Chapter 2: Network Connectivity and Protocols

2.1 OSI Model

The Open Systems Interconnection (OSI) model began as a reference model, but has since

been implemented. It was created by the International Organization for Standardization

(ISO) to provide a logical framework for how data communication processes should interact

across networks. Standards were created for the computer industry allowing different networks

to work together efficiently. The OSI model is not a protocol; it is a model for understanding and

designing a network architecture that is flexible, robust, and interoperable. There are 7 layers in

the OSI model. Each layer is responsible for a particular aspect of data communication. For

example, one layer may be responsible for establishing connections between devices, while

another layer may be responsible for error checking during transfer.

The layers of the OSI model are divided into two groups: the upper layer and lower layer.

The upper layers focus on user applications and how files are represented on the computers prior

to transport. For the most part, network engineers are more concerned with the lower layers. It's

the lower layers that concentrate on how the communication across a network actually occurs.

2.2.1 Working of OSI Reference Model

Fig 2.1 Working of OSI Reference Model

Information being transferred from a software application in one computer system to software

application in another must pass through each of the OSI layers. Each layer communicates with

three other OSI layers i.e., the layer directly above it, the layer directly below it, and its peer

layer in other networked systems.

for example, in above Fig 2.1 a software application in System 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); the presentation layer reformats the data if

required such that B can understand it. The formatted data is passed to the session layer (Layer

5), which in turn requests for connection establishment between session layers of A and B, it

then passes the data to the transport layer. The transport layer breaks the data into smaller units

called segments and sends them to the Network layer.

The Network layer selects the route for transmission and if, required breaks the data packets

further. These data packets are then sent to the Data link layer that is responsible for

encapsulating the data packets into data frames. The Data link layer also adds source and

destination addresses with error checks to each frame, for the hop. The data frames are finally

transmitted to the physical layer. In the physical layer, the data is in the form of a stream of bits

and this is placed on the physical network medium and is sent across the medium to System B.B

receives the bits at its physical layer and passes them on to the Data link layer, which verifies

that no error has occurred.

The Network layer ensures that the route selected for transmission is reliable, and passes the

data to the Transport layer. The function of the Transport layer is to reassemble the data packets

into the file being transferred and then, pass it on to the session layer.

The session layer confirms that the transfer is complete, and if so, the session is terminated. The

data is then passed to the Presentation layer, which may or may not reformat it to suit the

environment of B and sends it to the Application layer.

Finally the Application layer of System B passes the information to the recipient Application

program to complete the communication process.

2.2. Layers in the OSI model

2.2.1 Physical Layer

The physical layer is also concerned with the following:

• Physical characteristics of interfaces and medium: Physical layer defines the characteristics

of the interface between the devices and the transmission medium. It also defines the type of

transmission medium.

• Representation of bits. The physical layer data consists of a stream of bits (sequence of 0s or

1s) with no interpretation. To be transmitted, bits must be encoded into signals i.e. electrical or

optical. The physical layer defines the type of encoding .

• Data rate. The transmission rate is the number of bits sent each second. So physical layer

defines the duration of a bit, which is how long it lasts.

• Synchronization of bits. The sender and receiver not only must use the same bit rate but also

must be synchronized at the bit level.

• Line configuration. The physical layer is concerned with the connection of devices to the

media. In a point-to-point configuration, two devices are connected through a dedicated link. In

a multipoint configuration, a link is shared among several devices.

• Physical topology. The physical topology defines how devices are connected to make a

network. Devices can be connected by using a mesh topology , a star topology (devices are

connected through a central device), a ring

topology , a bus topology or a hybrid topology .

• Transmission mode. The physical layer also defines the direction of transmission between two

devices: simplex, half-duplex, or full-duplex. In simplex mode, only one device can send; the

other can only receive. The simplex mode is a one-way communication. In the half-duplex

mode, two devices can send and receive, but not at the same time. In a full-duplex (or simply

duplex) mode, two devices can send and receive at the same time.

2.2.3.2 Data Link Layer

Responsibilities of the data link layer include the following:

• The data link layer is responsible for moving frames from one hop (node) to the next.

• Framing. The data link layer divides the stream of bits received from the network layer into

manageable data units called frames.

• Physical addressing. If frames are to be distributed to different systems on the network, the

data link layer adds a header to the frame to define the sender and/or receiver of the frame. If

the frame is intended for a system outside the sender's network, the receiver address is the

address of the device that connects the network to the next one.

