What Is an Internetwork?
An internetwork is a collection of individual networks, connected by intermediate networking devices,
thatfunctionsasasinglelargenetwork.Internetworkingreferstotheindustry,products,andprocedures
thatmeetthechallengeofcreatingandadministeringinternetworks.Figure1-1illustratessomedifferent
kinds of network technologies that can be interconnected by routers and other networking devices to
create an internetwork.
History of Internetworking
The first networks were time-sharing networks that used mainframes and attached terminals. Such
environments were implemented by both IBM’s Systems Network Architecture (SNA) and Digital’s
network architecture.
Local-area networks (LANs) evolved around the PC revolution. LANs enabled multiple users in a
relatively small geographical area to exchange files and messages, as well as access shared resources
such as file servers and printers.
Wide-area networks (WANs) interconnect LANs with geographically dispersed users to create
connectivity. Some of the technologies used for connecting LANs include T1, T3, ATM, ISDN, ADSL,
Frame Relay, radio links, and others. New methods of connecting dispersed LANs are appearing
everyday.
Today, high-speed LANs and switched internetworks are becoming widely used, largely because they
operate at very high speeds and support such high-bandwidth applications as multimedia and
videoconferencing.
Internetworking evolved as a solution to three key problems: isolated LANs, duplication
of resources, and a lack of network management. Isolated LANs made electronic communication
between different offices or departments impossible. Duplication of resources meant that the same
hardwareandsoftwarehadtobesuppliedtoeachofficeordepartment,asdidseparatesupportstaff.This
lack of network management meant that no centralized method of managing and troubleshooting
networks existed.
Open System Interconnection Reference Model
The Open System Interconnection (OSI) reference model describes how information from a software
application in one computer moves through a network medium to a software application in another
computer. The OSI reference model is a conceptual model composed of seven layers, each specifying
particular network functions. The model was developed by the International Organization for
Standardization (ISO) in 1984, and it is now considered the primary architectural model for
intercomputer communications. The OSI model divides the tasks involved with moving information
betweennetworkedcomputersintosevensmaller,moremanageabletaskgroups.Ataskorgroupoftasks
isthenassignedtoeachofthesevenOSIlayers.Eachlayerisreasonablyself-containedsothatthetasks
assignedtoeachlayercanbeimplementedindependently.Thisenablesthesolutionsofferedbyonelayer
to be updated without adversely affecting the other layers. The following list details the seven layers of
the Open System Interconnection (OSI) reference model:
• Layer 7—Application
• Layer 6—Presentation
• Layer 5—Session
• Layer 4—Transport
• Layer 3—Network
• Layer 2—Data link
• Layer 1—Physical
Characteristics of the OSI Layers
The seven layers of the OSI reference model can be divided into two categories: upper layers and lower
layers.
The upper layers of the OSI model deal with application issues and generally are implemented only in
software. The highest layer, the application layer, is closest to the end user. Both users and application
layerprocessesinteractwithsoftwareapplicationsthatcontainacommunicationscomponent.Theterm
upper layer is sometimes used to refer to any layer above another layer in the OSI model.
ThelowerlayersoftheOSImodelhandledatatransportissues.Thephysicallayerandthedatalinklayer
areimplementedinhardwareandsoftware.Thelowestlayer,thephysicallayer,isclosesttothephysical
network medium (the network cabling, for example) and is responsible for actually placing information
on the medium.
Protocols
TheOSImodelprovidesaconceptualframeworkforcommunicationbetweencomputers,butthemodel
itself is not a method of communication. Actual communication is made possible by using
communication protocols. In the context of data networking, a protocol is a formal set of rules and
conventions that governs how computers exchange information over a network medium. A protocol
implements the functions of one or more of the OSI layers.
Awidevarietyofcommunicationprotocolsexist.SomeoftheseprotocolsincludeLANprotocols,WAN
protocols,networkprotocols,androutingprotocols.LANprotocolsoperateatthephysicalanddatalink
layersoftheOSImodelanddefinecommunicationoverthevariousLANmedia.WANprotocolsoperate
atthelowestthreelayersoftheOSImodelanddefinecommunicationoverthevariouswide-areamedia.
