Network Models Network uses a combination of hardware and software to send data from one location to another Performing a task is performed on different.

Network ModelsNetwork Models

Network uses a combination of hardware and software to send data from one location to another

Performing a task is performed on different layers [Higher ---> Lower]

LAYERED TASKS Concept of layers in our daily life

– Two friends communicate through postal mail

Layered TasksLayered Tasks

Layered TasksLayered Tasks

Hierarchy Task must be done in the order given in the hierarchy

– Sender site from up to down– Receiver site from down to up

Services Sender site each layer uses the services of the layer immediately

below it

Higher Middle Lower Carrier

The OSI ModelThe OSI Model

1947 established ISO : International Standards Organization ISO model covers network communications is OSI : Open System

Interconnection Introduced in the late 1970s. Open system: a set of protocols that allows any two different systems

to communicate regardless of their underlying architecture. Purpose of the OSI : to show how to facilitate communication between

different systems without requiring changes to the logic of the underlying hardware and software.

OSI model not a protocol, it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable.

ISO is the organization. OSI is the modelISO is the organization. OSI is the model

THE MODELTHE MODEL

Is a layered framework, for design of computer systems that allows communication across all types of computer systems

Consists of seven layers each defines a part of the process of moving information across network

Understanding of the fundamentals of the OSI model provides a solid basis for exploring data communications.

Seven layers of the OSI modelSeven layers of the OSI model

Layered Architecture Layered Architecture

When device A sent message to device B Message may pass intermediate nodes Only the first three layers of the model involve

Layered ArchitectureLayered Architecture

At single machine Each layer defines functions distinct from other layers Each calls upon the services of the layer below it

– Layer 3 uses services from layer 2– Layer 3 provides services for layer 4

Between Machines Layer x on one machine communicates with layer x on another

machine by protocols.– Communication is governed by an agreed-upon series of rules and

conventions called protocols– Process that communicate a given layer on one machine are called Peer-

to-Peer process– Communication between machines is therefore peer-to-peer process using

the protocols appropriate to a given layer

Peer-to-Peer process Peer-to-Peer process

On the sending machine Each layer in sending device

– adds its own information to the message it receives from the layer just above it

– pass the whole package to the layer below it– At layer 1 the message converted to a form that can be transferred

to the receiving machine

On receiving machine– Message is unwrapped layer by layer

layer 2 removes the data meant for it, then passes the rest to layer 3 and so on.

Interfaces Between LayersInterfaces Between Layers

Between any pair of adjacent layers sending and receiving done by interface

Each one defines what information and services must provide for layer above it.

Well-defined interfaces and layer functions provide modularity to a network ( could modified or replace without affect surrounding )

Organization of the LayersOrganization of the Layers Seven layers could be belonged to three subgroups Network support layers: layers 1,2 and 3

– deal with physical aspects of moving data from one device to another Such as electrical specifications, physical connections, physical addressing,

and transport timing and reliability User support layer: layer 5, 6, 7

– It allows interoperability among unrelated software systems Layer 4, transport layer

– It links the two subgroups– Ensures that what the lower layers transmitted is in a form that the upper layers can

use The upper OSI layers are implemented in software Lower layers implemented in hardware and software except for the physical

layer which is mostly hardware.

Organization of the LayersOrganization of the Layers

Organization of the LayersOrganization of the Layers

Each layer adds a header to data except layer 2 adds also trailer When data passes physical layer (layer 1) changed into an

electromagnetic signal and transported through physical link When reaching destination, the signal passes into layer 1 and it

transformed back into digital form. When data reaches the next higher layer

– header and trailers corresponding sending layer are removed– Action appropriate to that layer are taken

In layer 7, message will be in appropriate format ( application )

The physical layer is responsible for movements ofindividual bits from one hop (node) to the next.

