MAKERERE UNIVERSITY College of Computing and Information Sciences School of Computing and...

MAKERERE UNIVERSITY College of Computing and Information Sciences School of Computing and Informatics Technology

BIT 1102: COMMUNICATIONS BIT 1102: COMMUNICATIONS TECHNOLOGY AND THE INTERNETTECHNOLOGY AND THE INTERNET

7. 7. Introduction to ISO/OSI ArchitectureIntroduction to ISO/OSI Architecture

Introduction

Open system interconnection (OSI)model is a reference model developed by international organization for standardization(ISO) in 1984, as a conceptual framework of standards for communication in the networks across different equipment and applications by different vendors.

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BIT 1102: COMMUNICATION TECHNOLOGY AND THE INTERNET

The OSI model defines the communication process into seven layers which divides the tasks involved with moving information between networked computers into seven smaller ,more manageable task groups.

A task is then assigned to each of the seven OSI layers.

3BIT 1102: COMMUNICATION TECHNOLOGY AND THE INTERNET

Each layer is responsibly self-contained so that the tasks assigned to each layer can be implemented independently.

This enables the solutions offered by one layer to be updated without adversely affecting the other layers.


Networking Architecture

Architecture is essentially a set of rules that describes the function of some portion of the hardware and software that constitute a stack of layers.

A networking architecture is designed to implement the functions associated with a particular contiguous set of layers of the OSI Reference Model, either formally or informally.


Networking Layers

A layer is a collection of related functions that provides services to the layer above it and receives services from the layer below it.

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The OSI Model Layers

The seven layers of the OSI reference model can be divided into two categories:

a) Lower layers

The lower layers of the OSI model handle data transport issues. Also called Media layers

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The OSI Model Layers

b) Upper 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 layer processes interact with software applications that contain a communications component.

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Layer 1 – The Physical Layer

Provides the physical interconnections through the network hardware.

This layer deals with the physical plugs, sockets and electrical specification of signals. This is the medium over which the digital signals are to be transmitted.

Provides low level services involving the transmission of raw bits

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Layer 1 – The Physical Layer cont’d

Involves twisted pair, coaxial cable, optical fiber, wireless, Network adapters, transmitters, repeaters or other media.

Physical layer specifications define. characteristics such as voltage levels, timing of voltage changes, physical data rates, Maximum transmission distances, and physical connectors.

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Main Functions of the physical Layer

Establishing and terminating the physical connections to the network.

Converting “raw bits”, (Zeros and ones) into the respective communication channel energy such as Light, electrical signals (Known as data encoding)

Guides transmission and reception of data.Perform services requested by the Data Link

Layer

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Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer- MAC address.

Packages raw bits from the Physical layer into frames

Layer 2 – The Data-Link Layer14


Layer 2: Data Link Layer

• Mainly concerned with reliable transmission of data including detecting and correcting errors that may have occurred at the physical layer

• This Layer provides low level services involving the transmission of raw bits

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Main Functions of the Data Link Layer

Provides low level error detection and control using Techniques like CRC – Discussed in details at an advanced level.

Media Access control sub-layer controls access to the shared medium to avoid conflict (for example what does the Ethernet protocol use to control Media access?)

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Main Functions of the Data Link Layer cont’d

The Data Link Layer responds to service requests from the Network layer and sends requests to the physical layer.

Responsible for “Framing of data” i.e. conversion of data into information understandable by different protocols.

Basic Addressing using MAC address(hardware address) enables frames to be forwarded to the destination.

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Defines 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, Eg if a frame fails to get to its destination e.g. when a wrong

destination address is given.

Tasks of the s/w at layer 2 are; Flow control, Error notification, Generating frames

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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.

Two most common network devices exist at the second layer of the OSI model: Bridges and switches.

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Common data Link Layer H/w and protocols

EthernetATM (This Uses fixed packets in the

transmission)FDDI

More Hardware includes: Switch, intelligent Hub,NIC (more advanced NICs handle both

physical and DLL functions)

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This layer responds to service requests from the transport layer and issues service requests to the data link layer

The network layer defines the network address(IP address), which differs from the MAC address.

Layer 3 – The Network Layer21


Layer 3 – The Network Layer cont’d

IP addresses are written as four dot-separated decimal numbers between 0 and 255, e.g., 192.168.2.1. (Assigned by either a DHCP or Manually configured.)

Handles the addressing and delivery of data- This aids routers in finding the destination.

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Functions of the Network LayerWhile the functions of the Physical and

Data Link Layer are local (they are concerned with the devices that are connected to each other), the Network layer defines functions that involve the complete transmission of data across inter connected networks.

The Primary function of the Network layer is the delivery of the data from the source to the destination.

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Functions of the Network Layer

Logical address also known as layer 3 address for example on the internet, an IP address is implemented at the network layer.

Ensures that the data packets are transmitted to the destination over the best possible route by the process known as routing. Also involved in breaking and re-assembling data packets.

Attaches network headers , which contain information about the destination address, to the data frames to make them “datagrams” or data packets that can be delivered across networks.

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Common Network Layer Protocols

The most well-known examples of layer-3 protocols are: Internet Protocol (IP), • Internet Control Message Protocol (ICMP), • Internetwork Packet Exchange, X.25 (IPX), • Asynchronous Transfer Mode(ATM).• Data gram Delivery Protocol.

