1. Data Link Control Layer General Concepts
Rudra Dutta
CSC 401- Fall 2011, Section 001

2. Data Link Layer Perspective
Second of the OSI layers
Utilizes (unreliable) bitpipe
Provides service to Networking layer
Equipments: switch, bridge, …
PDUs: “Frames”
We want to understand:
Descriptive
Frame structures, protocols (Ethernet, WiFi, WiMax)
Algorithmic
Frame delineation, error detection, ARQ, MAC (CSMA-CD, CSMA-CA), transparent bridging
Copyright Fall 2011, Rudra Dutta, NCSU

3. Data Link Layer Services
Services Provided to the Network Layer
Framing
Logical bit groupings – more use at higher layers
Error Control
Error control overlaps with physical layer
DLC – retransmission strategies
Flow Control
Matches dissimilar endpoint processing speeds
Slow receiver should not be swamped by fast sender
Also a backward error correction mechanism
Optionally, mediate contention for shared medium
Such as Ethernet
Copyright Fall 2011, Rudra Dutta, NCSU

4. Utilizing Unreliable Bitpipe
Sometimes sender sends bits, sometime not
Bitpipe can sometime lose bits, or “manufacture” bits
How does the receiver (receiving DLC) know when bits are being received, and when not?
Must create own (DLC layer) convention (protocol) about transmission of bits
Start with preamble, end with conclusion
Creates logical groups at DLC layer - “frames”
Also serves as logical groups to encapsulate higher layer PDUs
Higher layers want service in multiple-byte chunks
Logical units determined by the logic of the higher layers
Frame delineation - must start with (and end with) an easily recognizable set of bits that are unlikely to arise randomly due to noise
Copyright Fall 2011, Rudra Dutta, NCSU

5. Framing with FLAGs FLAG is a distinctive bit pattern
Such as (0160)
Must not appear inside data
Copyright Fall 2011, Rudra Dutta, NCSU

6. Bit Stuffing Bit stuffing method Delineate by pattern of many bits
Prevent pattern from occurring in data by few bits
Must be completely reversible, i.e. destuffable
Copyright Fall 2011, Rudra Dutta, NCSU

7. Example
FLAG is ‘0160’
Rule: “Stuff a ‘0’ after any occurrence of ‘015’ in the bit string transmitted.”
in SDU
Rule: “Copy bits as received into the frame. If the last 6 bits to be seen were ‘015’, then discard the next bit if it is a ‘0’, otherwise copy as usual. If the last 8 bits to be copied were ‘0160’, recognize it as a frame-delimiting FLAG.”
copied
Copyright Fall 2011, Rudra Dutta, NCSU

8. Ethernet Frame ... 10101010 (7 times) Source address Dest. address
Type
FCS
Preamble
SFD
...
Payload
Copyright Fall 2011, Rudra Dutta, NCSU

9. Error Control Errors happen Dual concerns from a communication POV
Detection
Can at least catch errors
ARQ strategies may come in
Correction (FEC)
Better if we can
Sometimes essential due to link characteristics
Involves error codes
Coding, not encryption
Copyright Fall 2011, Rudra Dutta, NCSU

10. Flow Control
Potential data generation and transmission may be faster than consumption
Receiver might be “overwhelmed”
Fast sender, slow receiver
At E2E layer (transport, or app)
NOT at DLC/Phy level
Working communication
 Must be matched at that level
Realistically, must stop and wait during transmission
Channel idle for some fraction of time
Possible to proactively use some of this time
Windowed flow control D
APP R
TRNS
NET
NET
DLC
DLC
DLC
PHY
PHY
PHY
Copyright Fall 2011, Rudra Dutta, NCSU

11. Flow Control and Error Control
Flow control  if sender “gets ahead” of receiver, must retransmit
Error control  may need to retransmit because of error in channel as well
Error must be detected
Only non-erroneous copy at the sender
Receiver must request a repeat
Automatic Repeat ReQuest
But – sender needs to know
“Three armies problem”
Requests must sometimes be implicit
Copyright Fall 2011, Rudra Dutta, NCSU

12. The Three Army Problem Acknowledgements are needed
But may not be available
Hence requests must sometimes be implicit
ARQ strategies - later
Copyright Fall 2011, Rudra Dutta, NCSU

13. Error Control

14. Error Control Errors happen Dual concerns from a communication POV
Detection
Can at least catch errors
ARQ strategies may come in
Correction (FEC)
Better if we can
Sometimes essential due to link characteristics
We shall discuss
Simple block codes
Many others: BCH, Convolution: Viterbi, Turbo, …
Copyright Fall 2011, Rudra Dutta, NCSU

