OSI Model- Cont

™  Data Link Layer (Layer 2) o    Transforms physical layer which is raw transmission facility to a reliable link o    Responsible for Node to Node Delivery o    Makes physical layer look error free to the upper layer Figure

•   Functions of Data Link Layer o    Framing    The data link divides the stream of bits from Network layer into manageable data units called "FRAMES". This process is known as Framing. o    Physical Addressing    Frames need to be transmitted to different systems on a network    Data Link layer adds a HEADER to Frame    Header defines the physical address of sender(Source address) and/or receiver address (Destination address)    If frame is intended for a device outside the network, the receiver address is the address of the device that connects one network to the other o    Flow Control    Data Link layer imposes Flow Control mechanisms to prevent overwhelming the receiver

o    Error Control    Data link layer adds reliability to physical layer by adding mechanisms to detect and retransmit lost or damaged frames   Also uses a mechanism to prevent duplication of frames    Error Control bits are added to the form in the TRAILER o    Access Control    Two or more devices may be connected to a single link    Data link protocols are necessary to determine which device will have the control of the link at a given time ™   NETWORK LAYER ™ o    Responsible for Source-to-Destination delivery o    DL Layer oversees the delivery of data between 2 systems on the same network o    Network Layer ensures that each packet gets from its point of origin to its final destination    Node -to Node vs Source to Destination    If the two systems are connected to the same network, there is no need for Network layer and node -to node delivery is enough    If two systems are connected to two different networks, there is often a need for Source-to destination delivery •     Function of Network Layer o    Logical Addressing    Physical   addressing   implemented   by   Data   link   layer   handles   addressing problem locally    If a packet is going from one network to another, we need another addressing system to help distinguish source & destination systems    Network layer adds Header to the data coming from upper layers that among other things include LOGICAL ADDRESS of the sender and receiver o    Routing    When  independent  networks  or  links  are  connected  together  to  create  an "internetwork",   the   internetworking   devices   route   packets   to   their   final destination    Routers are those internetworking devices    One of the functions of Network layer is to define this route    Example Network Layer •We want to send data from a node with network address 'A' and physical address 10, located on one LAN to •A  node  with  network  address  P  and  physical  address  95  located  on another LAN •Because the two nodes are present on two different networks, we cannot use physical address only •We   need   a   Network   address   that   can   pass   us   from   the   Network boundaries •The packet therefore contains the logical address which remains the same from source to destination •The physical address will change when packet moves from one network to the other •The box with R is a Router