1. 8-Reliability and Channel Coding
Dr. John P. Abraham
Professor
UTPA

2. sources of transmission errors
All data communication systems are susceptible to errors. Devices fail. Equipment that does not meet the engineering standards. We need to have ways to control and recover from errors
Interference
Distortion
Attenuation

3. Effect of transmission errors on data
Single bit error
Result from very short-duration interference
Burst Error
Multiple bits in a block of bits are changed. Results from longer-duration interference
Erasure
Does not correspond to either a logical 1 or 0. Can result from distortion or intereference.

4. Link Layer – LLC & MAC LOGICAL LINK & MEDIUM ACCESS CONTROL
The following topics fall under Layer 2 of OSI or the Data Link Layer.
Towards the end of this presentation, I will present more on data link layer.
This layer deals with issues of:
Framing, physical addressing, error control and reliability, flow control and medium access control.
Chapter 13 of your book handles some of these issues.

5. Strategies for Handling channel Errors. Channel Coding
Forward Error Correction (FEC)
Add additional bits for detection and correction.
Automatic Repeat reQuest (ARQ)
Sender and receiver exchange messages to indicate to insure all data arrives correctly.

7. Single parity bit
Explain odd or even parity
Show examples

8. RAC Detection

9. CRC introduced by Hamming
Used in high speed data networks
3 key aspects:
CRC supports Arbitrary length message
Excellent error detection
Fast Hardware implementation, CPU can do CRC computation fast

10. Theory behind CRC Given a from of k-bit
Transmitter generates n-bit sequence called FCS (frame check sequence)
The k+n bit is divisible by some predetermined number.
The receiver divides the total by the predetermined number. If no remainder, then the block is fine.

11. Error Correction Techniques
Hamming code
Chapter 8 ends here

12. Chapter 9 – Transmission modes
Two fundamental categories
Parallel –multiple bits at the same time
Serial – one bit at a time

13. Parallel transmission
High throughput
Transmission through buses uses parallel transmission
Control bus, address bus and data bus
Parallel printer ports using parallel transmission

14. Serial transmission My handwritten notes
Parallel data is converted to serial data by UART (Universal Asynchronous Receiver and Transmitter) found which is connected to RS232 serial ports.
USART (Synchronous) chip handles transmission for synchronous networks.
Bits are arranged in order based on little Endian or big Endian
Timing of serial transmission is very important determine boundaries.

15. Asynchronous transmission
A computer can send any time. No timing coordination exists between sender and receiver.
There must enough information in data send to determine boundaries.
So, extra overhead is required such as preemble, start and stop bits, etc.
Try mode from command prompt
Try changing it – mode: 2400, o, 8
Alternatively go to serial port from device manager and set it from there.

16. Synchronous transmission
Bits are transmitted continuously without idle bits.
Synchronization is achieved various ways. Some of these are explained my notes.

17. Frames It is unusual for a computer to have data to send continuously.
So it is easier to collect bits into blocks of bits and then create a frame from the blocks.
Now sending computer can add distinguishing marks at the beginning and end of each frame.
Character counting
Bit stuffing etc. had been used.

18. Isochronous transmission
The speed is predetermined.
Steady bit flow for multimedia applications. Buffering handles speed differences

19. Simplex, half-duplex, full duplex transmission
Simplex – send one direction only (radio)
Half – duplex: Send and receive, but channel (cb radio)
Full duplex: Send and receive two channels

20. DCE and DTE
Already discussed
Chapter 9 ends here

21. Chapter 10 Modulation and Modems
I would refer you to my notes for this one
We discussed this in class as well (remember PSK, ASK, and FSK)

22. Chapter 11 Multiplexing, Demultiplexing
We already discussed FDM, TDM and statistical
Wavelength Division Multiplexing is used for Fiber. We divide light into its components
There are telephone multiplexing technologies not discussed here
end

23. ARQ Receiving side send Ack back if the message arrived correctly.
If timed out the message is resent
ARQ is provided if only error detection is implemented without error correction.
There are 3 types of ARQ
Stop and wait
Go-back-n
Selective repeat

24. Sliding Window Protocols
For full duplex two transmission lines are required
One to send data and the other to receive data and/or ACKs
Sending ACKs in a packet of its own is waste of resources
May use pggybacking
The acknowledgement is attached to an outgoing data frame(ack field in the frame header)
Data link layer must wait until it has some thing to send before it can ack.
This may cause time out and the frame may be re-sent.

25. The essence of sliding window protocols is that at any instant of time, the sender maintains a set of sequence numbers corresponding to frames it is permitted to send. These frames are said to fall within the sending window. Similarly, the receiver also maintains a receiving window corresponding to the set of frames it is permitted to accept. It is NOT necessary for the sender and receiver to have same number limits.
Sequence numbers of frames sent but not acknowledged are kept in the senders window.
Th receivers window corresponds to the frames it may accept.
See page 434 in your text book.

26. One bit sliding window protocol
uses stop and wait scheme, since the sender transmits a frame and waits for its acknowledgement before sending the next one.
The acknowledgement field of the frame contains the number of the last frame received without error. If this number agrees with the sequence number of the frame the sender is trying to send, the sender knows that it is done and can proceed with the next frame.

27. A protocol using Go-Back-N
Takes into account of round trip transmission time.
Enough frames should be send to stuff the band with to avoid channel idling
Consider a 50-kbps satellite channel with 500 m-sec round-trip propagation delay
Let us send 1000 bit frames
At t=0 starts sending the first frame, at t=20 msec the frames has been send
Receiver fully receives only at t=270msc, and ack will be received at t=520 msec.
Sender would be blocked 96% of time- only 4% of available band width was used.

28. Go-Back-N continued.
Instead of sending 4 frames, the sender should have sent 26 frames. This will take t=520 msec.
at t=520 the acknowledgement for the first frame will be received. The senders maximum window size is 26.
This technique is called pipelining.
If a frame in the middle of a stream gets damaged, what should the receiver do? Should all frames starting at that point be re-sent?
Go-Back-N simply discards all frames including the damaged one, not sending acks. The time out at the sender will resend all subsequent frames.
Can waste a lot of bandwidth in an unreliable channel.

29. Selective Repeat
Receiver stores all good frames after the damaged frame.
Notifies the sender of the bad frame, and sender re-sends that one.
This technique requires the data link layer to have large amount of memory for the cache.

30. Data Link Layer
Framing: bits from physical layer are grouped into units called frames. To distinguish one frame from the other there are frame delimiting techniques used such as byte or bit stuffing. Discussed in later chapters. A frame contains a header (control information) and the payload (data).
Physical addressing – a 48 bit addressing scheme for Ethernet.
Error control and reliability CRC, checksum, etc.- already discussed above.
Flow control – sliding window discussed above.
Medium Access – CSMA-CD and others discussed later.
Bridging – connecting two LANS into one.

31. Point to Point Protocol (PPP)
Used by ADSL (PPP over Ethernet) or POTS.
Derived from High-Level Data Link Control (HDLC)
PPP logs-in to authenticate using LCP (link control protocol)
PPPoE used in ADSL runs in two stages: The discovery stage and the PPP session stage.
The discovery stage, user station discovers the MAC address of the access concentrator at the ISP.
In the session stage a session ID is assigned.