Transport layer
The transport layer is responsible for transportation of data whether it is reliable or unreliable.
Segmentation of data takes place at the Transport layer, multiplexing of upper layer applications, and a means
to conceal network dependant information from the upper layers are also provided here.
The most important part to remember here, is that reliable connection oriented transportation is provided, and
it is done with TCP.
TCP at the Transport layer provides:
a) Three way handshake
b) Acknowledgements
c) Synchronization numbers
d) Error correction
e) Flow control (Windowing)
Network layer -
The primary responsibility of the network layer is to determine the best path and provide packet switching. (Packet switching is the means to make a packet from one interface to another). Reliability is not a function of the Network layer, it is provided by the Transport layer with TCP.
The network layer also takes the segments from the Transport layer, ads the IP header information for the source and the destination and sends them to the Data Link layer as Packets.
Data link layer
The Data Link layer translates Network information into frames (groups of bits) for the physical layer for transmission across the network.
The Data Link layer has 2 sublayers, MAC and LLC. This is where the source and the destination hardware address is added. Then the Packets from the Network layer are then Framed.
Data Link layer sublayers
- LLC (Logical Link Control) Sublayer 802.2
Defines whether the service is connection oriented or connectionless, provides error (detection) control, and frame synchronization of bits for the Physical layer (flow control).
- MAC (Media Access Control) Sublayer 802.3
Media access defines how transmission media is accessed, and moving data on the physical topology. The physical (MAC) ID is added to each Data Link Frame header and identifies the source and destination addresses.
Key terms; media access, logical and physical topology.
Physical - The Physical layer takes the framed bits from the Data Link layer and converts them into either electrical or light impulses for the Physical topology. At Physical layer, the interface between the DTE and DCE is determined.
Some of the standard interfaces that are defined at the Physical layer are:
EIA/TIA-232, EIA/TIA-449, V.24, V.35, X.21, G.703, HSSI (High Speed Serial Interface).
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