Transcript Introduction

CEN 4500
Data Communications
Chapter 3: The Data Link Layer
Instructor: S. Masoud Sadjadi
http://www.cs.fiu.edu/~sadjadi/Teaching/
sadjadi At cs Dot fiu Dot edu
Recap: Data Link Layer
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Data link layer is the second layer in the
hierarchy of the hybrid reference model
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Is to transform a raw transmission facility
into a line that appears free of undetected
transmission errors.
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Recap: Data Link Layer
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We will study algorithms for achieving reliable,
efficient communication between two adjacent
machines at the data link layer.
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Adjacent machines are two machines that are connected
by a communication channel that acts conceptually like a
wire (e.g., coaxial cable, telephone line, or point-to-point
wireless channel)
A channel is called “wire-like” if the bits are delivered in
exactly the same order in which they are sent.
What makes this layer essential is the fact that
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communication channels make errors occasionally,
they have only a finite data rate, and
there is a non-zero propagation delay
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Agenda
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Design Issues
Error Detection and Correction
Elementary Data Link Protocols
Sliding Window Protocols
Protocol Verification
Example Data Link Protocols
Summary
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Data Link Layer Design Issues
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Functions of the Data Link Layer
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Providing a well-defined service to the network layer
Dealing with transmission errors
Regulating the flow of data so that slow receivers are not
swamped by fast senders
Solution
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Data link layer takes the packets from the network layer
and encapsulates them into frames for transmission.
A frame contains header, payload (packet), and trailer.
Frame management is what data link layer is all about.
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Functions of the Data Link Layer
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Relationship between packets and frames.
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Services Provided to Network Layer
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The principle service is transferring data from the network
layer on the source machine to the network layer on the
destination machine.
(a) Virtual communication.
(b) Actual communication.
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Reasonable Service Possibilities
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Unacknowledged connectionless service
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Acknowledged connectionless service
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When error-rate is very low, so that the recovery
is left to the higher layers
Also for real-time traffic, such as voice
Most LANs use this on the data link layer
Useful over unreliable channels, such as wireless
Acknowledged connection-oriented service
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Each frame is numbered to guarantee exactly
once and in-order delivery of frames
A reliable bit stream
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Services Provided to Network Layer
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Placement of the data link protocol.
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Framing
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Physical layer accepts a raw bit stream and attempt to
deliver it to the destination.
It is up to the data link layer to detect and, if necessary,
correct errors.
The usual approach is for data link layer to break the
bit stream up into discrete frames and compute the
checksum for each frame.
Framing Methods
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2.
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Character count: count can be garbled
Flag bytes with byte stuffing: starting and ending delimiters
Starting and ending flags, with bit stuffing
Physical layer coding violations
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Framing: Character Count
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A character stream.
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(a) Without errors.
(b) With one error.
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Framing: Flag Bytes w/ Byte Stuffing
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What if the flag byte’s bit pattern happens occurs in
the date? Insert a escape byte (ESC)
(a) A frame delimited by flag bytes.
CEN 4500,
S. Masoud
Sadjadi before and after
(b) Four examples
of byte
sequences
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Framing: Bit Stuffing
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Each frame starts and ends with a special bit pattern,
01111110 (in fact, a flag byte).
Whenever the sender’s data link layer encounters
five consecutive 1s in the data, it automatically
stuffs a 0 bit into the outgoing bit stream
Bit stuffing
(a) The original data.
(b) The data as they appear on the line.
CEN 4500, S. Masoud Sadjadi
(c) The data as they are stored in receiver’s memory after destuffing.
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Framing: Physical Layer Coding Violation
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This method is only applicable to networks in
which the encoding on the physical medium
contains some redundancy.
For example, some LANs encode 1 bit of data
by using 2 physical bits
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A 1 bit is a high-low pair
A 0 bit is a low-high pair
So, every data bit has a transition in the middle:
easy for the receiver to locate the bit boundaries.
High-high and low-low are not used for data!
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Agenda
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Design Issues
Error Detection and Correction
Elementary Data Link Protocols
Sliding Window Protocols
Protocol Verification
Example Data Link Protocols
Summary
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Error Background
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Transmission errors will be with us for a while
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The local loops are still twisted copper pairs
Wireless communication is becoming more common
Error Property
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As a result of the physical process that generates them,
errors on some media (e.g., radio) tend to come in bursts
rather than singly.
