1. Data Link Networks Hardware Building Blocks Nodes & Links CS380
Int. to Comp. Networks
Data Link Networks - Part I

2. PROBLEM: Physically connecting Hosts
5 Issues
4 Technologies
Encoding
- encoding for physical medium
Framing
- delineation of bit stream
Error Detection
- identify frame errors
Reliable Delivery
- link integrity despite errors
Media Access Control
- multiple host access
Point-to-point Links
CSMA (Carrier Sense Multiple Access)
- Ethernet
- IEEE 802.3
Token Ring
- FDDI
- IEEE 802.5
Wireless
- IEEE
Network Card
CS380
Int. to Comp. Networks
Data Link Networks - Part I

3. Data Link Networks - Part I
Nodes
general-purpose computers;
e.g., desktop workstations, special- purpose hardware, PC
CPU
Network
Cache (T
o network)
adaptor
Finite memory
Connects to network via a network adaptor
Fast processor, slow memory
I/O bus
Memory
CS380
Int. to Comp. Networks
Data Link Networks - Part I

4. Data Link Networks - Part I
Network Node Memory
Moore’s Law
Doubling processor speeds in 18 months
Memory Latency
Only 7% improvement each year
Network nodes run at memory speeds, not CPU speeds
Memory accesses needed to be considered carefully
Two scarce resources: bandwidth and memory
CS380
Int. to Comp. Networks
Data Link Networks - Part I

5. Data Link Networks - Part I
Links
Electromagnetic Spectrum
CS380
Int. to Comp. Networks
Data Link Networks - Part I

6. Data Link Networks - Part I
Links
Sometimes you install your own
Sometimes leased from the phone company
(Note: T1 also called DS1, STS-1 also called OC-1)
CS380
Int. to Comp. Networks
Data Link Networks - Part I

7. Data Link Networks - Part I
Last-Mile Links
From home to the network service provider.
Service
Bandwidth
POTS
(Plain Old Telephone Service)
ISDN
(Integrated Services Digital Network)
xDSL
(Digital Subscriber Line)
CATV
(CAble TV)
Kbps
Kbps
16 Kbps-55.2Mbps
20-40 Mbps
CS380
Int. to Comp. Networks
Data Link Networks - Part I

8. Data Link Networks - Part I
Point-to-Point Links
Encoding
Framing
Error Detection
Reliable Transmission
CS380
Int. to Comp. Networks
Data Link Networks - Part I

9. Data Link Networks - Part I
Encoding
Signals propagate over a physical medium
modulate electromagnetic waves by varying the voltage
Network adaptor handles encoding
Encoded bits to signals (sending)
Decodes signals to bits (receiving)
CS380
Int. to Comp. Networks
Data Link Networks - Part I

10. Adaptors Signal travel between signalling components;
Node
Adaptor
Adaptor
Node
Bits
Signal travel between signalling components;
Bits flow between adaptors
CS380
Int. to Comp. Networks
Data Link Networks - Part I

11. Data Link Networks - Part I
Modem and Codec
Modem = Modulator + Demodulator
Codec = Encoder + Decoder
Encoder Modulator Demodulator Decoder
Media
CS380
Int. to Comp. Networks
Data Link Networks - Part I

12. Data Link Networks - Part I
NRZ Encoding
Encode binary data onto signals
e.g., 0 as low signal and 1 as high signal
known as Non-Return to zero (NRZ)
Problem: Consecutive 1s or 0s
Low signal (0) may be interpreted as no signal
High signal (1) leads to baseline wander
Unable to recover clock
Bits
NRZ 1
CS380
Int. to Comp. Networks
Data Link Networks - Part I

13. Alternative Encodings
Non-return to Zero Inverted (NRZI)
make a transition from current signal to encode a one; stay at current signal to encode a zero
solves the problem of consecutive ones
Manchester
transmit XOR of the NRZ encoded data and the clock
only 50% efficient.
CS380
Int. to Comp. Networks
Data Link Networks - Part I

