Chapter 6 - Link Layer and LANs Flashcards
What are the links in a network?
- wired links
- wireless links
- LANs
What is the packet in the link layer?
-A frame
- The frame will encapsulate the datagram
What is the data-link layer responsibility?
transferring datagram from one node to a physically adjacent node over a link
What are the link layer services?
Framing, link access:
- encapsulate datagram into frame, adding header, trailer
- channel access if shared medium
- MAC addresses used in frame headers to identify source and destination (different from IP)
Reliable delivery between adjacent nodes
Flow control
error detection
error correction
half-duplex and full-duplex
Where is the link layer implements?
Each and every host
How do adaptors communicate?
Sending side:
- encapsulates datagram in frame
- adds error checking bits, rdt, flow control etc.
Receiving side:
- looks for errors, rdt, flow control etc.
- extracts datagram, passes to upper layer at receiving side
How does error detection work?
- add erros detection and correction bits to the data
What is a single parity check?
- add single parity bit to a block of data bits
-can be a 1 or a 0 - ensures that sum of all bits yeilds either an even or odd result
- exclusive or operation
what is even parity?
designed to yield an even result
what is odd parity?
- designed to yield an odd result
What is two dimensional bit parity?
- rectangle of bits with i and j columns
- then horizontal parity check is appended to each row and vertical parity to each column
Internet checksum?
- 1s compliment addition
- sender puts checksum into UDP checksum field
- receiver will add all segments
- if all values equals one, no error, but if not all one, then there is error
Cyclic redundancy?
- more powerful error-detection
- data bits as binary
What are the type of links for a multiple access links protocol
point to point:
- PPP for dial up access
- point to point link between ethernet switch, host
broadcast:
- old fashioned ethernet
- upstream HFC
- 802.11 wireless LAN
What is a multiple access protocol?
- distributed algorithm that determines how nodes share channel ie determine when node can transmit
- communication about channel sharing must use channel itself
What are the three MAC protocols classes?
channel partitioning:
- divide into smaller pieces (time slots, frequency)
- allocate piece to node for exclusive use
random access:
- channel not divided, allow collisions
- recover from collisions
taking turns:
- node takes turns, but nodes with more to send can take longer turns
What is Time division multiple access?
- access to channel in rounds
- each station gets fixed length slot in each round
- unused slots go idle
What is frequency division multiple access?
- channel spectrum divided into frequency bands
- each station assigned fixed frequency band
- unused transmission time in frequency bands go idle
What is slotted ALOHA?
- node obtains fresh frame, transmits in next slot
- if no collision: node can send new frame in next slot
- if collision: node retransmits frame in each subsequent slot with prob. p until success
Asumptions with slotted ALOHA
- all frames same size
- time divided into equal size slots
- nodes start to transmit only slot beginning
- nodes are synchronized
- if 2 or more nodes transmit in slot, all nodes detect collision
Pros and cons of slotted ALOHA
Pros:
- single active node can continuously transmit at full rate of channel
- highly decentralized: only slots innodes need to be in sync
-simple
cons:
- collisions, wasting slots
- idle slots
- nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Efficiency of slotted ALOHA
long run fraction of successful slots (many nodes, all with many frames to send)
what is Pure (unslotted) ALOHA?
- simpler, no synchronization
- when frame first arrives, transmit immediately
- collision probability increases
Efiiciency of Pure ALOHA
- worse than slotted ALOHA
What is carrier sense multiple access?
- listen before transmit
- if channel sensed idle: transmit entire frame
- if channel sensed busy: defer transmission
Collisions of CSMA
- propagation delay means two nodes may not hear each others transmission
- entire packet transmission time wasted
