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Communications functions can be divided into subset activities
concept of layered communications
Each activity can be looked as a layer
A layer provides a specific service (function) to other layers
A layer has a specific protocol (control messages) to connect to other systems
IBM’s proprietary SNA
ISO’s Reference Model for Open Systems Interconnection (OSI model)
The OSI Model Lower Layer connects one node to another Upper Layer directly assist the user 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical Layer Function Network Choosing the next node and the link to it Data Link Controls the flow of messages on the chosen link Physical Connecting to the physical medium that provides the link Layer Function Application Provides services directly to an application program Presentation Presentation of information to user in a format that the user will understand Session Controls the user to user dialogue – its direction and synchronization Transport Raises the quality of service provided by the network to the level required by user
The Internet Protocol Suite Fiber Telnet DNS UDP TCP IP Ethernet Wi-Fi Co-ax HTTP Radio Application Transport Network Link Physical
TCP/IP Transmission Control Protocol/Internet Protocol
TCP/IP allows networks of different types to interconnect
The protocol field in the header information identifies which higher level TCP/IP protocol sent the data. When data arrives at its destination this field tells IP which protocol module to pass it on to.
The time-to-live (TTL) field, specifies how long the packet is allowed to reamin in the internet delivery system and is decremented by by every router that the packet passes through. When it reaches zero the packet is discarded and the sender. This prevents packets from traveling the Internet forever.
The checksum is an error detection checksum covering only the fields of the header.
Version Header length Service type Total length Identification Flags Fragment Offset Time to live Protocol Header checksum Source address Destination address IP Options Padding
Address Resolution Protocol (ARP) finds out the physical address corresponding to an IP address
When an IP Packet is received, an ARP request is brodcasted on the network. When a host recognizes an ARP request containing its own IP address, it sends an ARP reply containing its hardware address. This address is cached.
There is also a Reverse ARP (RARP) protocol. This is used by a host to find out its own IP address if it has no way of doing this except via the network.
Internet Control Message Protocol:
Defines the format of control messages that are sent to the sender indicating that a problem has occurred
TCP is a sliding window protocol , and does not wait for acknowledgement
To prevent overflow of receiver buffer:
An acknowledgement is sent containing the with the window size set to zero.
Later a windows update is sent, specifying the new window size.
TCP can specify an acknowledgement delay in the hope that within that time some data will need to be sent the other way, and the two can ride together
The Nagle algorithm allows a TCP segment containing less data than the receiver’s advertised window size can only be sent if the previous segment has been acknowledged. This allows aggregation of small amounts of data
Two different application one using UDP and another using TCP may use the same port number. The two data streams are distinguished by the protocol field in the IP address header
The checksum is optional.
Source Port Destination Port Message length Checksum
Unicast, Broadcast, Multicast Unicast One sender – one receiver Broadcast Sends data to all possible receivers Multicast Sends data to interested receivers Applications of IP Multicast: Pay TV File Transfer Financial Information But is complex to implement