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5: DataLink Layer 5a-
Chapter 5: The Data Link Layer
Our goals:
ļ² understand principles behind data link layer services: ļ error detection, correction ļ sharing a broadcast channel: multiple access ļ link layer addressing ļ reliable data transfer, flow control:done!
ļ² instantiation and implementation of various link layer technologies
Overview:
ļ² link layer services ļ² error detection, correction ļ² multiple access protocols and LANs ļ² link layer addressing, ARP ļ² specific link layer technologies: ļ Ethernet ļ hubs, bridges, switches ļ IEEE 802.11 LANs ļ PPP ļ ATM
5: DataLink Layer 5a-
Link Layer: setting the context
5: DataLink Layer 5a-
Link Layer Services
ļ² Framing, link access:
ļ encapsulate datagram into frame, adding header, trailer ļ implement channel access if shared medium, ļ āphysical addressesā used in frame headers to identify source, dest
- different from IP address!
ļ² Reliable delivery between two physically connected
devices:
ļ we learned how to do this already (chapter 3)! ļ seldom used on low bit error link (fiber, some twisted pair) ļ wireless links: high error rates
- Q: why both link-level and end-end reliability?
5: DataLink Layer 5a-
Link Layer Services (more)
ļ² Flow Control:
ļ pacing between sender and receivers
ļ² Error Detection:
ļ errors caused by signal attenuation, noise.
ļ receiver detects presence of errors:
- signals sender for retransmission or drops frame
ļ² Error Correction:
ļ receiver identifies and corrects bit error(s)
without resorting to retransmission
5: DataLink Layer 5a-
Error Detection
EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields
- Error detection not 100% reliable!
- protocol may miss some errors, but rarely
- larger EDC field yields better detection and correction
5: DataLink Layer 5a-
Parity Checking
Single Bit Parity:
Detect single bit errors
Two Dimensional Bit Parity :
Detect and correct single bit errors
d data bits parity bit
0111000110101011 1
even or odd parity
5: DataLink Layer 5a-
Checksumming: Cyclic Redundancy Check
ļ² based on r+1 bit pattern (generator) G known to tx and rx
ļ² treat data bits D as a binary number
ļ² obtain r-bit CRC R such that <D,R> divisible (modulo 2) by G
ļ² receiver divides <D,R> by G. non-zero remainder implies error
ļ² can detect all burst errors less than r+1 bits
ļ² widely used in practice (ATM, HDCL)
R = remainder[ D. 2 r]
G Docsity.com
5: DataLink Layer 5a-
CRC Example
R = remainder[ D. 2 r]
G
G = 1001 (r=3)
D = 101110
5: DataLink Layer 5a-
Multiple Access Protocols
ļ² single shared communication channel
ļ² two or more simultaneous transmissions by nodes:
interference
ļ only one node can send successfully at a time
ļ² multiple access protocol:
ļ distributed algorithm that determines how stations share channel, i.e., determine when station can transmit ļ communication about channel sharing must use channel itself! ļ what to look for in multiple access protocols:
- synchronous or asynchronous
- information needed about other stations
- robustness (e.g., to channel errors)
- performance
5: DataLink Layer 5a-
MAC Protocols: a taxonomy
Three broad classes:
ļ² Static Channel Partitioning
ļ divide channel into smaller āpiecesā (time slots, frequency) ļ allocate piece to node for exclusive use
ļ² Random Access
ļ allow collisions
ļ ārecoverā from collisions
ļ² āTaking turnsā (Dynamic channel partitioning)
ļ tightly coordinate shared access to avoid collisions
Goal: efficient, fair, simple, decentralized
5: DataLink Layer 5a-
Channel Partitioning (CDMA)
CDMA (Code Division Multiple Access)
ļ² unique ācodeā assigned to each user; ie, code set partitioning
ļ² used mostly in wireless broadcast channels (cellular, satellite,etc)
ļ² all users share same frequency, but each user has own āchippingā sequence (ie, code) to encode data
ļ² encoded signal = (original data) X (chipping sequence)
ļ² decoding: inner-product of encoded signal and chipping
sequence
ļ² allows multiple users to ācoexistā and transmit simultaneously with minimal interference (if codes are āorthogonalā)
5: DataLink Layer 5a-
CDMA Encode/Decode
5: DataLink Layer 5a-
Random Access protocols
ļ² When node has packet to send
ļ transmit at full channel data rate R.
ļ no a priori coordination among nodes
ļ² two or more trasnmitting nodes -> ācollisionā,
ļ² random access MAC protocol specifies:
ļ how to detect collisions ļ how to recover from collisions (e.g., via delayed retransmissions)
ļ² Examples of random access MAC protocols:
ļ slotted ALOHA ļ ALOHA ļ CSMA and CSMA/CD
5: DataLink Layer 5a-
Slotted Aloha
ļ² time is divided into equal size slots (= pkt trans. time)
ļ² node with new pkt: transmit at beginning of next slot
ļ² if collision: retransmit pkt in future slots with
probability p, until successful.
Success (S), Collision (C), Empty (E) slots
