Networking Standards

Standards Organizations
In this chapter, we will first focus our attention on two national and two
international standards organizations. The national standards organizations
we will briefly discuss in this section are the American National Standards
Institute (ANSI) and the Institute of Electrical and Electronics Engineers
(IEEE). The work of both organizations has been a guiding force in the rapid
expansion in the use of local area networks due to a series of standards they
have developed. Due to the importance of the work of the IEEE in developing
LAN standards, we will examine those standards as a separate entity in the
next section in this chapter. In the international arena, we will discuss the
role of the International Telecommunications Union (ITU), formerly known
as the Consultative Committee for International Telephone and Telegraph
(CCITT), and the International Standards Organization (ISO), both of which
have developed numerous standards to facilitate the operation of local and
wide area networks.
Because of the importance of the ISO’s Open Systems Interconnection (OSI)
Reference Model and the IEEE’s 802 Committee lower layer standards, we
will examine each as a separate entity in this chapter. Because a series of
Internet standards define the manner by which the TCP/IP protocol suite
can transport data between LANs and WANs, we will also discuss what are
referred to as Requests For Comments (RFCs). Because we must understand
the OSI Reference Model before examining the effect of the efforts of the
IEEE and ANSI upon the lower layers of that model and the role of RFCs,
we will look at the OSI Reference Model before examining the role of other
standards.
National Standards Organizations
The two national standards organizations we will briefly discuss are the American
National Standards Institute and the Institute of Electrical and Electronics
Engineers. In the area of local area networking standards, both ANSI and the
IEEE work in conjunction with the ISO to standardize LAN technology.
The ISO delegated the standardization of local area networking technology
to ANSI. The American National Standards Institute, in turn, delegated
lower-speed LAN standards—initially defined as operating rates at and below
50 Mbps—to the IEEE. This resulted in ANSI’s developing standards for the
100-Mbps fiber distributed data interface (FDDI), while the IEEE developed
standards for Ethernet, Token-Ring, and other LANs. Because the IEEE developed
standards for 10-Mbps Ethernet, that organization was tasked with the
responsibility for modifications to that LAN technology. This resulted in the
IEEE becoming responsible for the standardization of high-speed Ethernet
to include isoENET, 100BASE-T, and 100VG-AnyLAN, the latter two representing
100-Mbps LAN operating rates. Another series of IEEE standards
beginning with the prefix of 1000 defines the operation of Gigabit Ethernet
over different types of copper and optical fiber media. In addition, when this
book revision occurred the IEEE was in the process of finalizing a standard for
10 Gbps Ethernet.
Once the IEEE develops and approves a standard, that standard is sent to
ANSI for review. If ANSI approves the standard, it is then sent to the ISO.
Then, the ISO solicits comments from all member countries to ensure that
the standard will work at the international level, resulting in an IEEE- or
ANSI-developed standard becoming an ISO standard.
ANSI
The principal standards-forming body in the United States is the American
National Standards Institute (ANSI). Located in New York City, this nonprofit,
nongovernmental organization was founded in 1918 and functions as the
representative of the United States to the ISO.
American National Standards Institute standards are developed through
the work of its approximately 300 Standards Committees, and from the
efforts of associated groups such as the Electronic Industry Association (EIA).
Recognizing the importance of the computer industry, ANSI established its
X3 Standards Committee in 1960. That committee consists of 25 technical
committees, each assigned to develop standards for a specific technical area.
One of those technical committees is the X3S3 committee, more formally
known as the Data Communications Technical Committee. This committee
was responsible for the ANSI X3T9.5 standard that governs FDDI operations,
and that is now recognized as the ISO 9314 standard.
IEEE
The Institute of Electrical and Electronics Engineers (IEEE) is a U.S.-based
engineering society that is very active in the development of data communications
standards. In fact, the most prominent developer of local area networking
standards is the IEEE, whose subcommittee 802 began its work in 1980 before
they had even established a viable market for the technology.
The IEEE Project 802 efforts are concentrated on the physical interface
between network devices and the procedures and functions required to
establish, maintain, and release connections among them. These procedures
include defining data formats, error control procedures, and other control
activities governing the flow of information. This focus of the IEEE actually
represents the lowest two layers of the ISO model, physical and link, which
are discussed later in this chapter.
