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The OSI Model

Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards An ISO standard that covers all aspects of network communications is the Open Systems Interconnec- tion (OST) model An open system is a model that allows any (wo different systems to communicate regardless of their underlying architecture Vendor-specific protocols close off communication between unrelated systems The purpose of the OSI model is to open communication between different systems without requiring changes to the logic of the underlying hardware and software The OSI model is not a protocol; it is a

model for understanding and designing a network architecture that ts flexible, robust, and interoperable ISO ts the organization OSE is the model 3.1 THE MODEL

The Open Systems Interconnection model is a layered framework for the design of net- work systems that allows for communication across all lypes of computer systems I consists of seven separate but related layers, each of which defines a segment of the pro- cess of moving information across a network (see Figure 3.1) Understanding the funda- mentals of the OSI model provides a solid basis for exploration of data communication

Layered Architecture

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44 CHAPTER 3) TTTE OSI MODEL

Figure 3.100 The OST model 7 Application | 6 Presentation 5 Session 3 Network 2 Data fink 4 Transport | | Physical Figure 3.2) OSI layers node r Boom De DE 7 Application Device Device A Intermediate B Application ~ 7-6 interface 6 Presentation Presentation | 6 6-5 interface 5 Session Session 5 “5-4 interface 5-4 interface 4 Transport Transport 4 | 4-3 interface -| 4-3 interface 3 Network Network Network 3 _|_ 3-2 interface | | 3-2 interface 2 Data dink Data link Dita link Data link 2 2-1 interface : : 2-1 interface Physical Physical Physical Physical |

Physical communication Physical communication

functions had: related uses and collected those functions into discrete groups that

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SECTION 3.1 THE MODEL 45

created an architecture that is both comprehensive and flexible Most important, the OSI model allows complete transparency between otherwise incompatible systems

A mnemonic for remembering the layers of the OST model is: Please Do Not Touch Steve's Pet Alligator (Physical, Data ink, Network, Transport, Session, Presentation, Application),

Peer-to-Peer Processes

Within a single machine, each layer calls upon the services of the layer just below it Layer 3, for example, uses the services provided by layer 2 and provides services for layer 4, Between machines, layer x on one machine communicates with layer x on another machine This communication is governed by an agreed-upon series of rules and conventions called protocols The processes on each machine that communicate at a given layer are called peer-to-peer processes Communication between machines is therefore a peer-to-peer process using the protocols appropriate to a given layer

At the physical layer, communication is direct: Machine A sends a stream of bits to machine B At the higher layers, however, communication must move down through the layers on machine A, over to machine B, and then back up through the layers Each layer in the sending machine adds its own information to the message it receives from the layer just above it and passes the whole package to the layer just below it This

information is added in the form of headers or trailers (contro! data added to the

beginning or end of a data parcel) Headers are added to the message at layers 6, 5, 4, 3, and 2 A trailer is added at layer 2 Headers are added to the data at layers 6, 5, 4, 3, and 2 Trailers are usually added only al layer 2, At layer | the entire package is converted to a form that can be transferred to the receiving machine At the receiving machine, the message is unwrapped layer by layer, with each process receiving and removing the data meant for it For example, layer 2 removes the data meant for if, then passes the rest to layer 3 Layer 3 removes the data

meant for it and passes the rest to layer 4, and so on

Interfaces between Layers

The passing of the data and network information down through the layers of the send- ing machine and back up through the layers of the receiving machine is made possible by an interface between each pair of adjacent layers Each interface defines what infor- mation and services a layer must provide for the layer above it Well-defined interfaces and layer functions provide modularity to a network As long as a layer still provides the expected services to the layer above it, the specific implementation of its functions

san be modified or replaced without requiring changes to the surrounding layers Organization of the Layers

The seven layers can be thought of as belonging to three subgroups Layers |, 2, and

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4o CHAPTER } THE OSIMODEL

specifications, physical connections, physical addressing, and transport liming and

reliability) Layers 5, 6, and 7—session, presentation, and application—can be

thought of as the user support layers; they allow interoperability among unrelated software systems Layer 4, the transport layer, ensures end-to-end reliable data trans- mission while layer 2 cnsures reliable (ansmission on a single link The upper OS] layers are almost always implemented in software, lower layers are a combination of hardware and software, except for the physical layer, which is mostly hardware