Routing protocols are network layer protocols that are responsible for exchanging information between
routers so that the routers can select the proper path for network traffic. Finally, network protocols are
the various upper-layer protocols that exist in a given protocol suite. Many protocols rely on others for
operation.Forexample,manyroutingprotocolsusenetworkprotocolstoexchangeinformationbetween
routers.ThisconceptofbuildinguponthelayersalreadyinexistenceisthefoundationoftheOSImodel.
OSI Model and Communication Between Systems
Information being transferred from a software application in one computer system to a software
applicationinanothermustpassthroughtheOSIlayers.Forexample,ifasoftwareapplicationinSystem
A has information to transmit to a software application in System B, the application program in System
A will pass its information to the application layer (Layer 7) of System A. The application layer then
passes the information to the presentation layer (Layer 6), which relays the data to the session layer
(Layer5),andsoondowntothephysicallayer(Layer1).Atthephysicallayer,theinformationisplaced
on the physical network medium and is sent across the medium to System B. The physical layer of
System B removes the information from the physical medium, and then its physical layer passes the
informationuptothedatalinklayer(Layer2),whichpassesittothenetworklayer(Layer3),andsoon,
until it reaches the application layer (Layer 7) of System B. Finally, the application layer of System B
passes the information to the recipient application program to complete the communication process.
Interaction Between OSI Model Layers
A given layer in the OSI model generally communicates with three other OSI layers: the layer directly
above it, the layer directly below it, and its peer layer in other networked computer systems. The data
link layer in System A, for example, communicates with the network layer of System A, the physical
layer of System A, and the data link layer in System B
OSI Layer Services
One OSI layer communicates with another layer to make use of the services provided by the second
layer. The services provided by adjacent layers help a given OSI layer communicate with its peer layer
in other computer systems. Three basic elements are involved in layer services: the service user, the
service provider, and the service access point (SAP).
In this context, the service user is the OSI layer that requests services from an adjacent OSI layer. The
service provider is the OSI layer that provides services to service users. OSI layers can provide services
to multiple service users. The SAP is a conceptual location at which one OSI layer can request the
services of another OSI layer.
OSI Model Layers and Information Exchange
The seven OSI layers use various forms of control information to communicate with their peer layers in
other computer systems. This control information consists of specific requests and instructions that are
exchanged between peer OSI layers.
Control information typically takes one of two forms: headers and trailers. Headers are prepended to
data that has been passed down from upper layers. Trailers are appended to data that has been passed
down from upper layers. An OSI layer is not required to attach a header or a trailer to data from upper
layers.
Headers, trailers, and data are relative concepts, depending on the layer that analyzes the information
unit.Atthenetworklayer,forexample,aninformationunitconsistsofaLayer3headeranddata.Atthe
data link layer, however, all the information passed down by the network layer (the Layer 3 header and
the data) is treated as data.
In other words, the data portion of an information unit at a given OSI layer potentially
cancontainheaders,trailers,anddatafromallthehigherlayers.Thisisknownasencapsulation.Figure
1-6 shows how the header and data from one layer are encapsulated into the header of the next lowest
layer.
OSI Model Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functional specifications for
activating, maintaining, and deactivating the physical link between communicating network systems.
Physical layer specifications define characteristics such as voltage levels, timing of voltage changes,
physical data rates, maximum transmission distances, and physical connectors. Physical layer
implementations can be categorized as either LAN or WAN specifications. Figure 1-7 illustrates some
common LAN and WAN physical layer implementations.
OSI Model Data Link Layer
The data link layer provides reliable transit of data across a physical network link. Different data link
layerspecificationsdefinedifferentnetworkandprotocolcharacteristics,includingphysicaladdressing,
network topology, error notification, sequencing of frames, and flow control. Physical addressing (as
opposed to network addressing) defines how devices are addressed at the data link layer. Network
topology consists of the data link layer specifications that often define how devices are to be physically
connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols that a
transmission error has occurred, and the sequencing of data frames reorders frames that are transmitted
out of sequence. Finally, flow control moderates the transmission of data so that the receiving device is
not overwhelmed with more traffic than it can handle at one time.