Note

Physical Layer Physical Layer

The physical layer is responsible for transmitting individual bits

from one node to the next

Physical LayerPhysical Layer

The major duties of this layer are: Physical characteristics of interfaces and medium :

– Defines : characteristic of the interface between the devices and the

transmission medium Type of transmission medium

Representation of bits : Defines the type of encoding (how 0s and 1s are changed to signals: electrical or optical)

Data rate ( transmission rate ) : number of bits sent each second (duration of a bit)

Synchronization of bits : synchronized at bit level of sender and receiver

Physical LayerPhysical Layer

Line configuration: is concerned with the connection of the devices to the media– Point-to-point configuration– Multipoint configuration

Physical topology: defines how devices are connected to make a network. – Mesh, star, ring, bus, or a hybrid. – Hybrid is a combination of two or more topologies

Transmission mode: defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex.

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

Note

Data Link LayerData Link Layer

Data Link LayerData Link Layer

The data link layer is responsible for transmitting frames from one node to the next

Framing : divided bits into manageable data units called frames Physical addressing : If frames distributed

– On same the network, header (physical address of sender and receiver) added to frame

– On different networks, receiver address is the address of device that connects networks

Flow control : controls transmission rate between sender and receiver Error control : Link layer adds reliability to the physical layer by adding trailer

to frame :– Mechanisms to detect and retransmit damaged or lost frames– Mechanisms to prevent duplication of frames

Access control : which device has control over at any given time

Hop-to-hop (node-to-node) deliveryHop-to-hop (node-to-node) delivery

ExampleExample

A node with physical address 10 sends a frame to a node with physical address 87

Frame contains physical address in the header

Rest of header contains other information needed at this level

The trailer contains extra bits needed for error detection

Note

The network layer is responsible for the delivery of individual packets from

the source host to the destination host.

Network LayerNetwork Layer

Responsible for the source-to-destination delivery across multiple networks Ensures that each packet gets from its point of origin to its final destination No need for network layer if systems on the same networks

Network LayerNetwork Layer

Duties of the network layer:– Logical addressing : physical address locally but for universal – A header added to the packet include logical address of sender and

receiver– Routing : internetwork connected by devices ( routers or gateways ) which

route or switch the packets to their final destination, network layer provide this mechanism

SourceSource--toto--destination deliverydestination delivery

ExampleExample

Send from A to P ( network address ) or from 10 to 95 ( physical address )

Different networks

Two addresses required

Network address is universal address

Network address = logical address

Logical address remain the same from source to destination ( A and P )

Physical address changes

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

Note

Transport LayerTransport Layer

Responsible for process-to-process delivery of the entire message Compare with Network layer

– Delivery of individual packets– Does not recognize any relationship between those packets

Ensures that whole message – Intact: not changed or broken– In order ( manage error control + flow control)

Transport LayerTransport Layer

Transport LayerTransport Layer Service-point addressing: transport layer gets the entire message to the correct process

on that computer (from specific process [ running program ] on one computer to a specific process [ running program ] on the other.

– Transport layer adds header called a service—point address (port address)– Network layer gets each packet to the correct computer– Transport layer gets the entire message to the correct process on that computer

Segmentation and reassembling: the message is reassembled– Correctly upon sequence number– Identify and replace packets that lost in the transmission

Connection control : either – connectionless : each packets treat independent and delivers it to destination– connection-oriented : make connection first then delivers packets then terminated

Flow control: not on single link (data link) But end to end. Error control: not on single link? But process-to-process

– Sending transport layer ensures message arrives at transport layer in receiving without error (damage, loss, or duplication)

– error correction is achieved by retransmission.

Reliable process-to-process Reliable process-to-process delivery of a messagedelivery of a message

The session layer is responsible for dialog controland synchronization.

Session LayerSession Layer The session layer is the network dialog controller. IT establishes, maintains, and synchronizes the interaction among

communicating systems.

Responsibilities: Dialog control:

– Allows two system to enter into a dialog– Allows the communication between two processes to take place in:

Half-duplex or Full-duplex

Synchronization:– Allows process to add checkpoints (synchronization points) to a stream of

data– Example : add checkpoint every 100 pages for sending 2000 pages – If crash happens during the transmission of page 523 retransmission began

at page 501.

