Table Of ContentSource: Communications Network Test and Measurement Handbook
Part
1
Introduction to Network
Technologies and Performance
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Introduction to Network Technologies and Performancel
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Source: Communications Network Test and Measurement Handbook
Chapter
1
Open Systems
Interconnection (OSI) Model
Justin S. Morrill, Jr.
Hewlett-Packard Co., Colorado Springs, Colorado
Aprotocolis 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, openrefers 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.
3
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Open Systems Interconnection (OSI) Modell
4 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 5
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 Xand
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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Open Systems Interconnection (OSI) Modell
6 Introduction to Network Technologies and Performance
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 er-
ror-free connection between network elements. This layer formats the data stream
into groups of bytes called frames of data for transmission and adds framing infor-
mation 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 inter-
action 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 re-
quires 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-portdelivery). 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
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Open Systems Interconnection (OSI) Modell
Open Systems Interconnection (OSI)Model 7
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 en-
cryption and decryption processes. In the automobile analogy, functions at this layer
can be compared to a system that mediates geographically localized differences be-
tween 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 TheApplication 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
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Open Systems Interconnection (OSI) Modell
8 Introduction to Network Technologies and Performance
layer is appended to the original message in a process called encapsulation. This in-
formation 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 num-
ber of parameters and procedures, providing multiple opportunities for processing
error.
Figure 1.2 Encapsulation of data.
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Source: Communications Network Test and Measurement Handbook
Chapter
2
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 net-
works, 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 net-
work(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 Abrief history of data networks
Data networks evolved from three areas: mainframe communications, personal com-
puter (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
9
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Data Communications Basics
10 Introduction to Network Technologies and Performance
minicomputer companies in the late 1970s and early 1980s expanded these capabil-
ities considerably. With the widespread adoption of Ethernet and the proliferation of
PCs, small networks emerged that enabled a workgroup to share expensive periph-
erals 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 intercon-
nects. 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 de-
scribe 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 pro-
tocol stack.Interconnects will base routing decisions on the lower layers. Some com-
mon 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 trans-
port 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 net-
work 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 pro-
tocol 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
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