communications network test and measurement handbook

Part Introduction to Network Technologies and Performance 1 Source: Communications Network Test and Measurement Handbook Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction to Network Technologies and Performancel Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 3 Chapter 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 interoperability 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 communications 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. 1 Source: Communications Network Test and Measurement Handbook Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 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, message 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 protocol 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 voltage 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, protocols 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 4 Introduction to Network Technologies and Performance Open Systems Interconnection (OSI) Modell Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. implemented in software, whereas the lower layers are implemented in a combination 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 specifications 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 Open Systems Interconnection (OSI) Model 5 Figure 1.1 Open Systems Interconnection (OSI) Modell Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. error-free transmission of a data parcel from one network link to the next. Using the automobile analogy, the Data Link layer might be compared to sensing changing con- ditions and modifying the inputs to the engine system to control it (for example, slowing the engine by limiting fuel and ignition). In most protocols, the Data Link layer (layer 2) is responsible for providing an error-free connection between network elements. This layer formats the data stream into groups of bytes called frames of data for transmission and adds framing information to be interpreted by the remote device to which the frames are sent. Data Link layer functions generally exchange acknowledgment frames with the peer processes (Data Link layer functions) of the device to which it is directly connected. This interaction confirms the receipt of data frames and requests retransmission if an error is detected. Another major function of this layer is flow control, a provision for pacing the rate of data transfer to prevent a fast sender from overrunning a slow receiver. 1.2.3 The Network layer (layer 3) The Network layer provides error-free transmission of a single data parcel end-to-end across multiple network links. Again with the automobile analogy, the Network layer might be compared to the operator’s subliminal steering, which keeps the car on the road, and negotiating turns at appropriate corners. Additionally, decisions to change speed and make detours to avoid traffic congestion and even emergency avoidance of accidents also equate to layer 3 functions. The driver controls these functions, but does so automatically without thinking consciously about them, and can deal simul- taneously with many other details that can be associated with higher-layer functions. In data communication, the Network layer, layer 3, is responsible for the switching and routing of information and for the establishment of logical associations between local and remote devices, the aggregate of which is referred to as the subnet. In some cases, this layer deals with communication over multiple paths to a specific destination. The Network layer also can deal with congestion through flow control and rerouting information around bottlenecked devices or links. Information perti- nent to layer 3 is appended to the frame from the Data Link layer. Once this addition is made, the result is a packet (named after a packet of mail that might be sent through a postal service). 1.2.4 The Transport layer (layer 4) The Transport layer is responsible for the end-to-end delivery of the entire message. With the automobile analogy, this layer might be compared to the plan that the driver executes in getting from the origin to the destination of the trip. Often this plan requires using a map and choosing the most appropriate path based on the time of day, the urgency of the arrival, and so forth. Transport layer (layer 4) responsibilities include the integrity of the data, the se- quencing of multiple packets, and the delivery of the entire message—not just to the appropriate machine but to the specific application on that machine for which the data is intended (i.e., port-to-port delivery). While the lower three layers tend to be technology-dependent, the Transport layer tends to be independent of the end users’ communications device technologies. This independence allows it to mediate 6 Introduction to Network Technologies and Performance Open Systems Interconnection (OSI) Modell Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. between the upper and lower layers, and to shield the upper layer functions from any involvement with the nuts and bolts of data transport. 1.2.5 The Session layer (layer 5) The Session layer is responsible for establishing, maintaining, and terminating ses- sions between users or applications (if they are peer-to-peer). This layer might be very loosely compared to traffic laws that establish right-of-way. The Session layer (layer 5) protocols establish conversations between different machines and manage applications on them with services of synchronization and mu- tual exclusion for processes that must run to completion without interruption. Pro- tocols at this layer are responsible for establishing the credentials of users (checking passwords, for example), and for ensuring a graceful close at the termination of the session. An example of a graceful close mechanism is one that guarantees that the user of an automatic teller machine actually receives the money withdrawn from his or her account before the session terminates. Another example is the behavior of a printer with a paper jam. The function that causes the printer to reprint the damaged page, rather than going on from the jam point, is a Session layer protocol. 