself. The largest packet size that can be sent defines the maximum transmission unit (MTU).

·         Physical layer--Handles the bit-level electrical/light communication across the network channel. The major concern at this level is what physical access method to use. The physical layer deals with four very important characteristics of the network: mechanical, electrical, functional, and procedural. It also defines the hardware characteristics needed to transmit the data (voltage/current levels, signal strength, connector, and media). Basically, this layer ensures that a bit sent on one side of the network is received correctly on the other side.

Data travels from the application layer of the sender, down through the levels, across the nodes of the network service, and up through the levels of the receiver. Not all of the levels for all types of data are needed--certain transmissions might not be valid at a certain level of the model. A sender-receiver OSI example is shown in Figure 2.2.

To keep track of the transmission, each layer "wraps" the preceding layer's data and header with its own header. A small chunk of data will be transmitted with multiple layers attached to it. On the receiving end, each layer strips off the header that corresponds to its respective level. Figure 2.3 illustrates how the data is wrapped by the OSI layers.

The OSI model should be used as a guide for how data is transmitted over the network. It is an abstract representation of the data pathway and should be treated as such.

POTS and the Use of SLIP or PPP

In any discussion of POTS, it is necessary to talk about the Serial Line Internet Protocol (SLIP) and the Point-to-Point Protocol (PPP). These two protocols are probably the ones most widely used on POTS lines connecting home/business users and the Internet. Also, as more and more enterprise networks are being built on TCP/IP-based platforms, SLIP and PPP protocols will provide the capabilities of POTS interconnection between LANs on these types of networks. Let us look briefly at both.

SLIP: --This is an older protocol that was developed to pass TCP/IP data transmission over serial lines. Because TCP/IP networks have become more important in recent years, it is important to know about this protocol. It was originally developed to be used over serial lines, and it provides a TCP/IP connection over POTS circuits. The SLIP protocol is not an Internet standard, and there are many different versions of SLIP floating around. However, it is capable of framing and transmitting IP datagram’s on serial connections. Because it is not a standard protocol, SLIP has no maximum packet size specified. Therefore, any size can be used as long as both ends of the connection are using compatible packet sizes.

PPP: --The PPP protocol is a standard protocol for use over serial line connections. The difference between PPP and SLIP is the fact that PPP is based on a standard developed by the ISO called High-Level Data Link Control (HDLC). HDLC is very common and has been incorporated into X.25, Frame Relay, and ISDN. Hence, PPP is more common and more widely accepted than SLIP. Once again, PPP provides a TCP/IP connection over POTS circuits just as SLIP does. The difference is the fact that PPP is a standard protocol and therefore more likely to be used.

What Is HDSL?

HDSL is mainly aimed at a better and more efficient way of transmitting T-1/E-1 over traditional copper lines (using no repeaters and being more efficient). While ADSL uses one wire to transmit both upstream and downstream, HDSL implements a second wire to transmit data in both directions (see Figure 14.4). HDSL transmits at speeds of 1.544Mbps to more than 2Mbps. typical applications include, but are not limited to, Internet service providers, private networks, PBX networks, and more.

Multiplexing

As the T-1 became available commercially, a large number of companies began producing hardware to maximize the benefits of the carrier system. Because the T-1 is made up of 24 separate channels, configuration of the system is very flexible. Voice, data, video, and other signals can all share the same T-carrier. Allowing all these signals to use the same transmission carrier is called multiplexing.

A T-1 multiplexer can simply manage the data input sources, or it can combine channels for high bandwidth applications. Today, for example, it is common for the entire T-1 to be used for a single 1.536Mbps data application.

Figure 16.6 shows a typical multiplexer installation with a T-1. Note that the analog-to-digital conversion of the voice lines can occur in either the PBX or the multiplexer, depending on the exact hardware installed.

Time Division Multiplexing

The T-carrier system uses time-division multiplexing (TDM). As the name implies, TDM uses units of time to divide the signals from each source. These units of time, or timeslots, are approximately ¹/8000th of a second, which corresponds to the sampling rate of an analog voice call.

On a T-1, there are 24 carrier channels, each transmitting 8,000 timeslots/sec. The multiplexer is responsible for interleaving these timeslots so that the receiving device will be able to properly route the contents of each timeslot.

If four devices are all sending data through a TDM, the data is combined sequentially and sent down the T-carrier. At the receiving end, the order tells the de-multiplexer which device should receive the data. If a specific device has no data to send in a specific timeslot, then the multiplexer adds a null carrier to keep the ordering consistent.

Because TDM allows some timeslots to go unused, it is not as efficient as it might be. A newer multiplexing technology, called Statistical Time Division Multiplexing (STDM), is being developed to fix this inefficiency.

Short topics

Asynchronous communications: --A type of data transmission in which each character transmitted (8 bits) is framed by a start and stop bit. These two control bits delineate the beginning and end of a character. Though there is more flexibility with asynchronous transmission, it is much less efficient because the addition of the control bits increases the packet size by 25percent.

