41902 Lecture Notes - Lecture 12: Wireless Lan, Link Layer, Channel Access Method
Network Fundamentals
Chapter 6: The Link Layer and LANs
Chapter 6.1: Introduction, service
- Hosts and routers: nodes
- Communication channels that connect adjacent nodes along communication path links
- Wired links, wireless links, LANs
- 2-layer packet: frame, encapsulates datagram
- Data-link layer has responsibility of transferring datagram from one note to physically
adjacent node over a link
- Datagrams transferred by different link protocols over different links: e.g. Ethernet on
first link, frame reply on intermediate link, etc.
- Each link protocol provides different services (e.g. may or may not provide rdt over link
Link layer services:
- Franking link access:
- Encapsulate datagram into frame, adding header, trailer
Channel access if shared medium
- Reliable delivery between adjacent nodes:
- [refer to Chapter 3]
- Flow control:
- Pacing between adjacent sending and receiving nodes
- Error detection:
- Errors caused by signal attenuation and noise. Receiver detects presence of
errors: signals sender for retransmission or drops frame
- Error correction:
- Receiver identifies and corrects bit error(s) without resorting to
retransmission
- Half-duplex and full-duplex:
- With half duplex, nodes at both ends of link can transmit, but not at same
time
Adaptors communicating:
- Sending side: Encapsulates datagram in frame and adds error checking bits, rdt, flow
control , etc.
- Receiving side: Looks for errors, rdt, control flow, etc, extracts datagram, passes to
upper layer at receiving side.
Chapter 6.2: Error detection, correction
- EDC = error Detection and Correction bits (redundancy)
- D = data protected by error checking, may include header fields
- Error detection is not 100% reliable! Protocol may miss some errors, but rarely. Larger
EDC field yields better detection and correction.
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Document Summary
Communication channels that connect adjacent nodes along communication path links. Data-link layer has responsibility of transferring datagram from one note to physically adjacent node over a link. Datagrams transferred by different link protocols over different links: e. g. ethernet on first link, frame reply on intermediate link, etc. Each link protocol provides different services (e. g. may or may not provide rdt over link. Encapsulate datagram into frame, adding header, trailer. Pacing between adjacent sending and receiving nodes. Errors caused by signal attenuation and noise. Receiver detects presence of errors: signals sender for retransmission or drops frame. Receiver identifies and corrects bit error(s) without resorting to retransmission. With half duplex, nodes at both ends of link can transmit, but not at same time. Sending side: encapsulates datagram in frame and adds error checking bits, rdt, flow control , etc. Receiving side: looks for errors, rdt, control flow, etc, extracts datagram, passes to upper layer at receiving side.
