Ch05 data link

MẠNG MÁY TÍNH
(COMPUTER NETWORKING)

J.F Kurose and K.W. Ross. Computer
Networking A Top Down Approach.
6th edition, Addision Wesley, 2013


Chapter 5: DATA LINK
LAYER


1. Introduction and Services
2. Error detection and correction
3. Elementary Data Link Protocols
4. Sliding Window Protocols
5. Multiple access protocols
6. Link-layer Addressing
7. PPP

1.3


Link layer: Introduction
 Some terminology:


Host and router are nodes



Communication channels that
connect adjacent nodes along
communication path are links





Wired links



Wireless links



LANs

Packet is a frame, ancapsulates
datagram

Data-link layer has responsibility of
transferring datagram from one node
to adjacent node over a link

1.4


 Datagram transferred by

Transportation analogy

different link protocols over
different links


 Trip from Princeton to

Lausanne

ex: Ethernet on first link,
fram relay on intermediate
links, 802.11 on last link

 Each link protocol provides

Limo: Princeton to JFK



Plane: JFK to Geneva



Train: Geneve to Lausanne

 Tourist = datagram

different services




ex: may or may not provide
rdt over link

 Transport segment =

communication link
 Transportation mode = link

layer protocol
 Travel agent = routing

algorithm
1.5


Link layer services (1/2)
 Framing, link access:


Encapsulate datagram into frame, adding header, trailer



Channel access if share medium



“MAC” addresses used in frame headers to identify source,
destination
 Different

from IP address!

 Reliable delivery between adjacent nodes:


Seldom used on low bit-error link (fiber, some twisted pair)



Wireless links: high error rates
 Q:

why both link-level and end-end reliability?

1.6


 Framing (1/3)

A character stream. (a) Without errors. (b) With one error.

1.7


 Framing (2/3)

(a) A frame delimited by flag bytes.
(b) Four examples of byte sequences before and after stuffing.
1.8


 Framing (3/3)

Bit stuffing
(a) The original data.
(b) The data as they appear on the line.
(c) The data as they are stored in receiver’s memory after destuffing.

1.9


Link layer services (2/2)
 Flow control


Pacing between adjacent sending and receiving nodes

 Error detection


Errors caused by signal attenuation, 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
retransmision

 Half-duplex and full-duplex


With half-duplex, nodes at both ends of link can transmit, but not at the
same time

1.10


Where is the link layer implemented?
 In each and every host
 Link layer implemented in “adaptor”


Ethernet card, PCMCI card,
802.11 card



Implements link, physical layer

 Attaches into host’s system buses
 Combination of hardware, software,

firmware

1.11


1. Introduction and Services
2. Error detection and correction
3. Elementary Data Link Protocols
4. Sliding Window Protocols
5. Multiple access protocols
6. Link-layer Addressing
7. PPP

1.12


Error-Detecting Codes
 3 different error-detecting codes


Parity



Checksums



Cyclic Redundancy Checks (CRCs)

1.13


Error-Detecting Codes

1.14


Error-Correcting Codes
 4 difference error-correcting codes:
1.

Hamming codes

2.

Binary convolutional codes

3.

Reer-Solomom codes

4.

Low-Density Parity Check codes

1.15

Error-Correcting Codes

1.16


1. Introduction and Services
2. Error detection and correction
3. Elementary Data Link Protocols
4. Sliding Window Protocols
5. Multiple access protocols
6. Link-layer Addressing
7. PPP

1.17


Elementary Data Link Protocols
 An Unrestricted Simplex Protocol (A Utopian Simplex Protoco)
 A Simplex Stop-and-Wait Protocol for an Error-Free Channel
 A Simplex Stop-and-Wait Protocol for a Noisy Channel

1.18


An Unrestricted Simplex Protocol
 Consists of two distinct procedures, a sender and a receiver


Sender: runs in the data link layer of the source node
 Is

in an while loop just pumping data out onto the line as fast as
it can. The body of loop consists of 3 actions:



–

Fetch a pkt from network layer

–

Construct an outbound frame

–

Send the frame

Receiver: runs in the data link layer of the destination node
 Wait

for something to happen

 No sequence number or ACK
 Only the info field of the frame is used by this protocol
 There are no errors or flow control restrictions here.
 The data portion is passed on to the network layer, and the data link

layer settles back to wait for the next frame
1.19


Unrealistic protocol!!!!
-does not handle
either flow control or
error correction

1.20


A Simplex Stop-and-Wait Protocol for an ErrorFree Channel
 Prevent the sender from flooding the receiver

 Communication channel is still assumed to be error free, and the data

traffic is still simplex
 Solutions:
1.

Receiver must have sufficient buffering and processing abilities to run
at the line rate and must be able to pass the frames that are received to
the network layer quickly enough  worst-case solution

2.

Receiver provides feedback to the sender after having passed a pkt to
its network layer  stop-and-wait protocol


Half-duplex would suffice

1.21


1.22


A Simplex Stop-and-Wait Protocol for a
Noisy Channel
 Consider the normal situation of a communication channel that makes

errors.


Frames may be either damaged or lost completely.

 Assume: if a frame is damaged in transit, the receiver hardware will detect

this when it computes the checksum
 Adding timer
 Sender: send a frame
 Receiver: only send an ACK if data were correctly received

1.23


Continued 

A positive acknowledgement with retransmission protocol.
1.24


1.25