1.2.6 The Presentation layer (layer 6) The Presentation layer ensures that the data is in a format acceptable to both com- municating parties. It creates host-neutral data representations and manages encryption and decryption processes. In the automobile analogy, functions at this layer can be compared to a system that mediates geographically localized differences between automobiles, such as speedometer calibration in miles per hour or kilometers per hour, or steering wheel placement on the right or left side. The Presentation layer (layer 6) is concerned with the syntax and semantics of the information that passes through it. At this layer, any changes in coding, formatting, or data structures are accomplished. Layer 6 is typically the layer used to accomplish encryption, if any, to prevent unauthorized access to the data being transmitted. 1.2.7 The Application layer (layer 7) The Application layer provides the user or using process with access to the network. In the automobile analogy, it is roughly comparable to the mission of the trip and to the interface between car and driver (speedometer, odometer, gearshift, etc.). The mission sets the context of operation, including the urgency and the conservative- ness or aggressiveness of the trip. This layer is concerned with network services for a specific application, such as file transfer between different systems, electronic mail, and network printing. 1.2.8 User data encapsulation by layer User data is formed and presented to the Application layer. From there it is passed down through the successively lower layers of the model to the Physical layer, which sends it across a link. At layers 7 through 2, information used by processes at each Open Systems Interconnection (OSI) Model 7 Open Systems Interconnection (OSI) Modell Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. layer is appended to the original message in a process called encapsulation. This information is added as headers at layers 7 through 2, and as a trailer at layer 2 (see Figure 1.2). When the encapsulated transmission reaches its destination, it is passed up through the layers in a reverse of the sending process. Each layer removes and processes the overhead bits (header and/or trailer) intended for it before passing the data parcel up to the next layer. This activity requires the precise exercise of a number of parameters and procedures, providing multiple opportunities for processing error. 8 Introduction to Network Technologies and Performance Figure 1.2 Encapsulation of data. Open Systems Interconnection (OSI) Modell Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 9 Chapter Data Communications Basics Marc Schwager Hewlett-Packard Australia Ltd., Victoria, Australia 2.1 Introduction The purpose of this chapter is to provide a basic understanding of the major compo- nents of a data communications network. This chapter focuses on the most common elements likely to be encountered in a data communications network. Voice networks, wireless networks, and proprietary networks such as those used in process control applications are not discussed. The treatment is necessarily brief; references listed at the end of the chapter for further information. 2.1.1 The network fabric The network fabric is the combination of devices, wires, computers, and software that interact to form a data communications network. There are many of these that are brought together to create the local area network (LAN) and wide area network (WAN) environments that are in common use. There are three interlinked con- cepts that this chapter addresses: the protocol stack (TCP/IP, SNA, etc.), network topologies (ring, star, etc.), and the interconnects. The latter are the devices that do most of the work in the network, such as routers, hubs, and switches. These three as- pects of networking will determine a large part of how network testing is approached. 2.1.2 A brief history of data networks Data networks evolved from three areas: mainframe communications, personal computer (PC) networks that share peripherals, and workstation networks that share data. The early data networks were built around point-to-point networks, that is, one mainframe was connected directly to another. IBM created protocols such as Remote Job Entry (RJE) to facilitate load sharing and job sharing between computers. The 2 Source: Communications Network Test and Measurement Handbook Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. minicomputer companies in the late 1970s and early 1980s expanded these capabilities considerably. With the widespread adoption of Ethernet and the proliferation of PCs, small networks emerged that enabled a workgroup to share expensive peripherals like laser printers. Engineering workstations were being developed that had in- tegral networking capabilities, which were used for data and task sharing. The end of the 1980s saw the widespread adoption of networking and the creation of internet- works. These large corporate, university, and government networks were essentially a consolidation and interconnection of the “islands” of networking that had evolved. These networks still carry many different protocols, and they connect many types of computer equipment. The network fabric must be extremely flexible and adapt- able to handle the task. This is one reason that there are so many different interconnects. It makes the job of managing today’s networks challenging, and to make things worse, traffic in a typical corporate network grew at around 40 percent per year in the 1990s. The great intermeshing of networks will continue through the foreseeable future, with the major focus on the consolidation of voice, data, and video over a worldwide, high-speed fiber infrastructure. 