14. Data Link Networks - Part I
Encodings (cont)
4-Bit
Symbol
5-bit
Code
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
11110
01001
10100
10101
01010
01011
01110
01111
10010
10011
10110
10111
11010
11011
11100
11101
4B/5B
every 4 bits of data encoded in a 5-bit code
5-bit codes selected to have no more than one leading 0 and no more than two trailing 0s
thus, never get more than three consecutive 0s
resulting 5-bit codes are transmitted using NRZI
achieves 80% efficiency
Others:
11111 – idle
00000 – dead …
CS380
Int. to Comp. Networks
Data Link Networks - Part I

15. Data Link Networks - Part I
Encodings (cont)
Bits
NRZ
Clock
Manchester
NRZI 1
CS380
Int. to Comp. Networks
Data Link Networks - Part I

16. Data Link Networks - Part I
Framing
Frames
Bits
Adaptor
Node B
Node A
Packet-switched networks
Break sequence of bits into frames (blocks of data)
What set of bits constitute a frame?
Where the frame begins?
Where the frame ends?
Typically implemented by network adaptor
Adaptor fetches (deposits) frames out of (into) host memory
Central challenge
- Use different protocols
CS380
Int. to Comp. Networks
Data Link Networks - Part I

17. Data Link Networks - Part I
Framing Protocol
Byte-oriented
View each frame as a collection of bytes (characters)
Sentinel approach
BISYNC (Binary Synchronous Communication) protocol - IBM
Byte counting
DDCMP ( Digital Data Communication Message Protocol) protocol - DEC
Bit-oriented
HDLC (High-Level Data Link Control) Protocol – IBM and then ISO
Clock-based
SONET (Synchronous Optical Network) – Bellcore and then ANSI
CS380
Int. to Comp. Networks
Data Link Networks - Part I

18. Byte-Oriented - Sentinel Approach
Frame begins at SYN (Synchronization)
Sentinel values between body
STX = Start of text
ETX = End of text
CRC (Cycle Redundancy Check) – checks for errors
BISYNC frame format
(Binary Synchronous Communication) – IBM
problem: ETX character might appear in the data portion of the frame
solution: Character stuffing – Escape the ETX character with a DLE (data line escape) character in BISYNC 8 8 8 8 8 16
SYN
SYN
SOH
STX
ETX
Header
Body
CRC
CS380
Int. to Comp. Networks
Data Link Networks - Part I

19. Byte-Oriented – Byte-Counting Approach
COUNT field specifies how many bytes contained in a frame
DDCMP frame format
( Digital Data Communication Message Protocol) - DEC 8 8 8 14 42 16
SYN
SYN
Class
Count
Header
Body
CRC
CS380
Int. to Comp. Networks
Data Link Networks - Part I

20. Data Link Networks - Part I
Bit-Oriented
Denote the beginning/end of a frame with the distinguished bit sequence
HDLC frame format
(High-level Data Link Control) – IBM and then ISO
problem: the pattern could appear anywhere in the body of the frame
solution: Bit Stuffing - When it is located in the body, it is preceded with an escape sequence of bits (like an escape character in C) 8 16 16 8
Beginning
Ending
Header
Body
CRC
sequence
sequence
CS380
Int. to Comp. Networks
Data Link Networks - Part I

21. Data Link Networks - Part I
Clock-Based
each frame is 125s long
 At STS-1 (= Mbps) rate, 810B long
e.g., SONET: Synchronous Optical Network
ITU standard for transmission over fiber
STS-n (STS-1 = Mbps)
c - concatenated
Each frame is 810 bytes long
CS380
Int. to Comp. Networks
Data Link Networks - Part I

22. Data Link Networks - Part I
Error
Long history of dealing with bit errors
Hamming
Reed/Solomon
Detecting Error is only one part of the problem, the other part is correcting errors
Two methods of error correction
Have the message retransmitted
Error-correcting codes (algorithms that all the recipient to reconstruct the correct message)
CS380
Int. to Comp. Networks
Data Link Networks - Part I

