Data and Computer Communications

Protocols and Architecture

By Ruhul Amin

Need For Protocol Architecture• E.g. File transfer

—Source must activate comms. Path or inform network of destination

—Source must check destination is prepared to receive

—File transfer application on source must check destination file management system will accept and store file for his user

—May need file format translation

• Each problem needs to address separately • Task broken into subtasks• Implemented separately in layers in stack

Key Elements of a Protocol• Syntax

—Data formats—Signal levels

• Semantics—Control information—Error handling

• Timing—Speed matching (e.g., indicates flow control)—Sequencing (e.g., avoid packet duplication)

Protocol Architecture • Implementation of File Transfer

—File transfer example can have 4 problems—Implementation of “File Transfer” could use

three modules• File transfer application (Top most layer)• Communication service module (Middle Layer)• Network access module (Lower Layer)

Implementation of File Transfer Architecture

Module-1 Determine weather file transfer app at sys Y is ready to receive file?

Layer1 Determine weather file translation is required or not?

Module-2 Determine weather Computer Y is ready to receive the Data?

Module-3 Determine weather a direct link or an indirect link exists between X & Y

Open Systems Open Systems Interconnection Interconnection

(OSI)(OSI)

OSI Definition • Open Systems Interconnection

• Developed by the International Organization for Standardization (ISO)

• It has seven layers

• TCP/IP is the refinement of OSI

• Questions —Why TCP/IP doesn’t have presentation & session

layers?—How many layers a reference model should have ?

OSI – Advantage of 7 Layers Model• Each layer performs a subset of the required

communication functions— Sub tasks are assigned to one of the layers depending on the

nature

• Each layer relies on the next lower layer to perform more primitive functions— such as application layer relies on transport layer to send the

data and commands..

• Each layer provides services to the next higher layer— Such as Network layer offer services to Transport layer that can

be used at N/W-Layer SAP

• Changes in one layer should not require changes in other layers— Layers are entirely independent and transparent to other layers

The OSI 7-Layers functioning

OSI as Framework for Standardization

Services are well defined: (Service Definition)

Protocols are well defined:

(Protocol Specification)

Service Primitives and Parameters• Services need to be well defined by means of

service definition

• Services between adjacent layers expressed in terms of primitives and parameters

• Primitives specify function to be performed—Just like Op-code in Assembly

• Parameters pass data and control info—Just like Op-rand in Assembly

TCP/IP Protocol Architecture• TCP/IP was originally developed by the

US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET)

• 5 Layers of TCP/IP.1. Application Layer2. Transport Layer or Host to host3. Internet Layer or Network Layer4. Network Access Layer or Data Link Layer5. Physical Layer

1- Physical Layer• Responsible to deal with interface

between source & destination• Physical interface between data

transmission device (e.g. computers) and transmission medium or network

• Characteristics of transmission medium• Signal levels• Data rates• etc.

2- Network Access Layer or Data Link Layer

• Exchange of data between end system and the network• Responsible to deliver frames reliably from hop to hop (hop

could be DLL device such as bridges or switches)• If within the LAN, upper layers can leave reliability issues

(error/flow control) on the DLL• Guarantee Error Free Delivery of Data from one hop to

the other (not N/W to N/W OR not end-to-end)• DLL is divided into 2 sub layersDLL is divided into 2 sub layers

Network

Host A

Router

Host B

Host C

Router

Host D

LAN-1 LAN-2

DLL DLL DLL

Functions of Data link Layer• Framing Divides the stream of bits into manageable data units

called ‘frames’• Physical Addressing adds a header to the frame of sender and the

receiver(on the same network) For different network ….receiver address is the address

of the device that connects the network to the next node.

• Flow control data rate control…..(both sender and receiver should

have the same data rate for efficient data transmission)

Function of Data link Layer• Error controlAdds reliability to physical layer by adding

mechanism to detect and retransmit damaged or lost frames.

