Kotebe University CollegeDepartment of Computer Science & Technology

Data Communication and Computer Networks (CoSc2061)Chapter Two

3. Layered Models3.1. The Open System Interconnection (OSI) modelReference Model offers a means of standardization which is acceptable worldwide. Since people using the computer network are located over a wide physical range and their network devices might have heterogeneous architecture. In order to provide communication among heterogeneous devices, we need a standardized model i.e. a reference model, which would provide us way how these devices can communicate regardless their architecture.

We have two reference models such as OSI model and TCP/IP reference model, however, the OSI model is a hypothetical one but the TCP/IP is absolutely practical model.

3.2. Purpose and Definition of OSI modelOSI (Open Systems Interconnection) is reference model for how applications can communicate over a network. A reference model is a conceptual framework for understanding relationships. The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools. Most vendors involved in telecommunications make an attempt to describe their products and services in relation to the OSI model. And although useful for guiding discussion and evaluation, OSI is rarely actually implemented, as few network products or standard tools keep all related functions together in well-defined layers as related to the model. The TCP/IP protocols, which define the Internet, do not map cleanly to the OSI model.

Developed by representatives of major computer and telecommunication companies beginning in 1983, OSI was originally intended to be a detailed specification of actual interfaces. Instead, the committee decided to establish a common reference model for which others could then develop detailed interfaces, which in turn could become standards. OSI was officially adopted as an international standard by the International Organization of Standards (ISO).

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3.3. Layers and peer to peer processes

Let’s see how these layers work in a Peer to Peer Communications Network. In this exercise we will package information and move it from Host A, across network lines to Host B.

Each layer uses its own layer protocol to communicate with its peer layer in the other system. Each layer’s protocol exchanges information, called protocol data units (PDUs), between peer layers.

This peer-layer protocol communication is achieved by using the services of the layers below it. The layer below any current or active layer provides its services to the current layer. The transport layer will insure that data is kept segmented or separated from one other data. At the network layer we get packets that begin to be assembled. At the data link layer those packets become frames and then at the physical layer those frames go out on the wires from one host to the other host as bits

Figure: Data Encapsulation

This whole process of moving data from host A to host B is known as data encapsulation – the data is being wrapped in the appropriate protocol header so it can be properly received.

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Let’s say we compose an email that we wish to send from system A to system B. The application we are using is Eudora. We write the letter and then hit send. Now, the computer translates the numbers into ASCII and then into binary (1s and 0s). If the email is a long one, then it is broken up and mailed in pieces. This all happens by the time the data reaches the Transport layer.

At the network layer, a network header is added to the data. This header contains information required to complete the transfer, such as source and destination logical addresses.

The packet from the network layer is then passed to the data link layer where a frame header and a frame trailer are added thus creating a data link frame.

Finally, the physical layer provides a service to the data link layer. This service includes encoding the data link frame into a pattern of 1s and 0s for transmission on the medium (usually a wire).

3.4. Functions of Layers of OSI Model

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Summary of functions of OSI layerThe OSI model consists of seven layers as shown in the following diagram. Each layer has a specific function, however each layer provide services to the layer above.

Physical Layer

The Physical layer is responsible for the following activities: Activating, maintaining and deactivating the physical connection. Defining voltages and data rates needed for transmission. Converting digital bits into electrical signal. Deciding whether the connection is simplex, half duplex or full duplex.

Data Link LayerThe data link layer performs the following functions:

Performs synchronization and error control for the information which is to be transmitted over the physical link.

Enables error detection, and adds error detection bits to the data which are to be transmitted.

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Network LayerFollowing are the functions of Network Layer:

To route the signals through various channels to the other end.

To act as the network controller by deciding which route data should take.

To divide the outgoing messages into packets and to assemble incoming packets into messages for

higher levels.

Transport LayerThe Transport layer performs the following functions:

It decides if the data transmission should take place on parallel paths or single path. It performs multiplexing, splitting on the data. It breaks the data groups into smaller units so that they are handled more efficiently by

the network layer.The Transport Layer guarantees transmission of data from one end to other end.