The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two
sublayers: Logical Link Control (LLC) and Media Access Control (MAC). Figure 1-8 illustrates the
IEEE sublayers of the data link layer.
The Logical Link Control (LLC) sublayer of the data link layer manages communications between
devices over a single link of a network. LLC is defined in the IEEE 802.2 specification and supports
both connectionless and connection-oriented services used by higher-layer protocols. IEEE 802.2
defines a number of fields in data link layer frames that enable multiple higher-layer protocols to share
a single physical data link. The Media Access Control (MAC) sublayer of the data link layer manages
protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses,
which enable multiple devices to uniquely identify one another at the data link layer.
OSI Model Network Layer
The network layer defines the network address, which differs from the MAC address. Some network
layer implementations, such as the Internet Protocol (IP), define network addresses in a way that route
selection can be determined systematically by comparing the source network address with the
destination network address and applying the subnet mask. Because this layer defines the logical
network layout, routers can use this layer to determine how to forward packets. Because of this, much
of the design and configuration work for internetworks happens at Layer 3, the network layer.
OSI Model Transport Layer
The transport layer accepts data from the session layer and segments the data for transport across the
network.Generally,thetransportlayerisresponsibleformakingsurethatthedataisdeliverederror-free
and in the proper sequence. Flow control generally occurs at the transport layer.
Flow control manages data transmission between devices so that the transmitting device does not send
more data than the receiving device can process. Multiplexing enables data from several applications to
be transmitted onto a single physical link. Virtual circuits are established, maintained, and terminated
by the transport layer. Error checking involves creating various mechanisms for detecting transmission
errors,whileerrorrecoveryinvolvesacting,suchasrequestingthatdataberetransmitted,toresolveany
errors that occur.
The transport protocols used on the Internet are TCP and UDP.
OSI Model Session Layer
The session layer establishes, manages, and terminates communication sessions. Communication
sessions consist of service requests and service responses that occur between applications located in
different network devices. These requests and responses are coordinated by protocols implemented at
the session layer. Some examples of session-layer implementations include Zone Information Protocol
(ZIP), the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol
(SCP), the DECnet Phase IV session layer protocol.
OSI Model Presentation Layer
The presentation layer provides a variety of coding and conversion functions that are applied to
application layer data. These functions ensure that information sent from the application layer of one
system would be readable by the application layer of another system. Some examples of presentation
layer coding and conversion schemes include common data representation formats, conversion of
character representation formats, common data compression schemes, and common data encryption
schemes.
Commondatarepresentationformats,ortheuseofstandardimage,sound,andvideoformats,enablethe
interchange of application data between different types of computer systems. Conversion schemes are
used to exchange information with systems by using different text and data representations, such as
EBCDIC and ASCII. Standard data compression schemes enable data that is compressed at the source
device to be properly decompressed at the destination. Standard data encryption schemes enable data
encrypted at the source device to be properly deciphered at the destination.
Presentation layer implementations are not typically associated with a particular protocol stack. Some
well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG).
QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video
compression and coding.
Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint
Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF is a standard for
compressing and coding graphic images. JPEG is another compression and coding standard for graphic
images, and TIFF is a standard coding format for graphic images.
OSI Model Application Layer
TheapplicationlayeristheOSIlayerclosesttotheenduser,whichmeansthatboththeOSIapplication
layer and the user interact directly with the software application.
This layer interacts with software applications that implement a communicating component. Such
application programs fall outside the scope of the OSI model. Application layer functions typically
include identifying communication partners, determining resource availability, and synchronizing
communication.
Whenidentifyingcommunicationpartners,theapplicationlayerdeterminestheidentityandavailability
of communication partners for an application with data to transmit.