In Figure 3.3, which gives an overall view of the OSI layers, L7 data means the data unit at layer 7, L6 data means the data unit at layer 6, and so on.The process starts out at layer 7 (the application layer), then moves from layer to layer in descending sequential order At each layer (except layers 7 and 1), a header is added to the data unit At layer 2, a trailer is added as well When the formatted data unit passes through the physical layer (layer 1), itis changed tnto an electromagnetic signal and transported along a physical link Figure 3.300 ¬ An exchange using the OST model 8 § — L7 data 7 | L7 data L7 data |H6 6 6 L7 data {HO — — ‘Lo data | H5 5 3 Lódaa | HS I P | ] L5 data |H4| 4 4 L5 data | H4 Ỉ — — L4 data H3 3 3 L4 data H8 _ T2 L3 data H2112 2 T2 L3 data H2 010101010101101010000010000 010101010101101010000010000 Bi Lư Transmission medium

Upon reaching its destination, the signal passes into layer 1 and is transformed

back into bits The data units then move back up through the OST layers As cach block

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SECTION 3.2 FUNCTIONS OF THE LAYERS 47

3.2 FUNCTIONS OF THE LAYERS

In this section we briefly describe the functions of cach layer in the OSE model

Physical Layer

The physical layer coordinates the functions required to transmit a bit stream over a physical medium It deals with the mechanical and electrical specifications of the inter- face and transmission medium It also defines the procedures and functions that physi- cal devices and interfaces have to perform for transmission to occur Figure 3.4 shows the position of the physical layer with respect to the transmission medium and the data link layer Figure 3.4 9 Physical layer From data link layer To data fink layer L2 data | | L2 data L | | : | ] 4:è | | i | MmwMww 4 layer Hà 0101000000010 | Transmission medium [1oro1a0n000010 | Physi

The physical layer is concerned with the following:

m = Physical characteristics of interfaces and media The physical layer defines the

characteristics of the interface between the devices and the transmission medium, It also defines the type of transmission medium (see Chapter 7)

m Representation of bits The physical layer data consist of a stream of bits (se- quence of Os and Is) without any interpretation To be transmitted, bits must be encoded into signals—electrical or optical The physical layer defines the type of encoding (how Os and Is are changed to signals)

m= Đ=âData rate The transmission rate—the number of bits sent each second—is also defined by the physical layer In other words, the physical layer defines the dura-

tion of a bit, which 1s how long it lasts

m = Synchronization of bits The sender and receiver must be synchronized at the bit

level In other words, the sender and the receiver clocks must be synchronized

mw = =Line configuration The physical layer is concerned with the connection of devices

to the medium In a point-to-point configuration, two devices are connected

together through a dedicated link In a multipoint configuration, a link 1s shared

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-48 CHAPTER 3 TI: OST MODEL

Physical topology The physical topology defines how devices are connected to make a network Devices can be connected using a mesh topology (every device connected to every other device), a star topology (devices are connected through a central device), a ring topology (every device is connected to the next, forming a ring) or a bus topology (every device on a common link)

Transmission mode The physical layer also delines the direction of transmission between two devices: simplex, half-duplex, or full-duplex In the 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

Data Link Layer

The data link layer transforms the physical layer, a raw transmisston facility, lo a reli-

able link and is responsible for node-to-node delivery It makes the physical layer

appear error free to the upper layer (network layer) Figure 3.5 shows the relationship of the data link layer to the network and physical layers

Figure 3.5 0 Data link layer From network layer To network layer L3 data L3 data | J pH ! | ' 4 T t T Ị Ra | 1 I t ; | | | Data Data link T2 H2 | Frame Frame | T2 H2 | link layer — layer V7 1OTO LO000000 10 [0101000000010

‘To physical layer, From physical layer

Specific responsibilities of the data link layer include the following:

kraming The data link layer divides the stream of bits received from the network layer into manageable data units called frames

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SECTION 3.2 FUNCTIONS OF THE LAYERS 49

w= Flow control If the rate at which the data are absorbed by the receiver is less (han the rate produced in the sender, the data link layer imposes a flow control mecha- ‘nism to prevent overwhelming the receiver

a = 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 mecha- nism to prevent duplication of frames Error control is normally achieved through a trailer added to the end of the frame

m 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

Example 3.1

In Figure 3.6 a node with physical address 10 sends a frame to a node with physical address 87 The two nodes are connected by a link At the data link level this frame contains physical (link) addresses in the header These are the only addresses needed The rest of the header contains other information needed at this level The trailer usually contains extra bits needed for error detection Figure 3.6 9 Data link layer (Example 3.1) pak Trailer Source Destination address address Network Layer