23. Data Link Networks - Part I
Error Detection
Basic idea – add extra (redundant) bits to a frame that can be used to determine if errors have been introduced.
Ethernet: 1500B data requires only 32-bits (CRC-32)
Sender applies algorithm to the message to come up with the extra bits
Receiver uses the same algorithm to check if the calculation comes up with the same result
Common error-detecting codes
Two-dimensional parity (ASCII) (link-level)
Checksum (internet) (not link-level)
CRC, Cyclic Redundancy Check, (link-level)
CS380
Int. to Comp. Networks
Data Link Networks - Part I

24. Two-Dimensional Parity 1
Catch all 1,2,3-bit and most 4-bit errors
In this example, use 14 redundant bits for a 42-bit message, which is much better than the obvious way of sending two copies of the same data 1
Used by BISYNC protocol (IBM) to transmitting ASCII characters
Data 1 1
Parity
byte
Parity
bits
CS380
Int. to Comp. Networks
Data Link Networks - Part I

25. Internet Checksum Algorithm
Not used in link-level (unlike parity and CRC)
Sender adds up all the word and then transmit the result of that sum (Checksum)
Received adds up all the words and compares its checksum to the sender’s checksum
Algorithm for the Internet
Treat the data as a sequence of 16-bit integers. Add the 16-bit integers using 16-bit ones complement arithmetic
take the ones complement of the result. That 16-bit number is the checksum.
CS380
Int. to Comp. Networks
Data Link Networks - Part I

26. CRC - Cyclic Redundancy Check
Add k bits of redundant data to an n-bit message
want k << n
e.g., Ethernet: k = 32 and n = 12,000 (1500 bytes)
Represent n-bit message as n-1 degree polynomial
e.g., MSG= as M(x) = x7 + x4 + x3 + x1
Let k be the degree of some divisor polynomial
e.g., C(x) = x3 + x2 + 1 when k = 3
CS380
Int. to Comp. Networks
Data Link Networks - Part I

27. CRC - Cyclic Redundancy Check
Transmit polynomial P(x) that is evenly divisible by C(x)
shift left k bits, i.e., M(x)xk
subtract remainder of M(x)xk / C(x) from M(x)xk
Receiver polynomial P(x) + E(x)
(E(x) – error in the transmission)
E(x) = 0 implies no errors
Divide (P(x) + E(x)) by C(x); remainder zero if:
E(x) was zero (no error), or
E(x) is exactly divisible by C(x)
CS380
Int. to Comp. Networks
Data Link Networks - Part I

28. Data Link Networks - Part I
CRC Example: k=3
Original Message M(x)
Generator
C(x)
1101
Message & k bits of 0
M(x)xk
1101
1001
Perform logical XOR
Once the reminder is obtained, subtract it from M(x)xk, this can be accomplished with the XOR
– 101 =
Send this message
Recipient divides received message by C(x), if the reminder is 0  no error (most likely)
1101
1000
1101
1011
1101
1100
1101
1000
1101
101
Remainder
M(x)xk / C(x)
CS380
Int. to Comp. Networks
Data Link Networks - Part I

29. Data Link Networks - Part I
Selecting C(x)
All single-bit errors, as long as the xk and x0 terms have non-zero coefficients.
All double-bit errors, as long as C(x) contains a factor with at least three terms
Any odd number of errors, as long as C(x) contains the factor (x + 1)
Any ‘burst’ error (i.e., sequence of consecutive error bits) for which the length of the burst is less than k bits.
Most burst errors of larger than k bits can also be detected
CS380
Int. to Comp. Networks
Data Link Networks - Part I

30. Common CRC Divisor Polynomials
CRC-8 (ATM)
CRC-10 (ATM)
CRC-12
CRC-16
CRC-CCITT (HDLC)
CRC-32 (Ethernet)
C(x)
x8+x2+x1+1
x10+x9+x5+x4+x1+1
x12+x11+x3+x2+x1+1
x16+x15+x2+1
x16+x12+x5+1
x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
CS380
Int. to Comp. Networks
Data Link Networks - Part I