• Flow control. If the rate at which the data are absorbed by the receiver is less than the rate at

which data are produced in the sender, the data link layer imposes a flow control mechanism to

avoid overwhelming the receiver.

• Error control. The data link layer adds reliability to the physical layer by adding mechanisms

to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize

duplicate frames. Error control is normally achieved through a trailer added to the end of the

frame.

• Access control. When two or more devices are connected to the same link, data link layer

protocols are necessary to determine which device has control over the link at any given time.

2.2.3.3 Network Layer

Responsibilities of the network layer include the following:

• The network layer is responsible for the delivery of individual packets from the source host to

the destination host.

• Logical addressing. The physical addressing implemented by the data link layer handles the

addressing problem locally. If a packet passes the network boundary, we need another

addressing system to help distinguish the source and destination systems. The network layer

adds a header to the packet coming from the upper layer that, among other things, includes the

logical addresses of the sender and receiver.

• Routing. When independent networks or links are connected to create internetworks (network

of networks) or a large network, the connecting devices (called routers or switches) route or

switch the packets to their final destination. One of the functions of the network layer is to

provide this mechanism.

2.2.3.4 Transport Layer

Responsibilities of the transport layer include the following:

• The transport layer is responsible for the delivery of a message from one process to

another.

• Service-point addressing. Computers often run several programs at the same

time. For this reason, source-to-destination delivery means delivery not only from

one computer to the next but also from a specific process (running program) on

one computer to a specific process (running program) on the other. The transport

layer header must therefore include a type of address called a service-point

address (or port address). The network layer gets each packet to the correct

computer; the transport layer gets the entire message to the correct process on

that computer.

• Segmentation and reassembly. A message is divided into transmittable segments, with each

segment containing a sequence number. These numbers enable the transport layer to reassemble

the message correctly upon arriving at the destination and to identify and replace packets that

were lost in transmission.

• Connection control. The transport layer can be either connectionless or connection-oriented. A

connectionless transport layer treats each segment as an independent packet and delivers it to

the transport layer at the destination machine. A connection-oriented transport layer makes a

connection with the transport layer at the destination machine first before delivering the

packets. After all the data are transferred, the connection is terminated.

• Flow control. Like the data link layer, the transport layer is responsible for flow control.

However, flow control at this layer is performed end to end rather than across a single link.

• Error control. Like the data link layer, the transport layer is responsible for error control.

However, error control at this layer is performed process-to-process rather than across a single

link. The sending transport layer makes sure that the entire message arrives at the receiving

transport layer without error (damage, loss, or duplication). Error correction is usually

achieved through retransmission.

2.2.3.5 Session Layer:

Specific responsibilities of the session layer include the following:

• Dialog control. The session layer allows two systems to enter into a dialog. It allows the

communication between two processes to take place in either half-duplex (one way at a time)

or full-duplex (two ways at a time) mode.

• Synchronization. The session layer allows a process to add checkpoints, or Synchronization

points, to a stream of data. For example, if a system is sending a file of 2000

pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page

unit is received and acknowledged independently. In this case, if a crash happens during the

transmission of page 523, the only pages that need to be resent after system recovery are

pages 501 to 523. Pages previous to 501 need not be resent.

2.2.3.6 Presentation Layer

The presentation layer is concerned with the syntax and semantics of the information exchanged

between two systems. Specific responsibilities of the presentation layer include the following:

• Translation. The processes (running programs) in two systems are usually exchanging

information in the form of character strings, numbers, and so on. The information must be

changed to bit streams before being transmitted. Because different computers use different

encoding systems, the presentation layer is responsible for interoperability between these

different encoding methods. The presentation layer at the sender changes the information

from its sender-dependent format into a common format. The presentation layer at the

receiving machine changes the common format into its receiver-dependent format.

• Encryption. To carry sensitive information, a system must be able to ensure privacy.

Encryption means that the sender transforms the original information to another form and

sends the resulting message out over the network. Decryption reverses the original process to

transform the message back to its original form.

• Compression. Data compression reduces the number of bits contained in the information.

Data compression becomes particularly important in the transmission of multimedia such as

text, audio, and video.

