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 Interconnection (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. Application Layer Transport Layer or Host to host Internet Layer or Network Layer Network Access Layer or Data Link Layer 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 layers Host A Router Host B Host C Host D LAN-1 LAN-2 DLL Network

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 control Adds 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) or Network 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 Network R R R R N/W Layer N/W Layer

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 Routing switchin or routing the packets to the final destination its 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 layer it 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 Layer It 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 Layer Its 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 formatreceiver 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