1. Getting Connected (Chapter 2 Part 1)
Networking
CS 3470, Section 1
Sarah Diesburg

2. Five Problems Encoding/decoding Framing Error Detection
Error Correction
Media Access

3. Five Problems of Chapter 2
The physical links carry signals (electromagnetic radiation)
How do we turn signals into bits that are recognized at the receiver?
This is known as the encoding problem R S 

4. Five Problems of Chapter 2
Delineating the sequence of bits into complete messages is called framing.
When does a frame start?
If there's a specific pattern to mark the frame's start/end, what happens if that same bit pattern is embedded in the data/payload?
Byte-oriented
Bit-oriented
Clock-based

5. Five Problems of Chapter 2
Data verification
Has the data been corrupted?
If data has been corrupted, can we take the appropriate action?
This is the error detection problem.
CRC
2-D parity
Checksums

6. Five Problems of Chapter 2
Error Recovery
Frames that are damaged will need to be retransmitted.
This is the reliability problem
ARQ
Stop and wait
Sliding window
Concurrent channels

7. Five Problems of Chapter 2
How do you arbitrate, or self-regulate access to a shared link?
This is the media-access problem
Ethernet
Token Ring
Wireless

8. Perspectives on Connecting

9. Perspectives on Connecting
All the links seem the same, yet can be characterized in different ways
We can characterize them by the services and bandwidth they provide
Service
Bandwidth (typical)
Dail-up
28-56 Kbps
ISDN (Integrated Services Digital Network)
Kbps
DSL (Digital Subscriber Line)
128 Kbps – 100 Mbps
CATV (CAble TV)
1 – 40 Mbps
FTTH (Fibre To The Home)
50 Mbps – 1 Gbps

10. Perspectives on Connecting
All the links seem the same, yet can be characterized in different ways
Another way to characterize links is by their physical makeup
Copper – DSL, coaxial, cat5e/cat6 cables
Optical fiber – FTTH
Air – Wireless

11. Links
Regardless of the form, links propagate signals in the form of electromagnetic waves
These links provide the foundation to propagate binary information/bits (0’s and 1’s) – otherwise known as encoding

12. Visualizing Links Network adaptors connect nodes to links
Why abbreviated NIC?
Signals travel between signaling components
NIC = network interface card, another term for network adaptor

13. Data Encoding (signals)
In this class, we can assume that it’s possible to transmit a pair of high (1) or low (0) signals
…which brings us to the problem of encoding binary data on these two signals

14. Tick, tock! Let's synchronize the clock
Encoding and decoding processes are driven by a clock
Every clock cycle the sender transmits a bit and the receiver recovers a bit 1 1
Clock

15. Encoding Encoding schemes NRZ (Non-return to zero)
NRZI (Non-return to zero inverted)
Manchester
4B/5B

16. NRZ Non-Return to Zero
The simplest thing to do is to map “1” onto the high signal and “0” onto the low signal
There are a few challenges to this approach 1 1
NRZ
Clock

17. NRZ Non-Return to Zero Baseline Wander caused by signal averaging
Receiver keeps average of signal it has received so far and uses average to distinguish highs and lows
E.g., average of 0 and 1 is 0.5 – anything higher than 0.5 is a “1”
Problem occurs when to many consecutive 1’s or 0’s cause average to change
Not to be confused with the mind wander phenomenon, which happens during long PowerPoint presentations.

18. NRZ Non-Return to Zero
Clock recovery required when signal remains constant too long
Receiver uses high-low transitions to mark the clock boundaries
What happens when we send a lot of consecutive 1’s or 0’s?

19. NRZI Non-Return to Zero Inverted
Transition on the half-cycles
1's indicated by a transition (low-to-high, high-to-low)
0's are where there is no transition
Takes care of problem of consecutive 1’s
Still a problem for consecutive zeros 1 1
NRZI
Clock

20. Manchester Encoding Transition on the half-cycles
low-to-high indicates a zero
high-to-low indicates a one
Receiver is able to synchronize clock every cycle 1 1
Manchester
Clock

21. Baud rate The baud rate is the rate at which the signal changes
The bit rate is the rate at which you can transmit information
For the same baud rate, NRZ and NRZI have twice the bit rate as Manchester.

22. Different Encoding Strategies So Far…

23. 4B/5B encoding 4-bit payload in a 5-bit gift box
Goal is to improve upon Manchester (50% efficiency), but to avoid baseline wander and clock drift
Insert extra bits into bit stream to break up long sequences of 0’s and 1’s
5-bit codes selected such that there are never more than three consecutive zero's.
Resulting codes transmitted through NRZI encoding

24. 4B/5B 5 bits (32 patterns) to represent 4 bits (16 patterns)
5-bit patterns with no more than 1 leading zero
5-bit patterns with no more than 2 trailing zero's
16 leftovers
7 not valid
Others control signals
11111 (idle)
00000 (dead)
00100 (bad)

25. Framing We know how to transmit bits on a link between two nodes
Now we need to figure out how to send distinct messages in frames
(Think packets at the link layer)
Why would we want to break up messages into frames instead of just a bit stream?

26. Framing Framing Protocols Framing Approaches Bi-sync HDLC PPP SONET
Sentinel Approach
Byte-counting approach

27. Sentinel Approach
Use sentinel characters to designate where frames start and end
Bi-sync frame format (IBM and mainframes)
HEADER
BODY
SYN
SYN
SOH
STX
ETX
CRC

28. Sentinel Approach SYN STX ETX SOH CRC Synchronization Start of Text
End of Text
SOH
Start of Header (Why no EOH?)
CRC
Cyclic Redundancy Check

29. Character Stuffing
How do you handle the situation where the body contains STX, ETX, SOH, etc?
Escape out ETX with at Data Link Escape Character (DLE)
Now, how do you deal with a body that has a DLE in it?
Also known as character stuffing
Examples in programming

30. Byte-counting Protocols
Just like with C strings (and files), we can detect the end of the string (or file) in two ways
Special character
An extra length field
Same is true in framing
In the byte-counting approach, we detect the end of the frame with an extra “Count” field

31. Byte-counting Protocols
DECNET DDCMP
SYN: 8
CLASS: 8
Count:14
Header: 42
What happens if the count field gets corrupted?
HEADER
CLASS
COUNT
BODY
SYN
SYN
CRC

32. Bit-oriented Protocols
Unlike byte-oriented protocols, these protocols don’t care about bytes
Could be transmitting
ASCII (7-bits)
Pixel values in an image …

33. Bit-oriented Protocols
High-Level Data Link Control (HDLC) protocol
Denotes beginning and end of a frame with the delimiter:
Also transmitted anytime link is idle to keep clocks synchronized
Still has bit stuffing problem if special delimiter occurs in body
5 1's; zero ALWAYS follows in the body.
Note that bit stuffing forces frames with the same payload length to have different realized sizes.

34. ...and then there's Sonet Synchronous Optical Network standard
Dominant standard for long-distance transmission of data over optical networks
Every frame is exactly the same size!
Has some special bit pattern to tell receiver where frame starts and ends, with no bit stuffing

35. Sonet How does the receiver know where each frame starts and ends?
Receiver looks for it consistently (once every fixed number of bytes)
Encoded using NRZ
To combat NRZ clock recovery problem, XORs data to be transmitted to a well-known bit patten
Can XOR encoded data with well-known bit pattern to decode