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A burst error does not imply that all the bits are wrong; it just
implies that at least the first and last are wrong
If errors were independent, most block would contain
errors
However, they are much harder to correct than are isolated
errors
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Error Handling
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Error Detection
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On channels that are highly reliable, such as fiber,
it is cheaper to detect errors and retransmit data
Error-detecting codes add enough redundant
information to detect errors.
Error Correction
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On channels such as wireless links that make many
errors, retransmission does not make much sense
Error-correcting codes need to include enough
redundant information not only to detect the error
but also to correct it
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Error-Detecting Codes: CRC
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Cyclic Redundancy Check (CRC)
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Also known as polynomial code
Based on treating bit strings as representations of
polynomials with coefficients of 0 and 1 only
A k-bit frame is regarded as the coefficient list for
a polynomial with k terms, ranging from xk-1 to x0.
For example, 110001 represents x5 + x4 + x0
Polynomial arithmetic is done modulo 2
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There are no carries for addition or borrows for
subtractions, both are identical to exclusive OR
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CRC
• The sender and receiver agree upon a
generator polynomial, G(x), both of
its high and low bits are 1
• To compute the checksum for some
frame with m bits, corresponding to the
polynomial M(x), the frame must be
longer than the generator polynomial
• We append a checksum to the end of
the frame, so that the resulting
polynomial is dividable by G(x)
Calculation of the polynomial code checksum.
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Error-Correcting Code: Hamming Code
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A frame consists of m data bits and r redundant, or
check, bits.
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n = m + r; called n-bit codeword.
Hamming Distance
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The number of bit positions in which two codewords
differ
If two codewords are a Hamming distance d apart, it will
require d single-bit errors to convert one into the other.
Given the algorithm to compute the check bits, that are 2m
legal codewords out of 2n codewords
The Hamming distance of the complete code (the list of
legal codewords) is the minimun distance of any two
codewords in the list.
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Error-Correcting Code: Hamming Code
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To detect d errors
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you need a distance d + 1 code
If you receive a frame with d error, it will have an illegal
codeword
To correct d errors
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you need a distance 2d + 1 code
The closer legal codeword is the one to pick
Example, Parity Bit
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The number of 1s are either even or odd
A code with single parity has distance 2
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Error-Correcting Code: Hamming Code
• How many check bit do
we need to be able to
correct a 1 bit error?
• For each 2m legal
codewords, we need to
have n illegal ones, so n +
1 codewords should be
reserved
• 2m(n + 1) < 2n
OR
• (m + r + 1) < 2r
Hamming code to correct burst errors.
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Agenda
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Design Issues
Error Detection and Correction
Elementary Data Link Protocols
Sliding Window Protocols
Protocol Verification
Example Data Link Protocols
Summary
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Elementary Data Link Protocols
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An Unrestricted Simplex Protocol
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A Simplex Stop-and-Wait Protocol
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Protocol 2
A Simplex Protocol for a Noisy Channel
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Protocol 1
Protocol 3
Try the simulator on your own and discuss it
in your study group!
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Assumptions for the Protocols
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The physical, data link, and network layers are
independent processes that communicate by passing
messages back and forth.
Machine A is trying to send a long message to
machine B, using a reliable, connection-oriented
service.
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Later, we will consider the case where B will try to send a
message to A simultaneously.
A has an infinite supply of data ready to be sent to B.
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In a realistic situation, the data link layer will not sit in a
tight loop waiting for an event, but it will be implemented
using interrupts
Machines do not crash
As far as the data link layer is concerned, the packet
passed across the interface to data link is pure data.
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Protocol Definitions
Continued 
Some definitions needed in the protocols to follow.
These are located in the file protocol.h.
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Protocol
Definitions
(ctd.)
Some definitions
needed in the
protocols to follow.
These are located in
the file protocol.h.
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Unrestricted
Simplex
Protocol:
Protocol 1
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Simplex
Stop-andWait
Protocol:
Protocol 2
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A Simplex Protocol for a Noisy Channel:
Protocol 3
A positive
acknowledgement
with retransmission
protocol.