Session LayerSession Layer

The presentation layer is responsible for translation, compression, and encryption.

Presentation LayerPresentation Layer

Concern with the syntax and semantic of the information exchanged between two systems

Presentation LayerPresentation Layer

Responsibilities Translation : Information should change to bits stream before transmit

– Working with different encoding methods ( different computers)– Presentation layer is responsible for interoperability between these

different encoding methods. – At sender

Change information ( sender-dependent format to common format) – At receiving

Change information (common format to receiver-dependent format) Encryption

– Encryption at sending and Decryption at receiving Compression :

– Data compression reduces the number of bits to be transmitted– Compression needs in transmitted multimedia

The application layer is responsible for providing services to the user.

Note

Application layerApplication layer

Enables the user (human or software) accessing the network It provides user interfaces and services ( electronic mail, remote file access

and transfer, World Wide Web)

Application layerApplication layer

Mail services : the basis for e-mail forwarding and storage

File transfer and access: lets user to access file in a remote host– Change or read– Retrieve files – Manage or control files

Remote log-in: log into a remote computer and access the resources Accessing the WWW

Summary of LayersSummary of Layers

TCP/IP PROTOCOL SUITETCP/IP PROTOCOL SUITE Invented before OSIInvented before OSI The layers in the The layers in the TCP/IP protocol suiteTCP/IP protocol suite do not exactly match those in the do not exactly match those in the

OSI model. OSI model. The original TCP/IP protocol suite was defined as having four layers: The original TCP/IP protocol suite was defined as having four layers:

– host-to-networkhost-to-network equivalent to equivalent to physical + data link physical + data link – internetinternet, equivalent to , equivalent to networknetwork – transporttransport, equivalent to , equivalent to part of duties of sessionpart of duties of session– applicationapplication. equivalent to . equivalent to session + presentation + applicationsession + presentation + application

However, when TCP/IP is compared to OSI, we can say that the TCP/IP However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: protocol suite is made of five layers:

– physicalphysical, , – data linkdata link, , – networknetwork, , – transporttransport, , – applicationapplication..

TCP/IP PROTOCOL SUITETCP/IP PROTOCOL SUITE

The first four layers provide functions that correspond to the first four layers of the OSI model:

– Physical standers– Network interfaces– Internetworking – Transport functions

The three topmost layers functions in the OSI model are represented in TCP/IP by application layer

TCP/IP and OSI modelTCP/IP and OSI model

TCP/IP PROTOCOL SUITETCP/IP PROTOCOL SUITE TCP/IP is a hierarchical protocol made up of interactive modules Each layer in TCP/IP provides specific function The modules are not necessarily interdependent, but in OSI module specifies

which functions belong to each of its layers. Each layer has independent protocols TCP/IP is a hierarchical protocol : each upper-level supported by one or more

lower-level protocols depending on the needs of the system. AT transport layer, TCP/IP defines three protocols

– Transmission Control Protocol (TCP)– User Datagram Protocol (UDP)– Stream Control Transmission Protocol (SCTP)

At network layer, the main protocol defined by TCP/IP is the Internetworking Protocol (IP)


Physical and Data Link Layers– TCP/IP does not define any specific protocol but it supports all the

standard protocols.– A network in TCP/IP internetwork can be local or a wide-area network

Network Layer– At Internetwork layer, TCP/IP supports the Internetworking Protocol.

IP uses four supporting protocols: ARP RARP ICMP IGMP


Internetworking Protocol (IP)– It is the transmission mechanism used by the TCP/IP protocols.– It is unreliable and connectionless protocol (a best-effort delivery service).

A best effort: IP provides no error checking or tracking. IP assumes the unreliability of the underlying layers and does its best to get a

transmission through to its destination, but with no guarantees.– IP transport data in packets separately called datagrams.– Datatgrams can travel along different routes and can arrive out of

sequence or be duplicated. – IP does not keep track of the routes and has no facility for reordering

datagrams when they arrive their destination.– IP provides bare-bones transmission service that free the user to add only

those facilities necessary for a given application and thereby allows for maximum efficiency. (covers the weakness in its functionality)


Address Resolution Protocol– Associated a logical address with a physical address.– On a typical physical network, LAN, each device is identified by a

physical or station address, usually imprinted on the network interface card (NIC).