2.2.3.7 Application Layer

The application layer enables the user, whether human or software, to access the network. It

provides user interfaces and support for services such as electronic mail, remote file access and

transfer, shared database management, and other types of distributed information services.

Specific services provided by the application layer include the following:

• Network virtual terminal. A network virtual terminal is a software version of a physical

terminal, and it allows a user to log on to a remote host. To do so, the application creates a

software emulation of a terminal at the remote host. The user's computer talks to the software

terminal which, in turn, talks to the host, and vice versa. The remote host believes it is

communicating with one of its own terminals and allows the user to log on.

• File transfer, access, and management. This application allows a user to access files in a

remote host (to make changes or read data), to retrieve files from a remote computer for use in

the local computer, and to manage or control files in a remote computer locally.

• Mail services. This application provides the basis for e-mail forwarding and storage.

• Directory services. This application provides distributed database sources and access for

global information about various objects and services.

2.3 TCP/IP Protocol architecture (internet model)

The TCP/IP protocol suite was developed prior to the OSI model. Therefore, the layers in the

TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP

protocol suite was defined as having four layers: host-to-network, internet, transport, and

application. However, when TCP/IP is compared to OSI, we can say that the host-to-network

layer is equivalent to the combination of the physical and data link layers. The internet layer is

equivalent to the network layer, and the application layer is roughly doing the job of the session,

presentation, and application layers with the transport layer in TCP/IP taking care of part of the

duties of the session layer.

Fig 2.2 of TCP / IP reference model

TCP/IP is composed of two major parts: TCP (Transmission Control Protocol) at the transport

layer and IP (Internet Protocol) at the network layer. TCP is a connection oriented protocol that

passes its data to IP, which is a connectionless one. TCP sets up a connection at both ends and

guarantees reliable delivery of the full message sent. TCP tests for errors and requests

retransmission if necessary, because IP does not. An alternative protocol to TCP within the

TCP/IP suite is UDP (User Datagram Protocol), which does not guarantee delivery. Like IP, it is

also connectionless, but very useful for real-time voice and video, where it doesn’t matter if a

few packets get lost.

2.3.1 Layers of TCP/IP reference model

2.3.1.1 Application Layer (application & presentation layer of OSI Model)

There are many common TCP/IP applications that almost every implementation provides:

• Telnet for remote login,

• FTP, the File Transfer Protocol,

• SMTP, the Simple Mail Transfer protocol, for electronic mail,

• SNMP, the Simple Network Management Protocol.

If we have two hosts on a local area network (LAN) such as an Ethernet, both running FTP, Fig

2.3 shows the protocols involved

Fig 2.3 Two hosts on LAN running FTP

2.3.1.2 Transport Layer (session & transport layer function of OSI MODEL)

The Transport Layer (also known as the Host-to-Host Transport Layer) is responsible for

providing the Application Layer with session and datagram communication services. TCP/IP

does not contain Presentation and Session layers, the services are performed if required, but they

are not part of the formal TCP/IP stack. For example, Layer 6 (Presentation Layer) is where data

conversion (ASCII to EBCDIC, floating point to binary, etc.) and encryption /decryption is

performed. Layer 5 is the Session Layer, which is performed in layer 4 in TCP/IP. Thus, we

jump from layer 7 of OSI down to layer 4 of TCP/IP.

2.3.1.3 Internet Layer(Network layer of OSI model)

The Internet Layer is analogous to the Network layer of the OSI model.

2.3.1.4 Link/Physical Layer(data link & physical layer of OSI model)

The Link/Physical Layer (also called the Network Access Layer) is responsible for placing

TCP/IP packets on the network medium and receiving TCP/IP packets of the network medium.

TCP/IP was designed to be independent of the network access method, frame format, and

medium. In this way, TCP/IP can be used to connect differing network types. The Network

Interface Layer encompasses the Data Link and Physical layers of the OSI Model.

Fig 2.4 four layer of TCP/IP suite

Fig 2.5 Comparison between OSI & TCP/IP reference model

2.4 Some of the drawbacks of OSI reference model are: • All layers are not roughly, of equal size and complexity. In practice, the session layer and

presentation layer are absent from many existing architectures.

• Some functions like addressing, flow control, retransmission are duplicated at each layer,

resulting in deteriorated performance.

• The initial specification of the OSI model ignored the connectionless model, thus, leaving

much of the LANs behind.

