Layer Responsibilities

Physical Layer

transforms a sequence of bits into signals for transmission.

• The signal will be different for different communication media.
• E.g., dial-up phone, dial-up cellular phone, fiber optic LAN, Ethernet.
• The details of communicating over the available communication channel (link) are transparent to higher layers.
• This function is normally implemented in hardware.

Data Link Layer

is responsible for orderly access to the communication link.

• The CSMA/CD protocol in the ethernet is an example of a data link layer protocol.
• On point-to-point links, the data link layer marks the beginning and end of packets.
• It provides the ability to transmit packets of bits from one host to the next.

Network Layer

facilitates the exchange of packets between to remote hosts.

• For that purpose, it provides the ability to route packets.
• I.e., at each intermediate host (router) a decision is made on which link the packet should be forwarded.

Transport Layer

reassembles packets into a sequence of information bits.

• The transport layer must be able to cope with an unreliable network layer.
• Most importantly, packets may be lost or arrive out of sequence.