1. CS 164: Slide Set 3: Chapter 2 Direct Link Networks

2. What are we looking at ? Networks in which there is no routing involved. We will look at the physical layer –signal representation, error correction. We will look at the link layer –Point to Point links –Multiple-access Ethernet, Token Ring

3. Network Adaptor and Device Driver Network adaptor sits on the Systems I/O and delivers data from the workstation’s memory to the network link. Device driver is the software module that manages this adaptor. – Issues commands such as from what memory location should outgoing data be transmitted, where should the incoming data be stored etc.

4. Example architecture

5. Links We have already seen -- –Twisted pair -- phone – Coaxial cable -- Cable TV – Optical Fiber – Free space -- IR etc.

6. Leased Lines Telephone lines -- long distance –DS1 or T1 line -- 1.544 Mbps –DS3 or T3 line --- 44.736 Mbps -- 30 T1 lines. T1 lines originally conceived for 24 digital voice circuits -- each of 64 Kbps. Leased line does not have to be a single unbroken piece of fiber -- logical connection.

7. Synchronous Transport Signal (STS) Optical signals -- also called OC for optical carrier. We have STS-1 (or OC-1), STS-3, STS-12, STS-24 and STS-48. STS-1 --> 840 Mbps.

8. Last Mile Links We talked about this -- ADSL, POTS --(Plain old telephone service) etc. 1.554─8.448 Mbps 16─640 Kbps Local loop Central office Subscriber premises

9. Signals Electromagnetic waves -- travel at speed of light Frequency -- rate at which wave oscillates (Measured in Hertz). Wavelength = speed of light/ frequency -- distance between maxima and minima of a wave.

10. The spectrum

11. Modulation Data is in bits -- we need to somehow translate this to signal variations. –This process is modulation. Vary either the amplitude, frequency or phase of the signal -- - dictated by the bit stream.

12. Encoding Represent binary data as signals. Let us ignore modulation for the moment. We have two signals -- high and low for representing 0s and 1s. – signals represent voltages. – 1 is high voltage, 0 is low voltage – As an example +5 V and -5 V.

13. The NRZ scheme.

14. Problem with NRZ Receiver keeps an average of the signal received so far. Compares incoming signal to this average -- if significantly higher -- high, if significantly lower, then low. If too many zeroes or ones, difficult to track this average -- the average wanders -- called the baseline wander. If there are clock drifts between the sender and receiver, this cannot be detected -- how many bits were transmitted ?

15. Other encoding schemes NRZI : Transition from current signal to encode a `1’. Stay at the same signal if it is a `0’. Solves problem with consecutive 1s but not zeroes.

16. Manchester Encoding X-OR the NRZ with a clock 0 --> represented as a low to high transition. 1 -- > represented as a high to low transition.

17. Problem with Manchester Encoding Doubles the rate of transitions. Half the time for the receiver to detect each pulse – Increase in complexity Bit rate = 1/2 Baud rate for Manchester encoding. Note -- baud rate represents signal rate and in some cases, bit rate could be higher than baud rate -- multiple bits mapped onto a signal.

18. 4B/5B encoding Insert extra bits into bit stream to break long sequences of 0’s and 1’s. Specifically every four bits of data encoded into a five bit code. Codes such that no more than 2 trailing zeroes and no more than 1 leading zero. (When codes are transmitted back to back no more than 3 consecutive zeroes. Resulting codes transmitted using NRZI. Specific codes -- 11111 -- Line idle 00000 -- Line dead 00100 -- Halt

19. Framing Blocks of data (each consisting of bits) exchanged between nodes that form a link-- these blocks are called frames. Network adaptor allows exchange of frames.

20. Point to Point Links Link Layer protocols are used. Popular Link layer protocols are BISYNC, PPP and DDCMP. The protocols deal with the transfer of frames across point-to- point links.

21. BISYNC Binary Synchronous Communication (IBM). STX and ETX denote start of text and end of text. If ETX appears in body of message, precede with DLE (Data Link Escape). If DLE appears precede with another DLE. Called Character Stuffing

22. DDMCP Digital Data Communication Message Protocol Instead of indicating end of text, frame length specified by a “count”. The danger is that count could get corrupted. Similar ETX in BISYNC could get corrupted. Framing errors could result -- error correction/ retransmissions

23. PPP Point to Point protocol -- most popular –Commonly run over dial up modem links. –Can be used over Ethernet too --> Look for RFC.

24. PPP Frame Usually contain default values Used for demultiplexing The protocol field identifies the higher layer protocol -- typically IP. Payload default is 1500 bytes but negotiable.

25. LCP -- Link Control Protocol Initial PPP set up. Can be used to negotiate payload size. LCP messages encapsulated into PPP frames. –Protocol field set to LCP

26. LCP Operations Dead Link Establishment Open Authenticate Network Layer Conf Terminating carrier detection or indication that PHY layer is present by system admin. Send Link Configuration Options Set Protocol = LCP Configure req/ack exchange Configure Network Layer Exchange IP addr.

27. Bit Oriented Protocols Previous protocols were byte oriented. Bit oriented protocols not concerned with byte boundaries. HDLC is an example.

28. HDLC High level data link control Both beginning and ending by a “distinguished” bit sequence. If sequence appears in text use character stuffing Analog of DLE -- bit stuffing -- upon 5 consecutive 1s, inserts a zero.

29. SONET Synchronous Optical Network Standard developed for transmission over fiber. SONET frame has special information that tells receiver where frame starts and ends -- no bit stuffing.

30. SONET or STS-1 Frames First three bytes overhead Total 810 bytes. 2 byte pattern at beginning -- receiver has to see this every 810 bytes -- if it does it concludes that it is in sync.

31. Multiplexing with SONET Time is the same but now, a different amount of data is transmitted. In each STS-3 frame we have 3x810 = 2430 bytes. Interleave bytes --1st byte of first STS-1, 1st byte of 2nd STS-1 and so on. –Ensures that bytes from each STS-1 are evenly paced and arrive at a smooth 51 Mbps rate at receiver. Bottomline: STS-3 channel could contain multiple low-data rate STS-1 channel.

32. STS-Nc frames. Payloads linked together instead of interleaving. Concatenation instead of multiplexing. –Cannot be multiplexed from different streams. – Called STS-Nc (As an example STS-3c). One of the fields in header used to denote concatenation. Read rest of stuff on SONET from book.

33. Error Detection How does one deal with bit errors ? Simplest thing to do -- send two copies of each bit. –If copies match, then data ok, else in error. –Too much redundancy --in most cases errors not that frequent (especially on fiber and coax). However, some redundancy will be needed.

34. Adding parity bits We can add parity bits. One dimensional parity -- One extra parity bit added to a 7 bit code to balance the number of 1s. –If number of 1’s is odd, parity bit is odd. –Else it is even. If one of the bits gets corrupted, then it can be detected. Multiple errors cannot be :(.

35. Two Dimensional Parity Do a similar thing across frames in addition to along bytes! Catches 1 bit, 2 bit, 3 bit errors and most 4 bit errors. How? -- both row and column parities are affected by a bit error.

36. Internet Checksum Simple algorithm to compute a checksum. Take all words -- add up and transmit result of the sum using 1’s complement arithmetic. Smaller number of error protection bits but less protection.

37. Summary We have so far seen –What is modulation ? –Encoding schemes. Link layer protocols --BISYNC, PPP, DDMCP, SONET, HDLC. Error detection using parity Internet Checksum. Sections 2.1 to 2.4.2

38. Next CRC --cyclic redundancy check. Retransmissions for reliable transmission. Multiple access channels.