Network Layer hardware include:Router (Recall router definition)Brouter, ATM Switch (Used to handle ATM Protocol)

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The major responsibility of this layer is data integrity- ensures that data transmitted between hosts is reliable and timely.

Note: The sent data is expected not to be altered before the destination.

Act as an Intermediary or a go-between layers 1-3 and the 5-7 to ensure delivery of data to the right application.

Layer 4 – The Transport Layer26


Functions of the Transport layerGenerally ensures reliable end-to-end data

delivery.Completes the Network Layer services, which

only cater for “best effort” communication by enhancing quality of service. (QoS) and ensuring accurate data delivery (as in the case of TCP).

However, some layer 4 protocols like User Data Protocol (UDP) don't ensure reliable transmission.

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Layer 4 – The Transport Layer cont’d

Provides connection Services ie “connection oriented” and “connection-less” where the former requires a logical link to be established before transmission occurs and terminated after.

Functions of the Transport layerConnection Control -The layer can offer connectionless

or connection oriented services. With connectionless transmission, the layer treats each Segment as an independent packet and delivers it to the transport layer at the destination.

With connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data has been transmitted the connection is terminated.

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Functions of the Transport layer

Flow control.Error ControlSegmentation 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 during transmission.

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Transport layer looks to “see” if data is coming from more than one application and integrates each application's data into a single stream for the physical network

Main Transport layer protocols: TCP, UDP, TLSUser Datagram Protocol (UDP)

a connectionless protocol that, like TCP, runs on top of IP networks.


Unlike TCP/IP- UDP/IP provides very few error recovery services, offering instead a direct way to send and receive datagram over an IP network.

Transmission Control Protocol (TCP)TCP keeps track of the packet delivery order and the packets that must be resent (guaranteed delivery - i.e. we have to ensure data has no errors)


The primary function of the session layer is to establish, manage and terminate a “session”.

A session generally refers to a logical link between software applications.

Layer 5 provides for Duplex communication that can be either simplex, half duplex or full Duplex.

Session Layer Protocols include NETBIOS , ASP (Apple Talk Session Protocol), SCP

(Session Control Protocol), ZIP (Zone Information Protocol), etc.

Layer 5 – The Session Layer32


Establishes, maintains and ends / terminates communication with the receiving device.

Provides the mechanism for managing the dialogue between end-user application processes.

Layer 5 – The Session Layer33


Layer 6 – The Presentation Layer

The Presentation Layer is primarily concerned with translating data with different formats for “presentation” to the application layer.

For example the network may have computers running UNX, Windows, MAC OS or data encoded in ASCII Or EBCDIC etc like GIF,JPEG, etc . The presentation layer caters for differences in these formats.

Note: In practice, the application layer may be able to handle these differences.

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Takes the data provided by the Application layer and converts it into a standard format that the other layers can understand (On sending).

Its responsible for defining the syntax (language rules)

Layer 6 – The Presentation Layer35


Encryption and compression- this allows applications to agree on standardized representations for data, like American Standard Code for Information Interchange- ASCII; codes that represent text in computers, communications equipment, and other devices that work with text


Directly interfaces with the Application or service that is using the network.

Examples include Email software, or requesting an Ms power point (.ppt) file that is stored on a server across a Network.

Common Application Layer Protocols include,

FTPTELNETDNS (Discussed in details under the Internet)

Layer 7 – The Application Layer37


This is the layer that actually interacts with the operating system or application whenever the user chooses to transfer files, read messages or perform other network-related activities

Responsible for providing end-user services, such as file transfers, electronic messaging, e-mail, virtual terminal access, and network management. Note:

This is the layer with which the user interacts.

Layer 7 – The Application Layer38


Makes sure that the other party is identified and can be reached.

If appropriate, authenticates either the message sender or receiver or both

Ensures agreement at both ends about error recovery procedures, data integrity, and privacy

Determines protocol and data syntax rules at the application level.

Protocols that are commonly used and exist at layer 7 of the OSI model are

HTTP (Web requests), FTP (File Transfer Protocol),


What goes on when data passes from Host A to Host B

1. The application ,presentation and session layer take user input and converts it into data.

2. The transport layer adds a segment header converting the data into segments.

3. The network layer adds a network header and converts the segments into packets /datagram

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4.The data link layer adds a frame header converting the packets /datagrams into frames

5. The MAC sub layer converts the frames into bits which the physical layer can put on the wire.

The above steps are known as the 5 steps of data encapsulation.


When the bit stream arrives at the destination ,the physical layer takes it off the wire and converts it into frames ,each layer will remove their corresponding header while the data flows up the OSI model until it is converted back to data and presented to the user.

This is also known as decapsulation.


READ MORE ON THE ENCAPSULATION AND DECAPSULATION CONCEPTS

Note:A handy way to remember the seven layers is the sentence "All people seem to need data processing." The beginning letter of each word corresponds to a layer.

All-----------------Application layer People-----------Presentation layer Seem -----------Session layer To-----------------Transport layer Need-------------Network layer Data--------------Data link layer Processing-----Physical layer


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