– ARP is used to find the physical address of the node when its Internet address is known.

Reverse Address Resolution Protocol– RARP allows a host to discover its Internet address when it knows only its

physical address. – RARP is used when a computer is connected to a network for the first

time or when a diskless computer is booted.


Internet Control Message protocol– ICMP is a mechanism used by a host and gateways to send notification of datagram

problems back to the sender.– ICMP sends query and error reporting messages.

Internet Group Message Protocol– IGMP is used to facilitate the simultaneous transmission of a message to a group of

recipients.

Transport Layer– Traditionally the transport layer represented in TCP/IP by TCP and UDP .– IP is host-to-host protocol, deliver a packet from one physical device to another. – UDP and TCP are transport level protocols responsible for delivery of a message

from a process to another process.– SCTP, has been devised to meet the needs of some newer applications.


User Datagram Protocol– UDP is the simple of the two standard TCP/IP transport protocol.– UDP is process-to-process adds:

port addresses Checksum error control Length information to the data from the upper layer

Transmission Control Protocol– TCP provides full transport-layer services to applications.– TCP is a reliable stream (connection-oriented) transport protocol.– At sending end of each transmission:

TCP divides a stream of data into smaller units called segments Each segment includes a sequence number for recording after receipt An acknowledgment number for the segments received Segments carried across the internet inside of IP datagrams.

– At receiving end TCP collects each datagram as it comes in and reorders the transmission based on

sequence numbers


Stream Control Transmission Protocol– SCTP provides support for newer applications as voice over the Internet.– SCTP is a transport layer protocol that combines the best features of UDP

and TCP.

Application Layer– Application layer in TCP/IP is equivalent to the combined session,

presentation, and applications layers in the OSI model.– Many protocols are defined at this layer.

ADDRESSINGADDRESSING

Four levels of addresses are used in an internet employing the TCP/IP Four levels of addresses are used in an internet employing the TCP/IP protocols:protocols:


Each address is related to a specific layer in the TCP/IP architecture.


Physical Addresses– It is known as the link address.– It is the address of a node as defined by its LAN and WAN.– It is included in the frame used by the data link layer.– It is the lowest-level address.– It has authority over the network (LAN or WAN).– The size and format of these addresses vary depending on the network.

The Ethernet uses a 6-byte (48 bit) physical address that is imprinted on NIC. LocalTalk (Apple) has 1-byte dynamic address.


In Figure 2.19 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.


As we will see in Chapter 13, most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below:

07:01:02:01:2C:4B

A 6-byte (12 hexadecimal digits) physical address.


Logical Addresses– It is necessary for universal communications.– Physical addresses are not adequate in an internetwork environment where

different networks can have different address format.– A universal addressing system is needed in which each host can be

identified uniquely, regardless of the underlying physical newtwork.– A logical address in the Internet is currently a 32-bit address.– No two publicly addressed and visible hosts on the Internet can have the

same IP address.

IP AddressesIP Addresses

Figure 2.20 shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection.

Port AddressesPort Addresses

The end objective of Internet communication is a process communicating with another process.

In TCP/IP architecture, the label assigned to a process is called a port address. A port address in TCP/IP is 16 bits in length.


Figure 2.21 shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination

AddressesAddresses

The physical addresses will change from hop to hop,but the logical addresses usually remain the same.


753

A 16-bit port address represented as one single number.

As we will see in Chapter 23, a port address is a 16-bit address represented by one decimal number as shown.

Specific AddressesSpecific Addresses

E-mail address

Universal Resource Locator (URL)

Network Models Network uses a combination of hardware and software to send data from one location to another Performing a task is performed on different.

Documents

layer layer

given layer slide

layer functions

unwrapped layer

osi model slide

machines layer x

surrounding slide

machine message