Session LayerThe Session layer performs the following functions:

Manages the messages and synchronizes conversations between two different applications.

It controls logging on and off, user identification, billing and session management.

Presentation LayerThe Presentation layer performs the following functions:

Data format and representation/conversion. For example, This layer makes it sure that the information is delivered in such a form that the

receiving system will understand and use it.

Application LayerThe Application layer performs the following functions:

It provides different services such as manipulation of information in several ways, retransferring the files of information, distributing the results etc.

The functions such as LOGIN or password checking are also performed by the application layer.

Detail of OSI Layers functionalityIntroduction to the OSI layer

Open Systems Interconnection (OSI) Each layer support the layers above it and offers services to the layers below Each layer performs unique and specific task A layer only has knowledge of its neighbour layers only A layer service is independent of the implementation

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How OSI was created and how An attempt for a framework for developing networking technologies OSI became a tool for explaining the Networking in general Before OSI was created people created their Software/Hardware as they wanted it to

be. There was not any compatibility. Now OSI is used as a rule set for all vendors to create their Software/Hardware by using the standards.

Protocol Reference Model of OSI1. OSI - layered framework for the design of network systems that allows

communication across all types of computer systems.

2. The OSI 7 Layers. (Brief functional overview)

3. Vertical and horizontal communication between the layers using interfaces. (Defines what information and services should the layer provide to the layer above it)

Layers Each layer contains a logical grouping of functions Each function receive an input(one or more) and produces an output

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Layer abstraction and the path of the message

4. Data Encapsulation a) PDU conception – each protocol on the diff. layer has its own format.b) Headers are added while a packet is going down the stack at each layer. c) Trailers are usually added on the second layer.

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Application layer (Layer 7) Applications and Services run on it Enables human network to interface the underlying data network Applications on that layer (E-mail clients, web browsers, Chats, etc.) – top-stack

applications (As people are on the top of the stack) Applications provide people with a way to create message Application layer services establish an interface to the network Protocols provide the rules and formats that govern how data is treated Protocols on the destination and the host must match

Application Layer Services◦ Provides user interfaces and support for services ◦ Resource sharing and device redirection ◦ Remote file access ◦ Remote printer access ◦ Inter-process communication ◦ Network management ◦ Directory services◦ Electronic messaging (such as mail) ◦ Network virtual terminals

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Presentation layer (Layer 6) Coding and conversion of Application layer data to ensure that data from the

source device can be interpreted by the appropriate application on the destination device.

Compression of the data in a manner that can be decompressed by the destination device.

Encryption of the data for transmission and the decryption of data upon receipt by the destination.

This is the layer at which application programmers consider data structure and presentation

Examples: GIF, JPEG, TIFF, etc. Sometimes n distinction is made between the presentation and application

layers. For example http/https. HTTP is generally regarded as an application layer protocol although it has Presentation layer aspects such as the ability t identify character encoding for roper conversion

Presentation Layer Services◦ Translation (connects different computer systems)◦ Compression (transmission efficiency)◦ Encryption (SSL security)

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Session layer (Layer 5) Functions at this layer create and maintain dialogs between source and

destination applications Authentication Permissions Session Restoration (Checkpoint or recovery)

Session Layer Services◦ Session establishment, maintenance and termination (Deciding who

sends, and when.) ◦ Session support (security, name recognition, logging )

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Transport layer (Layer 4) Tracking the individual communication between applications on the source and

destination hosts Segmenting data and managing each piece Reassembling the segments into streams of application data Identifying the different applications Conversation Multiplexing Segments Connection-oriented conversations Reliable delivery Ordered data reconstruction Flow control TCP – Web Browser UDP – Video Streaming Applications

Transport Layer Services◦ Connectionless and connection-oriented services ◦ Process-Level Addressing ◦ Multiplexing and Demultiplexing ◦ Segmentation, Packaging and Reassembly ◦ Connection Establishment, Management and Termination ◦ Acknowledgments and Retransmissions ◦ Flow Control

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Network layer (Layer 3) Addressing (IPV4) Encapsulation (Inserts a header with source and destination IPs) Routing (Move a packet over the Internet) Decapsulation (Open the packet and check the destination host) IP is connectionless