When determining resource availability, the application layer must decide whether sufficient network
resources for the requested communication exist. In synchronizing communication, all communication
between applications requires cooperation that is managed by the application layer.
Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and
Simple Mail Transfer Protocol (SMTP).
ISO Hierarchy of Networks
Large networks typically are organized as hierarchies. A hierarchical organization provides such
advantages as ease of management, flexibility, and a reduction in unnecessary traffic. Thus, the
International Organization for Standardization (ISO) has adopted a number of terminology conventions
foraddressing networkentities.Key termsdefinedin thissectioninclude endsystem(ES), intermediate
system (IS), area, and autonomous system (AS).
An ES is a network device that does not perform routing or other traffic forwarding functions. Typical
ESs include such devices as terminals, personal computers, and printers. An IS is a network device that
performs routing or other traffic-forwarding functions. Typical ISs include such devices as routers,
switches, and bridges. Two types of IS networks exist: intradomain IS and interdomain IS. An
intradomain IS communicates within a single autonomous system, while an interdomain IS
communicateswithinandbetweenautonomoussystems.Anareaisalogicalgroupofnetworksegments
and their attached devices. Areas are subdivisions of autonomous systems (AS’s). An AS is a collection
ofnetworksunderacommonadministrationthatshareacommonroutingstrategy.Autonomoussystems
are subdividedintoareas,andanASissometimescalledadomain.
Internetwork Addressing
Internetwork addresses identify devices separately or as members of a group. Addressing schemes vary
depending on the protocol family and the OSI layer. Three types of internetwork addresses are
commonly used: data link layer addresses, Media Access Control (MAC) addresses, and network layer
addresses.
Data Link Layer Addresses
A data link layer address uniquely identifies each physical network connection of a network device.
Data-link addresses sometimes are referred to as physical or hardware addresses. Data-link addresses
usually exist within a flat address space and have a pre-established and typically fixed relationship to a
specific device.
End systems generally have only one physical network connection and thus have only one data-link
address. Routers and other internetworking devices typically have multiple physical network
connections and therefore have multiple data-link addresses.
Standards Organizations
A wide variety of organizations contribute to internetworking standards by providing forums for
discussion, turning informal discussion into formal specifications, and proliferating specifications after
they are standardized.
Most standards organizations create formal standards by using specific processes: organizing ideas,
discussingtheapproach,developingdraftstandards,votingonallorcertainaspectsofthestandards,and
then formally releasing the completed standard to the public.
Some of the best-known standards organizations that contribute to internetworking standards include
these:
• International Organization for Standardization (ISO)—ISO is an international standards
organization responsible for a wide range of standards, including many that are relevant to
networking.Itsbest-knowncontributionisthedevelopmentoftheOSIreferencemodelandtheOSI
protocol suite.
• American National Standards Institute (ANSI)—ANSI, which is also a member of
the ISO, is the coordinating body for voluntary standards groups within the United States. ANSI
developed the Fiber Distributed Data Interface (FDDI) and other communications standards.
• Electronic Industries Association (EIA)—EIA specifies electrical transmission standards,
including those used in networking. The EIA developed the widely used EIA/TIA-232 standard
(formerly known as RS-232).
• Institute of Electrical and Electronic Engineers (IEEE)—IEEE is a professional organization
that defines networking and other standards. The IEEE developed the widely used LAN standards
IEEE 802.3 and IEEE 802.5.
• International Telecommunication Union Telecommunication Standardization Sector
(ITU-T)—Formerly called the Committee for International Telegraph and Telephone (CCITT),
ITU-T is now an international organization that develops communication standards. The ITU-T
developed X.25 and other communications standards.
• Internet Activities Board (IAB)—IAB is a group of internetwork researchers who discuss issues
pertinent to the Internet and set Internet policies through decisions and task forces. The IAB
designates some Request For Comments (RFC) documents as Internet standards, including
Transmission Control Protocol/Internet Protocol (TCP/IP) and the Simple Network Management
Protocol (SNMP).
Internetworking Basics
