LECTURE 4

TCP/IP and OSI model

It’s not necessary to implement all 7 layers for every protocol. Usually many OSI layers are lumped together. TCP/IP is a good example for this case.

TCP/IP has 4 layers:

    1.  Network Access: Associated with physical layer + data link + some of the Network layers
            a. Dealing with the exchange of data between end system ( server, station ) and the network which attached.
            b. The software at this layer depends on the type of network to be used  ( X.25, Ethernet , etc.. ) but the application above this layer don’t have to know about the network underneath.
            c. Responsible for end-to end system on the same network ( used MAC address or physical address)
            d. The smallest unit at this level is packet.

   2.  (IP) Internet: The rest of the network layer.
         - Routing the data to different networks ( by routers )
         - The smallest unit at this level is IP Datagram
         - IP Datagram header composes of:
                * Destination subnetwork host IP address
                * Facilitesi request : Priority, etc…
                * Somemore field here

   3. (TCP) Host-to-host layer:

         Associated with the transport  layer and some of the session layers. Provide a reliable mechanism for data exchange  between 2 hosts.  The smallest unit at this level is TCP segment.
        The TCP header composes of:
            - Destination port
            - Sequence number
            - Checksum
            - Some more field here

   4. Process :  Include the rest of the OSI model.
        Some popular processes are Email, Telnet, FTP, Web Browser. 

Note: Packet, IP Datagram, TCP segment are called PDU ( Protocol data unit)

 LLC and MAC

The data link under IEEE 802 composes of : 

LLC:
   Provide one or more service access points (SAP), thus multi-access, shared medium nature of the link can be supported.
     Relieved some details of the link access by the MAC sub-layer underneath.
     The same LLC can support many MAC protocol.

MAC:
     On transmission, assemble the data into a frame with address and  error detection field.
     On reception, disassemble  the frame and perform address recognition and error correction.
     Govern the access to the LAN transmission media.

LLC services

     1.  Connectionless service:
            - Not required to establish a logical connection.
            - Good for highly interactive traffic.
     2. Connection-oriented:
            - Good for a reliable data exchange services.
     3. Multiplexing:
            - A single physical link should be able to provide data transfer with multiple logical endpoint on that link. 

Addressing

 IP address:  Identify a physical interface on the Internet. This is the universal address for Internet, recognize by the routers.

MAC address (is NIC address most of the time)
        - Identify a physical interface from a station to a  LAN.
        - A single station may have more than one MAC address ( for ex: Dual-home server  , proxy server).
        - MAC address only good for a single network  or sub-network.,.
        - Recognize by the bridge.

LLC address ( or SAP address )
        - Identify an LLC user. The LLC address is associated with a particular user within a station.
        - In some cases, the SAP refers to a process executing on the station. In some cases the SAP refers to a hardware port.

Note:
        -Group address ( multicast)
        -Broadcast address ( every station on the subnet)

MAC ( Medium access control)

 MAC governs the access to the transmission medium for several stations sharing the  link nicely.

Central control:
        - Simple, fairly distributed bandwidth
        - No need a great deal of coordination among stations

Disadvantage:
        - Single point of failure.
        - May become a bottle-neck, reducing performance if the stations are very fast.

Distributed control:
        - More complicated to implement
        - Required a great deal of coordination among stations.
        - No single point of failure.
        - No bottle-neck (unless you have a very slow router)

Synchronous:

          - FDM, TDM, circuit switching
          - Not good in WAN, LAN due to the unpredictable nature of network.

 Asynchronous

 A. Round-robin( token ring )   -Polular
            Control may be centralized (polling) or distributed  ( token ring ) 

B. Reservation
        - Good for stream (voice, audio/video on the net )
        - Time on the medium divides into slot like TDM.
        - Can be centralized or distributed 

C.Contention:   -Popular
        - Good for bursty traffic, simple hardware
        - Ethernet LAN is a good example.
        - Good for moderate load, the network collapses under heavy load.  

The formula of Ethernet-Internet success:
        - Cheap UTP + Cheap NIC card + Free TCP/IP + Demand of WWW contents : Information, entertainment , E-commerce ,  Internet Phone , etc…

 Bridge

        - Working at the MAC layer.
        - All devices use the same protocol, very little processing is required. 

Why we need the bridge:
        - Reliability
        - Performance
        - Security
        - Geography 

Note:
        - The bridge make no modification on the frame, just repeat it.
        - Must has the addressing differentiation and routing intelligence.
        - A bridge may connect more than one LAN.

 Router

        - Working at layer 3 of the OSI model or the IP layer in TCP/IP.
        - It can connect dissimilar networks together ( for ex: Ethernet and Token ring )
        - Router must be able to deal with a variety of differences among networks

                a. Addressing scheme:  Different MAC addresses
                b. Maximum frame size:  Segmentation the packet into smaller size. Example: X.25 and Ethernet
                c. Independent of software + hardware interface, network reliability.