International Standards Organizations
Two important international standards organizations are the International
Telecommunications Union (ITU), formerly known as the Consultative
Committee for International Telephone and Telegraph (CCITT), and the
International Standards Organization (ISO). The ITU can be considered a
governmental body, because it functions under the auspices of an agency of
the United Nations. Although the ISO is a nongovernmental agency, its work
in the field of data communications is well recognized.
ITU
The International Telecommunications Union (ITU) is a specialized agency of
the United Nations headquartered in Geneva, Switzerland. The ITU has direct
responsibility for developing data communications standards and consists
of 15 study groups, each with a specific area of responsibility. Although
the CCITT was renamed as the ITU in 1994, it periodically continues to be
recognized by its former mnemonic. Thus, the remainder of this book will refer
to this standards organization by its new set of commonly recognized initials.
The work of the ITU is performed on a four-year cycle known as a study
period. At the conclusion of each study period, a plenary session occurs.
During the plenary session, the work of the ITU during the previous four
years is reviewed, proposed recommendations are considered for adoption,
and items to be investigated during the next four-year cycle are considered.
The ITU’s eleventh plenary session met in 1996 and its twelfth session
occurred during 2000. Although approval of recommended standards is not
intended to be mandatory, ITU recommendations have the effect of law in
some Western European countries, and many of its recommendations have
been adopted by communications carriers and vendors in the United States.
Perhaps the best-known set of ITU recommendations is its V-series, which
describes the operation of many different modem features—for example, data
compression and transmission error detection and correction.
ISO
The International Standards Organization (ISO) is a nongovernmental entity
that has consultative status within the UN Economic and Social Council. The
goal of the ISO is to ‘‘promote the development of standards in the world with
a view to facilitating international exchange of goods and services.’’
The membership of the ISO consists of the national standards organizations
of most countries. There are approximately 100 countries currently
participating in its work.
Perhaps the most notable achievement of the ISO in the field of communications
is its development of the seven-layer Open Systems Interconnection
(OSI) Reference Model.
The ISO Reference Model
The International Standards Organization (ISO) established a framework for
standardizing communications systems called the Open Systems Interconnection
(OSI) ReferenceModel. The OSI architecture defines the communications
process as a set of seven layers, with specific functions isolated and associated
with each layer. Each layer, as illustrated in Figure 2.1, covers lower layer
processes, effectively isolating them from higher layer functions. In this way,
each layer performs a set of functions necessary to provide a set of services to
the layer above it.
Layer isolation permits the characteristics of a given layer to change without
impacting the remainder of the model, provided that the supporting services
remain the same. One major advantage of this layered approach is that users
can mix and match OSI-conforming communications products, and thus tailor
their communications systems to satisfy particular networking requirements.
The OSI Reference Model, while not completely viable with many current
network architectures, offers the potential to connect networks and networking
devices together to form integrated networks, while using equipment from
different vendors. This interconnectivity potentialwill be of substantial benefit
to both users and vendors. For users, interconnectivity will remove the
shackles that in many instances tie them to a particular vendor. For vendors,
the ability to easily interconnect their products will provide them with access
to a larger market. The importance of the OSI model is such that it was adopted
by the ITU as Recommendation X.200.
Layered Architecture
As previously discussed, the OSI Reference Model is based on the establishment
of a layered, or partitioned, architecture. This partitioning effort is
Application----------------Layer 7
Presentation---------------Layer 6
Session--------------------Layer 5
Transport-----------------Layer 4
Network------------------Layer 3
Data Link-----------------Layer 2
Physical------------------Layer 1

Figure 2.1 ISO Reference Model.
derived from the scientific process, in which complex problems are subdivided
into several simpler tasks.
As a result of the application of a partitioning approach to communications
network architecture, the communications process was subdivided into seven
distinct partitions, called layers. Each layer consists of a set of functions
designed to provide a defined series of services. For example, the functions
associated with the physical connection of equipment to a network are referred
to as the physical layer.
With the exception of layers 1 and 7, each layer is bounded by the layers
above and below it. Layer 1, the physical layer, is bound below by the
interconnecting medium over which transmission flows, while layer 7 is the
upper layer and has no upper boundary. Within each layer is a group of
functions that provide a set of defined services to the layer above it, resulting
in layer n using the services of layer n − 1. Thus, the design of a layered
architecture enables the characteristics of a particular layer to change without
affecting the rest of the system, assuming that the services provided by the
layer do not change.