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Continued 
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A Simplex Protocol for a Noisy Channel :
Protocol 3 (ctd.)
A positive acknowledgement with retransmission protocol.
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Agenda
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Design Issues
Error Detection and Correction
Elementary Data Link Protocols
Sliding Window Protocols
Protocol Verification
Example Data Link Protocols
Summary
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Sliding Window Protocols
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Need for both way transmission of data
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Using two simplex channels: two expensive
Using one channel in both directions (intermixed), with
temporarily delaying the ack (piggybacking)
A One-Bit Sliding Window Protocol
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Protocol 4
A Protocol Using Go Back N
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Protocol 5
A Protocol Using Selective Repeat
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Protocol 6
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Sliding Window Protocols (2)
•On the sender side: the frames that are sent or can be sent (no ack yet).
•On the receiver side: the frames that it may accept (discards other frames).
A sliding window of size 1, with a 3-bit sequence number.
(a) Initially.
(b) After the first frame has been sent.
(c) After the first frame has been received.
(d) After the first acknowledgement has been received.
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A One-Bit Sliding Window Protocol:
Protocol 4
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Continued 
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A One-Bit Sliding Window Protocol :
Protocol 4 (ctd.)
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A One-Bit Sliding Window Protocol :
Protocol 4 (2)
Two scenarios for protocol 4. (a) Normal case. (b) Abnormal case,
if both sides starts sending simultaneously (similar situation can
occur as a result of premature timeouts). The notation is (seq, ack,
packet number). An asterisk indicates where a network layer
accepts a packet.
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A Protocol Using Go Back N :
Protocol 5
Pipelining and error recovery. Effect on an error when
(a) Receiver’s window size is 1.
(b) Receiver’s window size is large.
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Sliding
Window
Protocol
Using Go
Back N :
Protocol 5
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Continued 
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Sliding Window Protocol Using Go Back N:
Protocol 5
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Continued 
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Sliding Window Protocol Using Go Back N:
Protocol 5
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Continued 
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Sliding Window Protocol Using Go Back N
: Protocol 5
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Sliding Window Protocol Using Go Back N:
Protocol 5
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Simulation of multiple timers in software.
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A Sliding Window Protocol Using Selective Repeat:
Protocol 6
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Continued 
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A Sliding Window Protocol Using Selective Repeat :
Protocol 6 (2)
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Continued 
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A Sliding Window Protocol Using Selective Repeat:
Protocol 6 (3)
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Continued 
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A Sliding Window Protocol Using Selective Repeat:
Protocol 6 (4)
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A Sliding Window Protocol Using Selective Repeat :
Protocol 6 (5)
(a) Initial situation with a window size seven.
(b) After seven frames sent and received, but not acknowledged.
(c) Initial situation with a window size of four.
(d) After four frames sent and received, but not acknowledged.
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Problem with non-sequential receive
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Suppose that we have a 3-bit seq. number
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The sender is permitted to send up to 7 frames before it is required to
wait for an ACK
Initially, we have (a)
The sender transmits 0, 1, 2, 3, 4, 5, and 6
The receiver accepts any frame between 0 to 6 (inclusive)
All seven frames arrive correctly, so the receiver acknowledges them
and advances its window (b)
Suppose that all the ACKs are lost
The sender times out and retransmits frame 0
The receiver accepts this frame as it is within 7, 0, …, 5
The receiver sends a piggybacked ACK for frame 6, since 0, …, 6
have been received previously.
The sender is happy that all its transmission are through and advances
its window, and send 7, 0, …, 5.
Receiver then sends from 7 and the old frame 0 to the network!!!
Therefore, the network gets an incorrect frame and the protocol fails!!
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Summary
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Data Link Layer
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The task of data link layer is to convert the raw bit stream
offered by the physical layer into a stream of frames for
use by the network layer.
Framing methods: character count, byte and bit stuffing.
It provides error control, flow control
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Protocols
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Protocols 1 to 6
Sliding window protocols
Protocol Verification: FSMs & Petri Nets
Examples: SDLC, HDLC, and PPP.
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