The network layer is responsible for the source-to-destination delivery of a packet possibly across multiple networks (links) Whereas the data link layer oversees the delivery of the packet between two systems on the same network (links), the network

layer ensures that each packet gets from its point of origin to its final destination If two systems are connected to the same link, there is usually no need for a net- work layer However, if the two systems are attached to different networks (links) with

connecting devices between the networks (links), there is often a need for the network

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A() CHAPTER § THỊ OSIAIODLEL

Eipeure 3.7 Network laver From transport layer To transport layer L4 data L4 data Network layer Network layer —~————— H3 ‡ Packet Packet HH3 —_ L3 data 13 data

To data link layer rom data fink Tayer

Specific responsibilities of the network layer include the following: m 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 disuinguish 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

n1 Routing When independent networks or links are connected together to create an

internetwork (a network of networks) or a large network, the connecting devices

(called routers or gateways) route the packets to their final destination One of the functions of the network layer is to provide this mechanism

example 3.2

Now imagine that in Figure 3.8 we want to send data from a node with network address A and physical address 10, located on one local area network, to a node with a network address P and physical address 95, located on another local area network Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses have only local jurisdiction What we need here are universal addresses that can pass through the bound- aries of local area networks The network (logical) addresses have this characteristic The packet at the network layer contains the logical addresses, which remain the same from the original source to the final destination (A and P, respectively, in the-lfigure) They will not change when we go from network to network However, the physical addresses will change when the packet moves frony one network to another The box with the R is a router (internetwork device), which

we will discuss in Chapter 21 ”

Transport Layer

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SECTION 3.2 FUNCTIONS OF THE LAYERS

Figure 3.8 9 Nenvork layer (Example 3.2) A l: = 10 87 Bus Ắ | Data | A | ati | A|IPfnẰ- 05 77 Bus

ual packets, it does not recognize any relationship between those packets It treats each one independently, as though each piece belonged to a separate message, whether or not it does The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source- to-destination level Figure 3.9 shows the relationship of the transport layer to the net- work and session layers

For added security, the transport layer may create a connection between the two end ports A connection is a single logical path between the source and destination that is associated with all packets in a message Creating a connection involves three steps:

connection establishment, data transfer, and connection release By confining transmis-

sion of all packets to a single pathway, the transport layer has more control over sequencing, flow, and error detection and correction,

Specific responsibilities of the transport layer include the following:

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CHAPTER 3 THE OSTMODELT,

Figure 3.90 Transport layer From session layer To session layer LS5 data L5 data 4 /\ \ F— 7 oe fais, Ri hột, ` Transpott " } e Transport layer 3 H4 H4 H4 H4 H4 Hai 9P layer ft, Ẫù ` # His L4 data 14 data L4 data L4 data 14 data 14 data To network bayer From network layer

m Segmentation and reassembly A message is divided into transmittable segments, 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 the transmission

m 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 Alter all the data are

transferred, the connection is terminated

nm 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

a Error control Like the data link layer, the transport layer is responsible [or error control However, error control at this layer is performed end to end rather than across a single link The sending transport layer makes sure that the enlire message arrives at the receiving transport layer-without error (damage, loss, or duplica- tion) Error correction is usually achieved through retransmission

Example 3.3

Figure 3.10 shows an example of a transport layer Data coming from the upper layers have service-point (por) addresses j and k (/ ts the address of the sending application and & 1s the address of the receiving application) Since the data size is larger than the network Jayer can handle the data are split into two packets, each packet retaining the service-point

addresses (j and &) Then in the network layer, network addresses (A and P) are added to cach

packet The packets may travel on diferent paths and arrive at the destination cither in order

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SECTION 3.2 FUNCTIONS OF THE LAYERS 53 responsible for removing the network layer headers The two packets are now passed to the transport layer, where they are combined for delivery to the upper layers

Figure 3.1000 Transport layer (Example 3) ¬ SN Transport Transport [_ Đaa |] min : lkị layer layer D + | J | k | Dau-2 || J | P Network Network [DaucL | j [KIA ñ

[Data] j | kK} ATP] layer layer [Dau-2 ]} i | k|ỊAIP

`*ở Data link Data link 4ì

klAIP tíP layer layer ie j|k : fÄ|[Daa]|¡ |k|A|»Í Session Layer

The services provided by the first three layers (physical, data fink, and network) are

not sufficient for some processes The session layer is the network dialog controller

It establishes, maintains, and synchronizes the interaction between communicating systems,

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 either in hall- duplex (one way at a time) or full-duplex (two ways at a time) For example, the dialog between a terminal connected to a mainframe can be half-duplex