2.5 Some of the drawbacks of TCP/IP model are: • TCP/IP model does not clearly distinguish between the concepts of service, interface, and

protocol.

• TCP/IP model is not a general model and therefore it cannot be used to describe any protocol

other than TCP/IP.

• TCP/IP model does not distinguish or even mention the Physical or the Data link layer. A

proper model should include both these layers as separate.

2.6 TCP/IP Layering (TCP/IP PROTOCOL SUITE) There are more protocols in the TCP/IP protocol suite. Fig 2.4 shows some of the additional

Protocols

2.6.1 Application layer protocols

2.6.1.1 File Transfer Protocol (FTP): is used for interactive file transfer.FTP uses TCP

as its transport. It has two well-known ports:

• FTP data port is 20

• FTP control port is 21

The commands and status information are sent over the control port; the requested files, etc. are

transferred on the separate data port. The standard for FTP, illustrates a typical FTP set-up, as

shown in Fig 2.6

Fig 2.6 of Typical FTP implementation

The user types commands such as “get /tmp/x” into the part of the client program that handles

the user interface. This part of the program communicates the user’s instructions to the “protocol

interpreter” (PI) – a separate part of the program that sends FTP protocol commands to the

server, and interprets responses from the server. The PI also controls the “data transfer process”

(DTP) component, instructing it as to when to connect to the server, which IP address and port

number to connect on, which files to transfer, etc. The actual file data, directory listings, and so

on are transferred over the data connection. The FTP protocol can handle “third-party” transfers,

from one remote server to another remote server, controlled by a client elsewhere as shown in

Fig 2.7 below, which is why the control and data connections are separate.

Fig 2.7 Client controls third-party transfer between two other machines

How FTP connections are established

A typical FTP session consists of two phases (Fig 2.8 below):

1. The user specifies to the client which server and port to connect to. The client establishes the

control connection. The user logs in, negotiates any transfer parameters with the server (e.g.

binary or text mode transfer), and sets any local parameters .

2. The user requests a file transfer (from server to client or vice versa). The data connection has

to be established now. The procedure is:

• the client opens an ephemeral port and listens on it, waiting for the server to connect.

• The client sends the FTP PORT command, telling the server the number of the port

the client is listening on.

• the server connects to that port and the connection is established

• the data are transferred over the connection

• The connection is closed.

Phase 2 is repeated for each transfer , using a new data connection each time.

Fig 2.8 show two phases of a typical “active mode” FTP session

The way the data connection is established causes problems when the connection is through a

firewall, from an internal client to an external server. The data connection is established from

outside the firewall (the server) to inside (the client) on a more or less random port (:1068 here).

The firewall doesn’t know whether this is the second phase of a genuine FTP session that an

internal client established or an outside hacker trying to break in and consequently many

firewalls block it. To overcome this problem, the FTP server can operate in passive mode (as

opposed to active mode

Fig 2.9 of a “passive mode” FTP session

2.6.1.2 Telnet:- “Telnet” stands for “telecommunications network protocol.” telnet is one of the oldest Internet

applications. It lets you connect your “terminal” to a remote host over the network

telnet is typically used to “remote login” from your own PC to another elsewhere on the

network. Remote login like this lets you use applications on the remote system as shown if fig

below It provides a text-only connection, usually to a “command-line prompt” like a UNIX shell,

or Windows Command/DOS prompt, as though you were sitting at a terminal wired directly to a

serial port on the remote machine. This was how networked computing was performed for years.

Telnet is a client/server application. The client takes the characters you input at the keyboard,

sends them to the server, and prints whatever output the server sends back. The server does more

work: it passes the input characters from the client to a shell process, which interprets them as

commands; then it reads the command output and sends it back to the client, to be printed on

your terminal’s output device.

Fig 2.10 of using telnet to log on remote host

Fig 2.11 below shows the typical arrangement of the Telnet client and server.

Fig 2.11 below shows the typical arrangement of the Telnet client and server.

1. The Telnet client interacts with both the user at the terminal and the TCP/IP protocols.

Normally everything we type is sent across the TCP connection, and everything received

from the connection is output to our terminal.

2. The Telnet server often deals with what's called a pseudo-terminal device, at least under

Unix systems. This makes it appear to the login shell that's invoked on the server, and to any

programs run by the login shell, that they're talking to a terminal device.