2.2 Protocols 2.2.1 Common protocol stacks Protocols are the language by which computers and other devices communicate on the network. A standard model, which takes a layered approach, has evolved to describe these protocols. Defined by the International Standards Organization, (ISO) it is called the Open Systems Interconnect (OSI) Reference Model. It has seven layers, each of which has a function to perform. A collection of these layers is called a protocol stack. Interconnects will base routing decisions on the lower layers. Some common protocol stacks are profiled here, with comments on their use. The OSI model. Table 2.1 shows the Open Systems Interconnect model. Note that functions such as error detection can occur in more than one layer of the protocol stack. While the OSI model covers seven layers in a complete implementation, there are many protocol stacks that are focused at the Network layer and below. This is the case in most of the following examples. X.25. Table 2.2 shows X.25, which is common in wide area networks. X.25 is a transport protocol stack, being defined only up through the Network layer. The use of hop- to-hop error recovery at both the Data Link layer and the Network layer makes X.25 a very robust protocol stack, and therefore a good choice when line quality is poor. Unfortunately this also makes it slow: X.25 can add 40 to 60 ms in traffic delay per network hop. Frame relay is preferable for connecting LANs over a wide area network. Frame relay. Like X.25, frame relay (described in Table 2.3) is a WAN transport protocol stack, being defined only up through the Network layer. The absence of hop-to- hop error recovery makes frame relay much faster than X.25. Error recovery is handled by the upper-layer protocols such as TCP/IP in a typical LAN environment. Due to its low latency, frame relay is often used for connecting LANs over a wide area network. Frame relay can deal gracefully with traffic bursts, and can specify quality 10 Introduction to Network Technologies and Performance Data Communications Basics Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. [...]... website Data Communications Basics 20 Introduction to Network Technologies and Performance 2.3.2 Bus The use of a “bus” created the first LAN networks Because any device on the network can talk, a method was developed to minimize collisions on the network The scheme employed on Ethernet networks is Carrier Sense Multiple Access with Collision Detection (CSMA/CD) A station will listen to the network to... analyzer is needed for testing Hook between the computer and the modem Noisy lines can cause problems A typical test is to generate some traffic and verify muxing/ demuxing properly by comparing input and output Testing protocols typically requires demuxing first Can be configured to loopback to check network continuity of the WAN or private side connection A good access point for testing Troubleshooting... capability resolves potential looping Bridged networks are very susceptible to broadcast storms These are hard to pin down, and can drastically reduce network performance If your network is growing, consider moving to routers which, while harder to configure provide more flexibility, security, and manageability the time to understand the protocol mechanism and errors There is a mechanism for removing... website Data Communications Basics Data Communications Basics 33 multimedia, to a desktop system Enterprise switches are generally focused on creating faster backbones Edge switches are used in the WAN, at the edge of the carrier networks, and central office or core switches are very large switches used to consolidate and transport traffic in the carrier networks ATM brings four main features to networking:... card, and a routing card Other common interfaces include Token-Ring, FDDI, ATM, and others Each of these cards will perform its associated functions of bridging, routing, switching, or repeating Modular hubs are industrial-grade units, and thus more expensive than rack -and- stack solutions for small networks Modular hubs normally have extensive SNMP-based network management systems for configuration and. .. control, priority control, error detection and correction, multiplexing data flows over one physical segment Network Controls the topology and access to the network This layer links logical (or network) addresses to physical addresses Data Link Detects and corrects errors in the received bit stream Physical addresses are in this domain Physical Transmits and receives the data Specifications deal with... given at the website Data Communications Basics Data Communications Basics 21 work, each router is connected to at least two other points This gives a measure of fault tolerance in case one path fails, as well as the opportunity to balance the network load 2.3.5 Virtual networks Virtual networks (Figure 2.8) have appeared relatively recently The physical topology of these networks is usually a hierarchical... Interconnects are the devices that comprise the network There are many categories, and the distinction between them becomes blurred as networking companies become more clever in their engineering and marketing Some of the major interconnects are profiled in this section The first section covers LAN devices and the second section covers WAN devices Figure 2.8 Virtual networks Downloaded from Digital Engineering... Companies All rights reserved Any use is subject to the Terms of Use as given at the website Data Communications Basics 22 Introduction to Network Technologies and Performance 2.4.1 LAN interconnects This section contains descriptions of and comments about devices commonly found on local area networks Tables 2.9 and 2.10 contain the following information on LAN interconnects: ■ Common name ■ Device function... application-to-terminal communications, while LU6 is for program-to-program communications PUs come in different types, including terminals (PU1), hosts (PU5), and a variety of others 2.2.2 Framing Data generally moves in frames, packets, or cells These packets are assigned address fields, which are used by various devices on the network for routing, bridging, and so on Let’s examine how the packets are formed and addressed . Part Introduction to Network Technologies and Performance 1 Source: Communications Network Test and Measurement Handbook Downloaded from Digital Engineering Library. understanding where and how a protocol fits into a network. 2. As a model for developing network architectures that are maximally functional and interoperable. 1 Source: Communications Network Test. Remote Job Entry (RJE) to facilitate load sharing and job sharing between computers. The 2 Source: Communications Network Test and Measurement Handbook Downloaded from Digital Engineering Library

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