Synchronization The session layer allows a process to add checkpoints (synchro- nization points) into 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 casc, if a crash happens during the transmission of page 523, retransmission begins al page 501: pages | to 500 need not be retransmitted Figure 3.11 illustrates the rela-

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CHAPTER 3

Th OSUMODEL,

Figure 3.11 0 Session layer

From presentation layer L6 data To presentation layer L6 data T / ị , 4 | / | / / 4 N Ị / I / it : ' ! / Ị / jt | I ! l 1! / if “ ry | : / Ị Session 7 if C lị , Session ; | aver Í i footy aver f hayes Ị layer i WS l 15 ! | syn syn syn —Ì syn LS data To transport layer Presentation Layer ee ee oe ES data From transport layer

The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems Figure 3.12 shows the relationship between the pre- sentation layer and the application and session layers

Mreure 3.12 0 Presentation laver + From application layer L7 data To application layer L7 data —————-— ——ảH— ——— , T T Presentation `X⁄ Presentation layer “ Encoded, encrypted, and n HG layer ayer Decoded, decrypted, and tc 16 ) compressed data - decompressed data | | ] | “a ^ | | ỳ | ' ị | l Í | L6 data To session layer Specific responsibilities of the presentation Jayer include the following: L6 data From session layer

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SECTION 3.2 FUNCTIONS OF THE LAYERS 55

information should be changed to bit streams before being transmitted Because different computers use different encoding systems, the presentation layer 1s

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 assure pri- vacy 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 m= Compression Data compression reduces the number of bits to be transmitted

Data compression becomes particularly important in the (ransmission of multime-

dia such as text, audio, and video

Application Layer

The application layer enables the user, whether human or software, to access the net- work It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distrib-

uted information services

Figure 3.13 shows the relationship of the application layer to the user and the presentation layer Of the many application services available, the figure shows only three: X.400 (message-handling services); X.500 (directory services); and file

transfer, access, and management (FTAM) The user in this example uses X.400 lo send

an e-mail message Note that no headers or trailers are added at this layer Figure 3.1300 Application layer e a User + User † Application Application layer layer X.500 TAM x.400 X.500 TAM X00 R| | rs T WH T t bef : | | | I L7 data L7 data

To presentation layer From presentation layer

Specific services provided by the application layer include the following:

m Network virtual (terminal A network virtual terminal is a software version of a

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CHAPTER 3 THE OSTMODEL

cation 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 you to log on

m file transfer, access, and management (FTAM) This application allows a user to access files in a remote computer (to make changes or read data), to retrieve files

from a remote computer; and to manage or control files in a remote computer m Mail services This application provides the basis for e-mail forwarding and

storage |

m Directory services This application provides distributed database sources and

access for global information about various objects and services

Summary of Layer Functions

The functions of the seven layers are summarized in Figure 3.14

Figure 3.14 0 Summary of layer functions To allow access to network resources To translate, encrypt, and compress data ° YP Presentation : a To establish, manage, Session _ ;

— ; and terminate sessions

To provide reliable end-to-

end message delivery and Š Transport

error recovery Sy : | To move packets from

Network | source to destination; to

To organize bits into ẳ _ | provide internetworking frames; to provide node- ị Data link

to-node delivery : ‘at \ To transmit bits over a medium:

Physical | to provide mechanical and

electrical specifications

3.3 TCP/IP PROTOCOL SUITE

The TCP/IP protocol suite, used in the Internet, was developed prior to the OSI model

Therefore, the layers in the Transmission Control Protocol/Internetworking Proto- col (TCP/IP) protocol suite do not match exactly with those in the OSI model The TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application The first four layers provide physical standards, network interface, inter- networking, and transport functions that correspond to the first four layers of the OS]

model The three topmost layers in the OSI model, however, are represented in TCP/IP

by a single layer called the application layer (see Figure 3.15)

TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality, but they are not necessarily interdependent Whereas

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SECTION 3.4 KEY TERMS AND CONCEPTS

Figure 3.15 TCP/1P and the OSI model Application Applications | j NFS Presentation SMTP FTP TELNET DNS SNMP TETP RPC Session | Transport TCP UDP | ICMP IGMP Network etwor Pp ARP RARP Data link = | Physical | 7 ‘se

TCP/IP protocol suite contain relatively independent protocols that can be mixed and

matched depending on the needs of the system The term hierarchical means that each upper-level protocol is supported by one or more lower-level protocols