3. Only a single TCP connection is used. Since there are times when the Telnet client must talk

to the Telnet server (and vice versa) there needs to be some way to delineate commands that

are sent across the connection, versus user data.

4. We show dashed boxes in Figure to note that the terminal and pseudo terminal drivers, along

with the TCP/IP implementation, are normally part of the operating system kernel. The

Telnet client and server, however, are often user applications.

5. We show the login shell on the server host to reiterate that we have to login to the server. We

must have an account on that system to login to it, using either Telnet or Rlogin.

2.6.1.3 SMTP: Simple mail transfer protocol Internet e-mail systems use TCP as their transport layer and are client/server applications. On a

typical LAN, Internet e-mail involves three separate steps:

1. Your client sends messages using SMTP (Simple Mail Transfer Protocol)

2. Your client receives messages from the server using POP3 (Post Office Protocol, version 3)

3. Your server (or your ISP’s server) sends and receives messages from other servers

using SMTP.

That is, SMTP is used everywhere except by clients retrieving their mail from their own (or their

ISP’s) server. All e-mail over the Internet is sent using SMTP. Even if your mail server is

Microsoft Exchange or Lotus, it uses SMTP to send to remote sites, and to receive from them

Fig 2.12 of Email protocols

2.6.1.4 DNS: (Domain Name Service)

a) DNS

Specifies the name syntax and rules for delegating authority over names Specifies the

implementation of distributed computing system that efficiently map names to addresses.

b) DNS Syntax

Set of labels separated by delimiter character (period)

Example: ecs.syr.edu

syr.edu is also a domain

The top-level domain is edu

c) Original Top-Level Domains

Fig 2.13 show domain name

d) Mapping Domain Names to Addresses

• Name server: supplies name-to-address translation

• Client: uses one or more name servers when translating a name

• DNS uses a set of on-line servers

• Servers arranged in tree

• Given server can handle entire subtree. For example, ECS manages domain names within the

ecs.syr.edu domain.

Fig 2.14 show domain tree structure

Fig 2.16 show inverse mapping process in DNS

2.6.2 Host To Host transports layer protocol:

• TCP (discuss in details in next chapter)

• UDP (discuss in details in next chapter)

2.6.3 Internet layer protocol:

• IP (showing in details in next chapter)

• ARP (showing in details in next chapter

• RARP (showing in details in next chapter)

• ICMP (showing in details in next chapter)

2.6.4 Network Interface layer protocol:

• Ethernet

• Frame Relay

• ATM

2.6.4.1 Ethernet:-

Ethernet protocol is a MAC sublayer protocol. Ethernet stands for cable and IEEE 802.3 Ethernet

protocol was designed to operate at 10 Mbps. Here, we will begin discussing the Ethernet with

various types of cabling. With the help of table 2.2 we will try to summaries the cabling used for

baseband 802.3 LANs.

The "10" in 10Base2 stands for "10 Mbps"; the "2" stands for "200 meters

Table 2.2 show Characteristics Ethernet cable

MAC frame structure for IEEE 802.3 with the help of Fig 2.20

Fig 2.20 of Ethernet Frame Format

Each frame has seven fields explained as follows:

• Preamble: The first field of 802.3 frame is 7 byte (56 bits) long with a sequence of

alternate 1 and 0 i.e., 10101010. This pattern helps the receiver to synchronies and gets the

beginning of the frame.

• Starting Delimiter (SD): The second field start delimiter is 1 byte (8 bit) long. It has

pattern 10101011. Again, it is to indicate the beginning of the frame and ensure that the

next field will be a destination address. Address, here, can be a single address or a group

address.

• Destination Address (DA): This field is 6 byte (48 bit) long. It contains the physical

address of the receiver.

• Source Address (SA): This filed is also 6 byte (48 bit) long. It contains the physical

address of the sender.

• Length of Data Field: It is 2 byte (16 bit) long. It indicates the number of bytes in the

information field. The longest allowable value can be 1518 bytes.

• Data: This field size will be a minimum of 46 bytes long and a maximum of 1500 bytes as

will be explained later.

• Pad: This field size can be 0 to 46 bytes long. This is required if, the data size is less than

46 bytes as a 802.3 frame must be at least 64 bytes long.

• Frame Checksum (FCS): This field is 4 bytes (32 bit) long. It contains information about

error detection. Here it is CRC-32.