Detection of duplicate frames..Its done by adding a trailer to the end of the

frame.• Access control controls the access over the same

transmission link (Multi point layout of nodes)

3- Internet Layer (IP) orNetwork Layer

• Systems may be attached to different networks (such as Host-A at LAN-1 & Host-D at LAN-2)

• Lowest layer that deals with end-to-end transmission• Implemented in end systems and router• Guarantee Error Free Delivery of Packets from one

N/W to the other N/W (not end-to-end)

Host A

Router-1

Host B

Host C

Router-2

Host D

LAN-1 LAN-2

DLL DLL DLL

Network

N/W LayerN/W Layer

R

R

R

R

Functions of Network layer• Logical Addresssing its necessary to transmit packets out

side one network to another… adds a header to the packet coming

from upper layer(transport layer) ..includes a logical address of the sender and the receiver to the packet.

Function of network Layer• Routingswitchin or routing the packets to the final

destinationits usually done through router or

switches..Routing of packets is needed when data

communication is taken place in internetworks or larger networks.

Functions of Transport layerit does process to process delivery of

messages• Service point addressing Delivery of packets not from source

computers to destination computers but from particular process to intended process.

port addressing (correct process on the destination computer)

Functions of Transport Layer• Segmentation and Re-assembly Messege is divided into transmittable

segments. adds a number to the segment segment number helps the destinations

device to reassemble segments correctly upon arriving as well as to identify and replace segments that were lost in the transmission

Transport layer functions cont….• Connection Control connectionless or connection oriented connectionless all segments are independent. A

segment is transmitted as a single unit regardless of its process..

Connection Oriented connection is established first….then

transmission of data occurs.. After data is being transmitted connection is terminated

Functions of tranpor layer conti..• Flow control End to End flow control…..rather than a

single link…• Error Control process to process …rather than a single

link. Error control for damage,loss or

duplication of messeges….usually achieved through retransmission of messeges

Functions of Session LayerIt acts as dialogue

controller(Establishes,mantains and synchronize the interaction among communicating devices)

• Dialogue control allows the communicating devices to

enter into a dialogue communicating modes(half duplex….or

full duplex)

Functions of Session Layer..• Synchronization adds a check point to a stream of data. it adds flexibility to the data transmission

Functions of presentation LayerIts concerned with the syntax and semantics of the

information exchanged between the two communicating systems…

Translation Information is usually exchanged in the form of

string,number or others….these must be transformed into bit stream before being transmitted…

Different computer systems use different encoding techniques…

sender-dependent code common formatreceiver dependent format

Functions of Presentation Layer• Encryption provide privacy to the information sent by

converting the original information into another form..(sender side)

Decryption ….a process to reverse the encrypted informtion(receiver side)

• Compression Reduced the size of the data….particularly

in the transmission of tex, audio, and video

Functions of Application Layer• Network Virtual Terminal a software version of physical terminal…

allows users to log on to remote host. Software Emulation….logically connected

not physically..• File transfer,Access and Management Ftp….transfer of files between local and

remote computers and to manage or control files in romote computers locally

Functions of Application Layer• Mail Services… E-mail….SMTP,POP etc.• Directory Services provides distributed data base sources

and access for global information about various objects and services

OSI v TCP/IP

TCP and UDP• TCP (connection oriented Protocol)

—Establish a reliable logical connection between the source & the destination hosts

—Need to know the source & destination port addresses —Logical connection is monitored by TCP —TCP segments (TCP PDU) can be transmitted through the

secure connection —TCP is reliable but allow relatively slower communication

• UDP (Connection Less Protocol)—Is a connection less protocol—Does not guarantee the reliable transmission of UDP

segments—UDP header has limited control information—Faster but not reliable at all

• Which protocol should I use then (TCP or UDP ???)

TCP/IP Concepts

PDUs in TCP/IP

Some Protocols in TCP/IP Suite