Network Layer Services◦ Logical Addressing ◦ Routing (where the packet is destinated to) ◦ Datagram Encapsulation ◦ Fragmentation and Reassembly (handling too big packets ) ◦ Error Handling and Diagnostics ( using status messages for example )

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End to end packet delivery

Data link layer (Layer 2) It is the role of the OSI Data Link layer to prepare Network layer packets for

transmission and to control access to the physical media. Allows the upper layers to access the media using techniques such as framing Controls how data is placed onto the media and is received from the media

using techniques such as media access control and error detection Frame - The Data Link layer PDU Node - The Layer 2 notation for network devices connected to a common

medium Media/medium - The physical means for the transfer of information between

two nodes Network - Two or more nodes connected to a common medium The Data Link layer is responsible for the exchange of frames between nodes

over the media of a physical network.

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Data Link Layer Services2.1. Logical Link Control (LLC)

◦ Establishment and control of logical links between local devices on a network.

2.2. Media Access Control (MAC) ◦ The procedures used by devices to control access to the network

medium.◦ Frame sequencing ◦ Frame acknowledgment ◦ Addressing◦ Frame delimiting◦ Frame error checking◦ PDU: frame

Physical layer (Layer 1) The role of the Physical layer is to encode the binary digits that represent Data

Link layer frames into signals and to transmit and receive these signals across the physical media that connect network devices.

Copper cable Fiber Wireless

Physical Layer Services• Definition of Hardware Specifications (of cables, connectors, wireless

radio transceivers, network interface cards )• Encoding and Signaling (bit representation) • Data Transmission and Reception (half duplex, full duplex ) • Topology and Physical Network Design (mesh, ring, bus)• PDU: bit

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OSI Summary

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The way people learn Networking A standard for software A standard for hardware Seven layers architecture Each layer independent on the others Similar to TCP/IP(TCP/IP explained) OSI is used as a model for developing network aware applications(Here I mean that people use its structure to

model software)

OSI Layers Brief SummaryTo assist you in quickly comparing the layers of the OSI Reference Model, and understanding where they are different and how they relate to each other, summary chart of OSI model is shown the next table. It shows each layer’s name and number, describes its key responsibilities, talks about what type of data is generally handled at each layer, and also what the scope of each layer is in approximate terms. The table also shows some of the more common protocols that are associated with each layer.

Table 18: OSI Reference Model Layer Summary

Group # Layer Name Key Responsibilities Data Type Handled Scope Common Protocols

and Technologies

Lower Layers 1 Physical

Encoding and Signaling; Physical Data Transmission; Hardware

Specifications; Topology and DesignBits

Electrical or light signals sent

between local devices

(Physical layers of most of the technologies

listed for the data link layer)

2 Data Link Logical Link Control; Media Access Control; Data Framing; Addressing;

Error Detection and Handling; Defining Requirements of Physical

Layer

Frames Low-level data messages

between local devices

IEEE 802.2 LLC, Ethernet Family; Token Ring; FDDI and CDDI; IEEE 802.11 (WLAN,

Wi-Fi); HomePNA; HomeRF; ATM; SLIP

and PPP

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3 NetworkLogical Addressing; Routing;

Datagram Encapsulation; Fragmentation and Reassembly; Error Handling and Diagnostics

Datagrams / Packets

Messages between local or remote devices

IP; IPv6; IP NAT; Ipsec; Mobile IP; ICMP; IPX;

DLC; PLP; Routing protocols such as RIP

and BGP

4 Transport

Process-Level Addressing; Multiplexing/Demultiplexing;

Connections; Segmentation and Reassembly;

Acknowledgments and Retransmissions;

Flow Control

Datagrams / Segments

Communication between software

processes

TCP and UDP; SPX; NetBEUI/NBF

Upper Layers

5 Session Session Establishment, Management and Termination Sessions

Sessions between local or remote

devices

NetBIOS, Sockets, Named Pipes, RPC

6 Presentation Data Translation; Compression and Encryption

Encoded User Data

Application data representations

SSL; Shells and Redirectors; MIME

7 Application User Application Services User Data Application data

DNS; NFS; BOOTP; DHCP; SNMP; RMON;

FTP; TFTP; SMTP; POP3; IMAP; NNTP;

HTTP; Telnet

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OSI Reference Model Layer Mnemonics

If you spend any amount of time at all dealing with networking design or implementation issues, or learning about how the various protocols operate, the names and numbers of the various layers will eventually become “second nature”, i.e., you know it from experience!

