Link Layer
Link layer has responsibility of transferring datagram from one node to physically adjacent node over a link.
Two types of links:
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Point to point
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Point-to-multipoint: shared medium
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Link Layer: To move a packet from one node (host or router) to the next node in the route, the network layer relies on the services of the link layer. The services provided by the link layer depend on the specific link-layer protocol that is employed over the link. For example, some link-layer protocols provide reliable delivery, from transmitting node, over one link, to receiving node. Examples of link layer protocols include Ethernet, WiFi, and the Point-to-Point Protocol (PPP). As datagrams typically need to traverse several links to travel from source to destination, a datagram may be handled by different link-layer protocols at different links along its route. A link-layer data unit is a frame.
Context
- layer-2 data unit: frame, encapsulates datagram
- datagram transferred by different link protocols over different links:
- e.g., WiFi on first link, Ethernet on next link
- each link protocol provides different services
- e.g., may or may not provide reliable data transfer over link
Transportation analogy
- trip from Waterloo to Lausanne
- limo: Waterloo to YYZ
- plane: YYZ to Geneva
- train: Geneva to Lausanne
- tourist = datagram
- each transport leg = communication link
- transportation mode = link-layer protocol
- travel agent = routing algorithm
Possible services / key responsibilities
- Framing, link access (compulsory)
- error detection (not even compulsory)
- error correction
- reliable delivery between adjacent nodes
- flow control