At the transport layer, TCP/IP defines two protocols: Transmission Control Proto-

col (TCP) and User Datagram Protocol (UDP) At the network layer, the main protocol

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58 CHAPTER 3 THE OSTMODEL

logical address presentation layer

network layer session layer

node-to-node delivery source address open system Open Systems Interconnection (OSI) peer-to-peer process physical address source-to-destination delivery trailer

Transmission Control Protocol/Internet- working Protocol (TCP/IP)

physical layer transmission rate

port address transport layer

3.5 SUMMARY

The International Standards Organization (ISO) created a model called the Open Systems Interconnection (OSI), which allows diverse systems to communicate The seven-layer OSI model provides guidelines for the development of universally

compatible architecture, hardware, and software

The physical, data link, and network layers are the network support layers The session, presentation, and application layers are the user support layers The transport layer links the network support layers and the user support layers The physical layer coordinates the functions required to transmit a bit stream over a physical medium

The data tink layer ts responsible for delivering data units from one station to the next without errors

The network Jayer is responsible for the source-to-destination delivery of a packet across multiple network links

The transport layer is responsible for the source-to-destination delivery of the

enlire Message

The session layer establishes, maintains, and synchronizes the interactions bet-

ween communicating devices,

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SECTION 3.6 PRACTICE SET 59

m@ The application layer enables the users to access the network

m TCP/IP, a five-layer hierarchical protocol suite developed before the OST model, is

the protocol suite used in the Internet

3 6 PRAC T ICE SET

Review Questions

I Which OSI layers are the network support layers? 2 Which OSI layers are the user support layers?

3 What is the difference between network layer delivery and transport layer delivery?

4 How are OSI and ISO related to each other? 5 List the layers of the OSI model

6 What ts a peer-to-peer process?

7 How does information get passed from one OSI layer to the next? 8 What are headers and trailers and how do they get added and removed? 9 Group the OSI layers by function

10 What are the concerns of the physical layer?

11 What are the responsibilities of the data link layer? 12 What are the responsibilities of the network layer? 13 What are the responsibilities of the transport layer?

I4 The transport layer creates a connection between the source and destination, What are the three events involved in a connection?

IS What is the difference between a service-point address, a logical address, and a

physical address?

16 What are the responsibilities of the session layer? 17 What is the purpose of the dialog controller?

|8 What are the responsibilities of the presentation layer? 19 What is the purpose of translation by the presentation layer? 20 Name some services provided by the application layer,

21 How do the layers of the TCP/IP protocol suite correlate to the layers of the OSI model?

Multiple Choice Questions

22 The model shows how the network functions of a computer ought to be organized

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62 CHAPTER 3 37 AO 41 44 Tt OST MODEL In the layer, trañskHtons from one character code to another occur, a transport b session c presentation d application The layer changes bits into electromagnetic signals a physical b data link c transport d presentation The layer can use the trailer of the frame for error detection a physical b data dink c transport do presentation

Why was the OSI model developed?

a Manufacturers disliked the TCP/IP protocol suite b The rate of data transfer was increasing exponentially

c Standards were needed to allow any two systems to communicale d none of the above

The physical layer ts concerned with the transmission of over the physi- cal medium a programs b dialogs c protocols d bits 2 Which layer functions as a liaison between user support layers and network sup- port layers? a network layer b physical layer c transport layer d sesston laycr What is the main function of the transport layer? a Hode-to-node delivery b end-to-end message delivery c synchronization

d updating and maintenance of routing tables Session layer checkpoints

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45,

SECTION 3.6

b detect and recover errors c control the addition of headers

d are involved in dialog control

Which of the following is an application layer service?

a network virtual terminal

b file transfer, access, and management c mail service d all of the above Exercises 46 41 49 Match the following to one of the seven OST layers: a Route determination b Flow control

c Interface to outside world

d Access to the network provided for the end user e ASCII changed to EBCDIC

[ Packet switching

Match the following to one of the seven OSI layers: a Reliable end-to-end data transmission

b Network selection c Frames defined

d User services such as e-mail and file transfer provided e ‘Fransmission of bit stream across physical medium Match the following to one of the seven OSI layers:

PRACTICE SET 63

a Direct communication with the user’s application program

b Error correction and retransmission

c Mechanical, electrical, and functional interface

d Responsibility for information between adjacent nodes e Reassembly of data packets

Match the following to one of the seven OS] layers: a Provides format and code conversion services b Establishes, manages, and terminates sessions c Ensures reliable transmission of data

d Provides log-in and log-out procedures

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