ATM (Asynchronous Transfer Mode):--A high-speed connection-oriented switching technology that uses 53-byte cells (packets) to simultaneously transmit different types of data, including video and voice. ATM is an attractive technology because it provides dedicated bandwidth at speeds ranging from 25Mbps to 655Mbps.

Backbone: --A network that interconnects individual LANs and that typically has a higher capacity than the LANs being connected. One exception is a T-1 backbone connecting a WAN connecting two 100Mbps Ethernet LANs at either end of the backbone. In this case, the LANs have a much higher capacity than the backbone.

Baseband: --A type of transmission that uses digital signals to move data. Because the signal is digital, the entire bandwidth of the cable is used.

BISDN (Broadband ISDN):--The next generation of ISDN service. BISDN is a fiber-optic-based service using asynchronous transfer mode (ATM) over SONET-based transmission circuits. The service is designed to handle high-bandwidth applications, such as video, at rates of 155Mbps, 622Mbps, and higher.

Broadband: --A type of transmission is using coaxial cable and analog or radio-frequency signals. Broadband uses a frequency band that is divided into several narrower bands, so different kinds of transmission can be transmitted at the same time.

Circuit switching: --A method of transmission in which a fixed path is established between the nodes communicating. This fixed path permits exclusive use of the circuit between the nodes until the connection is dropped. The public telephone network uses circuit switching.

CSMA/CD (Carrier Sense, Multiple Accesses with Collision Detection):--The medium access method used in Ethernet to avoid having more than one host transmitting on a LAN segment at a time. The transmitting host first listens for traffic on the cable and then transmits, if no traffic is detected. If two hosts transmit at the same time, a collision occurs. Each host then waits for a random length of time before listening and transmitting again.

Data link layer: --Layer 2 of the seven-layer OSI model. The data link layer is concerned with managing network access, for example, performing collision sensing and network control. Also, if the data link layer detects an error, it arranges to have the sending computer resend the corrupt packet.

Frequency division multiplexing (FDM):--The technique of dividing a specific frequency range into smaller parts, with each part maintaining enough bandwidth to carry one channel.

Half-duplex: --A method of two-way transmission, but data can only travel in one direction at a time. Contrast to full-duplex.

Full-duplex: --The capability of having two-way data transmission in both directions (send and receive) simultaneously. Contrast to half-duplex.

HDLC (High-Level Data Link Control):--The most widely used synchronous data link protocol in existence. It supports half-duplex and full-duplex transmission, point-to-point configurations, and switched or non-switched channels.

Multiplexer: --A device used to combine data transmitted from many low-to-medium speed devices onto one or more high-speed paths for retransmission. There are various techniques for achieving this, such as time division, frequency division, statistical time division, and wavelength division multiplexing. A multiplexer is sometimes called a concen- trator.

OSI model: --A concept developed by ISO and CCITT used to develop standards for data networking that promote multivendor equipment interoperability. The OSI model is separated into seven layers that relate to the interconnection of computer systems. See also application layer, presentation layer, session layer, transport layer, network layer, data link layer, and physical layer.

Packet-switched network: --A networking technique where data is broken into small packets and then transmitted to other networks over a WAN to computers configured as packet switches where the data is then reassembled. The packets get routed and rerouted, depending on the size of the network or the distance the packets travel to their destination

Physical layer: --Layer 1 of the seven-layer OSI model, which specifies the physical medium of a network. It is the wire on which data is transmitted and it is the connectors, hubs, and repeaters that comprise the network. Some refer to the physical layer as the hardware layer.

PPP (Point-to-Point Protocol):--A point-to-point circuit is a network configuration where a connection exists only between two points. PPP is the protocol for transmitting routing information over synchronous or asynchronous point-to-point circuits. The routing information allows different vendor's equipment to interoperate over point-to-point circuits.

Presentation layer: --Layer 6 of the seven-layer OSI model. The presentation layer makes sure that data sent to the application layer is in the correct format. If some conversion were required between different data types, it would take place at this layer. Translation and byte reordering is sometimes necessary when different computers (for example, IBM, Apple, NeXT) want to share information.

SDSL (Single line Digital Subscriber Line):--HDSL over a single telephone line. This name has not been set by any standards group, and may not stick. SDSL operates over POTS and would be suitable for symmetric services to the premises of individual customers.

Session layer: --Layer 5 of the seven-layer OSI model. The session layer defines the session type between two computers and controls the dialogue between the applications on those two computers. For example, when a user accesses another computer, a session that allows computer applications to inform each other of any problems is created and controlled by Layer 5.

SLIP (Serial Line Internet Protocol):--An Internet protocol used to run IP over serial lines, such as telephone circuits, and connecting two computers. Though similar to PPP, SLIP supports only IP and is not as efficient as PPP.