Minimum and Maximum Length of Frame

Minimum frame length = 64 bytes = 512 bits

Maximum frame length= 1518 bytes = 12144 bits

2.6.4.2 Frame relay:- Frame Relay is a virtual-circuit wide-area network. with the following features:

• Frame Relay operates at a higher speed (1.544 Mbps and recently 44.376 Mbps). This

means that it can easily be used instead of a mesh of T-l or T-3 lines.

• Frame Relay operates in just the physical and data link layers. This means it can easily be

used as a backbone network to provide services to protocols that already have a network

layer protocol, such as the Internet.

• Frame Relay allows bursty data.

• Frame Relay allows a frame size of 9000 bytes, which can accommodate all local-area

network frame sizes.

• Frame Relay is less expensive than other traditional WANs.

• Frame Relay has error detection at the data link layer only. There is no flow control or error

control. There is not even a retransmission policy if a frame is damaged; it is silently

dropped. Frame Relay was designed in this way to provide fast transmission capability for

more reliable media and for those protocols that have flow and error control at the higher

layers.

Architecture Frame Relay provides permanent virtual circuits and switched virtual circuits. Fig 2.21 shows an

example of a Frame Relay network connected to the Internet. The routers are used, to connect

LANs and WANs in the Internet. In the figure, the Frame Relay WAN is used as one link in the

global Internet

Fig 2.21 shows an example of a Frame Relay network connected to the Internet

Virtual Circuits

Frame Relay is a virtual circuit network. A virtual circuit in Frame Relay is identified by a

number called a data link connection identifier (DLCI).

2.6.4.3 ATM Asynchronous Transfer Mode (ATM) is the cell relay protocol designed by the ATM Forum

and adopted by the ITU-T. In fact, ATM can be thought of as the "highway" of the information

superhighway.

Design Goals

Among the challenges faced by the designers of ATM, six stand out.

• Foremost is the need for a transmission system to optimize the use of high-data-rate

transmission media, in particular optical fiber. In addition to offering large band-widths,

newer transmission media and equipment are dramatically less susceptible to noise

degradation. A technology is needed to take advantage of both factors and thereby

maximize data rates.

• The system must interface with existing systems and provide wide-area interconnectivity

between them without lowering their effectiveness or requiring their replacement.

• The design must be implemented inexpensively so that cost would not be a barrier to

adoption. If ATM is to become the backbone of international communications, as intended,

it must be available at low cost to every user who wants it.

• The new system must be able to work with and support the existing telecommunications

hierarchies (local loops, local providers, long-distance carriers, and so on).

• The new system must be connection-oriented to ensure accurate and predictable delivery.

Last but not least, one objective is to move as many of the functions to hardware as possible (for

speed) and eliminate as many software functions as possible (again for speed).

Cell Networks Many of the problems associated with frame internetworking are solved by adopting a concept

called cell networking. A cell is a small data unit of fixed size. In a cell network, which uses the

cell as the basic unit of data exchange, all data are loaded into identical cells that can be

transmitted with complete predictability and uniformity. As frames of different sizes and formats

reach the cell network from a tributary network, they are split into multiple small data units of

equal length and are loaded into cells. The cells are then multiplexed with other cells and routed

through the cell network. Because each cell is the same size and all are small, the problems

associated with multiplexing different-sized frames are avoided.

Architecture ATM is a cell-switched network. The user access devices, called the endpoints, are connected

through a user-to-network interface (UNI) to the switches inside the network. The switches are

connected through network-to-network interfaces (NNIs). Fig 2.22 shows an example of an

ATM network.

Fig 2.22 shows an example of an ATM network.

Virtual Connection Connection between two endpoints is accomplished through transmission paths (TPs), virtual

paths (VPs), and virtual circuits (VCs). A transmission path (TP) is the physical connection

(wire, cable, satellite, and so on) between an endpoint and a switch or between two switches.

Think of two switches as two cities. A transmission path is the set of all highways that directly

connect the two cities.

A transmission path is divided into several virtual paths. A virtual path (VP) provides a

connection or a set of connections between two switches. Think of a virtual path as a highway

that connects two cities. Each highway is a virtual path; the set of all highways is the

transmission path. Cell networks are based on virtual circuits (VCs). All cells belonging to a

single message follow the same virtual circuit and remain in their original order until they reach

their destination.