OSI Layers
The best way to gain an understanding of the OSI layers is to examine
a network structure that illustrates the components of a typical wide area
network. Figure 2.2 illustrates a network structure that is typical only in the
sense that it will be used for a discussion of the components upon which
networks are constructed.
The circles in Figure 2.2 represent nodes, which are points where data
enters or exits a network or is switched between two networks connected by
one or more paths. Nodes are connected to other nodes via communications
cables or circuits and can be established on any type of communications
medium, such as cable, microwave, or radio.
From a physical perspective, a node can be based on any of several types of
computers, including a personal computer, minicomputer, mainframe computer,
or specialized computer, such as a front-end processor. Connections to
network nodes into a wide area network can occur via terminal devices, such
as PCs and fixed logic devices, directly connected to computers, terminals
connected to a node via one or more intermediate communications devices,
or paths linking one network to another network. In fact, a workstation on
an Ethernet local area network that provides access into a wide area network
can be considered a network node. In this situation, the workstation can be a
bridge, router, or gateway, and provides a connectivity mechanism between
other stations on the Ethernet local area network and the wide area network.
The routes between two nodes—such as C-E-A, C-D-A, C-A, and C-B-A, all
of which can be used to route data between nodes A and C—are information
paths. Due to the variability in the flow of information through a wide area
network, the shortest path between nodes may not be available for use,
or may be inefficient in comparison to other possible paths. A temporary
connection between two nodes that is based on such parameters as current
network activity is known as a logical connection. This logical connection
represents the use of physical facilities, including paths and temporary nodeswitching
capability.
The major functions of each of the seven OSI layers are described in the
following seven paragraphs.
Layer 1—The Physical Layer
At the lowest or most basic level, the physical layer (level 1) is a set of rules
that specifies the electrical and physical connection between devices. This
level specifies the cable connections and the electrical rules necessary to
transfer data between devices. Typically, the physical link corresponds to
previously established interface standards, such as the RS-232/V.24 interface.
This interface governs the attachment of data terminal equipment, such as the
serial port of personal computers, to data communications equipment, such
as modems.
Layer 2—The Data Link Layer
The next layer, which is known as the data link layer (level 2), denotes
how a device gains access to the medium specified in the physical layer.
It also defines data formats, including the framing of data within transmitted
messages, error control procedures, and other link control activities. Because
it defines data formats, including procedures to correct transmission errors,
this layer becomes responsible for the reliable delivery of information. An
example of a data link control protocol that can reside at this layer is the ITU’s
High-Level Data Link Control (HDLC).
Because the development of OSI layers was originally targeted toward wide
area networking, its applicability to local area networks required a degree of
modification. Under the IEEE 802 standards, the data link layer was initially
divided into two sublayers: logical link control (LLC) and media access control
(MAC). The LLC layer is responsible for generating and interpreting commands
that control the flow of data and perform recovery operations in the event of
errors. In comparison, the MAC layer is responsible for providing access to
the local area network, which enables a station on the network to transmit
information.
With the development of high-speed local area networks designed to operate
on a variety of different types of media, an additional degree of OSI layer
subdivision was required. First, the data link layer required the addition
of a reconciliation layer (RL) to reconcile a medium-independent interface
(MII) signal added to a version of high-speed Ethernet, commonly referred
to as Fast Ethernet. Next, the physical layer used for Fast Ethernet required
a subdivision into three sublayers. One sublayer, known as the physical
coding sublayer (PCS) performs data encoding.Aphysicalmedium attachment
sublayer (PMA) maps messages from the physical coding sublayer to the
transmission media, while a medium-dependent interface (MDI) specifies the
connector for the media used. Similarly, Gigabit Ethernet implements a gigabit
media-independent interface (GMII), which enables different encoding and
decoding methods to be supported that are used with different types of media.
Later in this chapter, we will examine the IEEE 802 subdivision of the data
link and physical layers, as well as the operation of each resulting sublayer.