Many people, however, especially those just learning about networks, find it difficult to recall the names of all the layers, and especially, what their exact order is. To help these people, a number of mnemonics (pronounced “neh-MON-iks) several memory aids were created. These are attractive phrases where each word starts with the first letter of an OSI model layer, arranged in the correct order.

 

Figure 18: OSI Reference Model Mnemonics

These mnemonics may help you to remember the order of the OSI Reference Model layers.

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I have seen a few of them in my time, some of which go in ascending layer number order, and some of which go in the other direction. These two go from physical layer to application layer:

o Please Do Not Throw Sausage Pizza Away o Please Do Not Touch Steve's Pet Alligator

And these ones go the other direction (application to physical):o All People Seem To Need Data Processing (this is a popular one) o All People Standing Totally Naked Don't Perspire (hmm, that's interesting…)

3.5. TCP/IP model (protocols and services)TCP/IP model is practical model and is used in the Internet. TCP/IP is acronym of Transmission Control Protocol and Internet Protocol.

The TCP/IP model combines the two layers (Physical and Data link layer) into one layer i.e. Host-to-Network layer. The following diagram shows the various layers of TCP/IP model:

Application Layer

This layer is same as that of the OSI model and performs the following functions: It provides different services such as manipulation of information in several ways,

retransferring the files of information, distributing the results etc. The functions such as LOGIN or password checking are also performed by the

application layer.

Protocols used: TELNET, FTP, SMTP, DN, HTTP, NNTP are the protocols employed in this layer.

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Transport LayerIt does the same functions as that of transport layer in OSI model. Here are the key points regarding transport layer:

It uses TCP and UDP protocol for end to end transmission. TCP is reliable and connection oriented protocol. TCP also handles flow control. The UDP is not reliable and a connection less protocol also does not perform flow

control.Protocols used: TCP/IP and UDP protocols are employed in this layer.

Internet Layer

The function of this layer is to allow the host to insert packets into network and then make them travel independently to the destination. However, the order of receiving the packet can be different from the sequence they were sent.

Protocols used: Internet Protocol (IP) is employed in Internet layer.

Host-to-Network Layer

This is the lowest layer in TCP/IP model. The host has to connect to network using some protocol, so that it can send IP packets over it. This protocol varies from host to host and network to network.

Comparison of OSI Reference Model and TCP/IP Reference Model

Following are some major differences between OSI Reference Model and TCP/IP Reference Model, with diagrammatic comparison below.

OSI(Open System Interconnection) TCP/IP(Transmission Control Protocol / Internet Protocol)

1. OSI provides layer functioning and also defines functions of all the layers.

1. TCP/IP model is more based on protocols and protocols are not flexible with other layers.

2. In OSI model the transport layer guarantees the delivery of packets

2. In TCP/IP model the transport layer does not guarantees delivery of packets.

3. Follows horizontal approach 3. Follows vertical approach.

4. OSI model has a separate presentation layer 4. TCP/IP does not have a separate presentation

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layer

5. OSI is a general model. 5. TCP/IP model cannot be used in any other application.

6. Network layer of OSI model provide both connection oriented and connectionless service.

6. The Network layer in TCP/IP model provides connectionless service.

7. OSI model has a problem of fitting the protocols in the model

7. TCP/IP model does not fit any protocol

8. Protocols are hidden in OSI model and are easily replaced as the technology changes.

8. In TCP/IP replacing protocol is not easy.

9. OSI model defines services, interfaces and protocols very clearly and makes clear distinction between them.

9. In TCP/IP it is not clearly separated its services, interfaces and protocols.

10. It has 7 layers 10. It has 4 layers

Diagrammatic Comparison between OSI Reference Model and TCP/IP Reference Model

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