15. Fundamental Concepts - Redundancy
Some data is actual data for error control
Note: Could be data + metadata from other POV
More data introduced expressly for the purpose of error control
“Metadata” for error control
Data and metadata must be distinguished
Usually a framing issue
Metadata introduces overhead
Only data is “useful” outside error control
Data
ECC
Copyright Fall 2011, Rudra Dutta, NCSU

16. Fundamental Concepts - Rate
Overhead  “rate” of a coding scheme
Rate of useful data per encoded data, < 1.0
Higher rate is more efficient
Unfortunately, less likely to be good coding
Expressed as a ratio of bits
Over long periods, if not constant
N bits
M bits
Copyright Fall 2011, Rudra Dutta, NCSU

17. Fundamental Concepts – Block
If a coding scheme always codes n bits to n+m bits,
We speak of n bit blocks that are independently coded
The code is called a block code
Alternative – “stream” type coding
Convolution
Copyright Fall 2011, Rudra Dutta, NCSU

18. Fundamental Concepts - Distance
Data given to error control can be anything
2n “datawords”
For each, a unique m bit code
“Codeword” (encoded data) is n+m bits
Hamming distance is a measure of strength
dmin = number of bit corruptions that can fool the code
> t for t-bit error detection
> 2t for t-bit error correction
Copyright Fall 2011, Rudra Dutta, NCSU

19. Efficient Use of Rate ? Lower rate means more overhead bits
But does not necessarily translate into higher distance
Choosing appropriate codewords “smartly” can increase distance
0  00, 1  10
(does not increase robustness)
0  00, 1  11
(does)
n = 1, m = 1
n = 2, m = 1
00 
01 
10 
11  ?
Copyright Fall 2011, Rudra Dutta, NCSU

20. Simple Repeat Correction is possible, but rate suffers
“k out of 2k-1” kind of voting process
E.g., if each bit is sent seven times,
1 bit blocks
21 datawords (‘0’ and ‘1’)
Only 2 valid codewords (‘ ’ and ‘ ’)
Hamming distance is 7
dmin is 7 if only detection is attempted
4 if correction is attempted from any seven-bit combination
Intermediate strategies possible
What is the probability of a bit being misinterpreted?
Copyright Fall 2011, Rudra Dutta, NCSU

21. Parity XOR all bits of the message
Append the resultant bit to the word (m = 1)
Detects all single bit errors, however long the word
In fact, all odd number of bits errors
Can not correct
Unfortunately, chances of errors increases
Coder
s1 s2 … sn
s1 s2 … sn c
c = s1 + s2 + … + sn , the modulo-2 sum
Copyright Fall 2011, Rudra Dutta, NCSU

22. Horizontal and Vertical Parity
Arrange message bits in a grid
Parity bits for each row, each column
Detects all single, double, triple errors
Some but not all quadruple errors
Which ones? What fraction of them? p
s11
s12
s13
s14 c1
s21
s22
s23
s24 c2
s31
s32
s33
s34 c3
s41
s42
s43
s44 c4 r1 r2 r3 r4 c q
Message = [ sij]
ci = si1 + si2 + … + sip
rj = s1j + s2j + … + sqj
n = pq
m = p + q + 1
Copyright Fall 2011, Rudra Dutta, NCSU

23. CRC – Polynomial View Message polynomial
u(x) = u0 + u1x + u2x2 + … + un-1 xn-1
CRC polynomial
r(x) = r0 + r1x + r2x2 + … + rm-1 xm-1
Code polynomial
c(x) = u(x) xm + r(x) r u
Copyright Fall 2011, Rudra Dutta, NCSU

24. CRC as Remainder of a Division
Generator polynomial :
g(x) = 1 + g1x + … + gm-1xm-1 + xm
Encoding Rule:
u(x) xm
r(x) = Rem { }
g(x)
Divisible by g
Copyright Fall 2011, Rudra Dutta, NCSU

25. Long Division Example n=3 and m=3 g(x) = 1 + x2 + x3 u(x) = 1 + x2
c(x) = ?
x3 + x2 + 1
x x3
x5 + x x2
x4 + x3+ x2
x4 + x x
Remainder
x2 + x
Copyright Fall 2011, Rudra Dutta, NCSU

26. Summary DLC - framing, flow control, error control Error control
Optionally medium access
Error control
Metadata must be added to enable error detection/correction
May be integrated with encoding of data – overlap with physical layer
Block codes
Encapsulates error detection/correction largely within DLC and physical layers
Copyright Fall 2011, Rudra Dutta, NCSU