Layer 3—The Network Layer
The network layer (level 3) is responsible for arranging a logical connection
between the source and destination nodes on the network. This responsibility
includes the selection and management of a route for the flow of information
between source and destination, based on the available data paths in the
network. Services provided by this layer are associated with the movement
of data packets through a network, including addressing, routing, switching,
sequencing, and flow control procedures. In a complex network, the source
and destination may not be directly connected by a single path, but instead
require a path that consists of many subpaths. Thus, routing data through the
network onto the correct paths is an important feature of this layer.
Several protocols have been defined for layer 3, including the ITU X.25
packet switching protocol and the ITU X.75 gateway protocol. X.25 governs
the flow of information through a packet network, while X.75 governs the flow
of information between packet networks. Other popular examples of layer 3
protocols include the Internet Protocol (IP) and Novell’s Internet Packet
Exchange (IPX), both of which represent layers in their respective protocol
suites that were defined before the ISO Reference Model was developed. In
an Ethernet environment the transport unit is a frame. As we will note later
in this book when we examine Ethernet frame formats in Chapter 4, the frame
on a local area network is used as the transport facility to deliver such layer 3
protocols as IP and IPX, which in turn represent the vehicles for delivering
higher-layer protocols in the IP and IPX protocol suites.
Layer 4—The Transport Layer
The transport layer (level 4) is responsible for guaranteeing that the transfer
of information occurs correctly after a route has been established through the
network by the network level protocol. Thus, the primary function of this layer
is to control the communications session between network nodes once a path
has been established by the network control layer. Error control, sequence
checking, and other end-to-end data reliability factors are the primary concern
of this layer, and they enable the transport layer to provide a reliable endto-
end data transfer capability. Examples of popular transport layer protocols
include the Transmission Control Protocol (TCP) and the User Datagram
Protocol (UDP), both of which are part of the TCP/IP protocol suite, and
Novell’s Sequence Packet Exchange (SPX).
Layer 5—The Session Layer
The session layer (level 5) provides a set of rules for establishing and terminating
data streams between nodes in a network. The services that this session
layer can provide include establishing and terminating node connections,
message flow control, dialogue control, and end-to-end data control.
Layer 6—The Presentation Layer
The presentation layer (level 6) services are concerned with data transformation,
formatting, and syntax. One of the primary functions performed by the
presentation layer is the conversion of transmitted data into a display format
appropriate for a receiving device. This can include any necessary conversion
between ASCII and EBCDIC codes. Data encryption/decryption and data compression/
decompression are additional examples of the data transformation
that can be handled by this layer.
Layer 7—The Application Layer
Finally, the application layer (level 7) acts as a window through which the
application gains access to all of the services provided by the model. Examples
of functions performed at this level include file transfers, resource sharing,
and database access. While the first four layers are fairly well defined, the
top three layers may vary considerably, depending on the network protocol
used. For example, the TCP/IP protocol, which predates the OSI Reference
Model, groups layer 5 through layer 7 functions into a single application
layer. In Chapter 5 when we examine Internet connectivity, we will also
examine the relationship of the TCP/IP protocol stack to the seven-layer OSI
Reference Model.
Figure 2.3 illustrates the OSI model in schematic format, showing the
various levels of the modelwith respect to a terminal device, such as a personal
computer accessing an application on a host computer system. Although
Figure 2.3 shows communications occurring via a modem connection on
a wide area network, the OSI model schematic is also applicable to local
area networks. Thus, the terminal shown in the figure could be replaced
by a workstation on an Ethernet network while the front-end processor
(FEP) would, via a connection to that network, become a participant on
that network.
Data Flow
As data flows within an ISO network, each layer appends appropriate heading
information to frames of information flowing within the network, while
removing the heading information added by a lower layer. In this manner,
layer n interacts with layer n − 1 as data flows through an ISO network.
Figure 2.4 illustrates the appending and removal of frame header information
as data flows through a network constructed according to the ISO
Reference Model. Because each higher level removes the header appended by
a lower level, the frame traversing the network arrives in its original form at
its destination.
As you will surmise from the previous illustrations, the ISO Reference
Model is designed to simplify the construction of data networks. This simplification
is due to the potential standardization of methods and procedures
to append appropriate heading information to frames flowing through a
network, permitting data to be routed to its appropriate destination following
a uniform procedure.