Communications Network Test and Measurement Handbook.pdf

Source: Communications Network Test and Measurement Handbook

Introduction to Network

Technologies and Performance

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Part

Introduction to Network Technologies and Performancel

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Source: Communications Network Test and Measurement Handbook

Open Systems

Interconnection (OSI) Model

Justin S. Morrill, Jr.

Hewlett-Packard Co., Colorado Springs, Colorado

A protocol is an agreed-upon set of rules and procedures that describe how multiple

entities interact. A simple example of a protocol in everyday life is the motoring rule

specifying that the vehicle to the right at an intersection has the right-of-way, other

things being equal. If this traffic protocol is violated, the result might be a serious

problem.

When the entities are network devices, protocols are necessary for interaction to

happen at all. If two devices follow different protocols, their communication will be

no more successful than a conversation between a person speaking French and a

person speaking Chinese. As there is more and more essential data traffic over a wide

variety of networks, the ability to guarantee protocol interoperability has become in-

creasingly vital. A number of standards have been developed to make that possible.

Among these standards, one has been designed to facilitate complete interoperabil-

ity across the entire range of network functions: the Open Systems Interconnection

(OSI) Reference Model, published by the International Standards Organization (ISO).

In computing and communications, open refers to a nonproprietary standard. An

open system is one in which systems from different manufacturers can interact

without changing their underlying hardware or software. The OSI model is such a

standard and is a useful framework for describing protocols. It is not a protocol itself,

but a model for understanding and defining the essential processes of a data com-

munications architecture.

Since its conception, the OSI model has become a vital tool in two ways:

1. As a point of reference for comparing different systems or understanding where

and how a protocol fits into a network.

2. As a model for developing network architectures that are maximally functional

and interoperable.

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Chapter

Open Systems Interconnection (OSI) Modell

Introduction to Network Technologies and Performance

1.1 Data Communications Protocols

In data communications, all interaction between devices is specified by protocols.

These protocols are an agreement between sender and receiver defining conven-

tions such as:

■ When a device may transmit.

■ The order of an exchange.

■ What kind of information must be included at any given point in the transmission

(such as which sections of a data package contain addressing, error control, mes-

sage data, etc.,) or which wire is reserved for which type of information, as in the

interface described below.

■ The expected format of the data (such as what is meant by a given sequence of bits).

■ The structure of the signal (such as what pattern of voltages represents a bit).

■ The timing of the transmission (for example, the receiving device must know at

which points to sample the signal in order to correctly separate the bits).

The EIA 232 (also known as RS-232) physical connection, commonly found on the

back of data terminals and personal computers, is specified by a protocol. This pro-

tocol is defined by the Electrical Industries Association (EIA), a standards-setting

organization that assigns, numbers, and publishes the standards for manufacturers.

The protocol includes the pin assignments for each signal and the loading and volt-

age levels that are acceptable. When a data communications connection fails, this

protocol is usually the first to be analyzed for violations or problems that may impair

the link operation.

As data communications have evolved, many manufacturers have decided to com-

ply with standard protocols in order to ensure that their equipment will interoperate

with that of other vendors. On the other hand, there are still proprietary protocols

used that limit interoperability to devices from the same vendor. In either case, pro-

tocols provide the descriptions, specifications, and often the state tables that define

the procedural interactions that allow devices to communicate properly.

1.1.1 Layered protocols

Because of the complexity of the systems that they define, data communications

protocols are often broken down into layers, also called levels (so called because

they are schematically stacked on top of one another in order of use). The functions

at each layer are autonomous and encapsulated so that other layers do not have to

deal with extraneous details, but can concentrate on their own tasks. Encapsulation

also provides a degree of modularity so that protocols at the same layer can be in-

terchanged with minimum impact on the surrounding layers.

1.2 The OSI Reference Model

The OSI model, shown in Figure 1.1, consists of seven layers: Physical, Data Link,

Network, Transport, Session, Presentation, and Application. The upper layers are

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Open Systems Interconnection (OSI) Modell

Open Systems Interconnection (OSI) Model

Figure 1.1

implemented in software, whereas the lower layers are implemented in a combina-

tion of software and hardware. Network test and measurement is concerned primar-

ily with the functions of the lower layers and not with the content of the message,

but with how well it is delivered.

Note: The layers of the OSI model may not be distinct in a specific protocol; in the

TCP/IP protocol suite, for example, the popular File Transfer Protocol (FTP) includes

functions at the Session, Presentation, and Application layers of the OSI model. Rather,

the OSI model represents a theoretical superset of what is generally found in practice.

1.2.1 The Physical layer (layer 1)

The Physical layer in a data communication protocol (also known as layer one or

level one) deals with the actual transmission of bits over a communication link. A

loose analogy for the physical layer is the function of the internal combustion engine

and the resulting source of mechanical motion in an automobile. The engine system

performs on its own as long as its lubrication, ignition, cooling, fuel, and oxygen sup-

ply elements are functioning properly, and as long as the operator avoids actions that

would damage the engine.

Protocols at layer one define the type of cable used to connect devices, the voltage

levels used to represent the bits, the timing of the bits, the specific pin assignments

for the connection, how the connection is established, whether the signal is electri-

cally balanced or is single-ended, and so on. The specifications of EIA 232 in North

America, or its V.24 European equivalent, are examples of Physical layer protocols.

Note: Numbering of protocols is done by the various standards bodies. The X and

V series are defined by the International Telecommunications Union (ITU) in Eu-

rope; the EIA standards are published by the Electrical Industry Association in the

United States. Other examples of Physical layer standards are the X.21 interface,

EIA 449 interface, V.35 modem, 10Base-T Ethernet LAN, and Fiber Distributed Data

Interface (FDDI) LAN.

The Physical layer elements interoperate with the media of connection and with

the next layer of abstraction in the protocol (layer 2, the Data Link layer). Its speci-

fications are electrical and mechanical in nature.

1.2.2 The Data Link layer (layer 2)

The Data Link layer provides error handling (usually in the form of error detection

and retransmission) and flow control from one network node to the next. It provides

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