1. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 1 Asynchronous and Synchronous Transmission Timing problems require a mechanism to synchronize the transmitter and receiver Two solutions –Asynchronous –Synchronous

2. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 2 Asynchronous Data transmitted one character at a time –5 to 8 bits Timing only needs maintaining within each character Resync with each character High overhead

3. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 3

4. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 4 Asynchronous - Behavior In a steady stream, interval between characters is uniform (length of stop element) In idle state, receiver looks for transition 1 to 0 Then samples next seven intervals (char length) Then looks for next 1 to 0 for next char Simple and cheap Overhead of 2 or 3 bits per char (~20%) Good for data with long intercharacter time (e.g. keyboard input)

5. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 5 Synchronous - Bit Level Block of data transmitted without start or stop bits Clocks must be synchronized Can use separate clock line –Good over short distances –Subject to impairments Embed clock signal in data –Manchester encoding –Carrier frequency (analog)

6. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 6 Synchronous - Block Level Need to indicate start and end of block Use preamble and postamble –e.g. series of SYN (hex 16) characters –e.g. block of 11111111 patterns ending in 11111110 plus other control information to form a frame More efficient (lower overhead) than async Character-oriented - the block of data is treated as sequence of characters (e.g. 8-bit characters) Bit-oriented - the block of data is treated as a sequence of bits

7. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 7

8. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 8

9. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 9 Line Configuration Topology –Physical arrangement of stations on medium –Point to point –Multi point Computer and terminals, local area network

10. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 10

11. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 11 Communication Modes Half duplex –Only one station may transmit at a time –Requires one data path Full duplex –Simultaneous transmission and reception between two stations –Requires two data paths (or echo canceling) Simplex

12. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 12

13. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 13 serial - a bit is transmitted immediately behind the preceding bit; timing accuracy between sender and receiver is required in order to sample the received sequence correctly parallel - separate channels are available to transmit consecutive bits (e.g. printer port)

14. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 14 Interfacing Data processing devices (or data terminal equipment, DTE) do not (usually) include data transmission facilities Need an interface called data circuit terminating equipment (DCE) –e.g. modem, NIC DCE transmits bits on medium DCE communicates data and control info with DTE –Done over interchange circuits –Clear interface standards required

15. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 15 Figure 5.4 Data Communications Interfacing

16. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 16 Characteristics of Interface Mechanical –Connection plugs Electrical –Voltage, timing, encoding Functional –Data, control, timing, grounding Procedural –Sequence of events

17. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 17 V.24/EIA-232-F ITU-T v.24 Only specifies functional and procedural –References other standards for electrical and mechanical EIA-232-F (USA ), formerly known as RS-232 EIA - Electronics Industries Association F version is the 6th revision –Mechanical ISO 2110, Electrical v.28 –Functional v.24, Procedural v.24

18. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 18 Mechanical Specification

19. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 19

20. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 20 Functional Specification (See Table 6.1 in Stallings)

21. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 21 Local and Remote Loopback

22. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 22 Procedural Specification E.g. Asynchronous private line modem When turned on and ready, modem (DCE) asserts DCE ready When DTE ready to send data, it asserts Request to Send –Also inhibits receive mode in half duplex Modem responds when ready by asserting Clear to send DTE sends data When data arrives, local modem asserts Receive Line Signal Detector and delivers data

23. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 23 Dial Up Operation (1)

24. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 24 Dial Up Operation (2)

25. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 25 Dial Up Operation (3)

26. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 26 Null Modem

27. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 27 Multiplexing Common application: long-haul communications Reasons of widespread use: –The higher the data rate, the more cost effective the transmission facility –Most individual data comm. devices require modest data rate support, e.g. 9600 - 64K bps –Efficient use of spectrum, e.g. Cable TV, GSM

28. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 28 Frequency-Division Multiplexing Different signals modulated onto different carrier frequencies Each channel has its own carrier frequency Guard bands for interference prevention Input signals can be digital or analog

29. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 29 FDM System

30. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 30 Example of FDM - Broadcast and Cable Television 4MHz signals fit in 6MHz bandwidth B&W TV signal - use AM modulation Freq. Allocation in US: Ch#254-60MHz Ch#360-66MHz :: Ch#69800-806MHz

31. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 31

32. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 32 Time-Division Multiplexing Each channel occupy one time slot Two types of TDM: –Synchronous TDM –Statistical TDM TDM and FDM can be used together in a system, e.g. GSM

33. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 33 Synchronous Time Division Multiplexing Data rate of medium exceeds data rate of digital signal to be transmitted Multiple digital signals interleaved in time May be at bit level or blocks Time slots preassigned to sources and fixed Time slots allocated even if no data Time slots do not have to be evenly distributed amongst sources

34. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 34 Synchronous TDM Each buffer: typically 1 bit or 1char. Source with faster rate can be assigned with more than one time slot per cycle

35. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 35 TDM Frames Data are organized into frames Each frames contain a cycle of time slots Slot length = buffer length Channel - sequence of slots dedicated to 1 source

36. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 36 TDM Link Control Two key data link control mechanisms: –Flow control –Error control These control mechanisms are usually provided, on a per-channel basis, by data link control protocol such as HDLC

37. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 37 Example of the use of data link control on TDM channels:

38. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 38 Statistical TDM Many timeslots in a synchronous TDM system may be wasted Statistical TDM can be used to improve utilization

39. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 39 The statistical TDM system will use a synchronous protocol such as HDLC. Within the HDLC frame, two possible formats can be used to carry the source data: one source per frame multiple sources per frame

40. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 40 Performance of Statistical TDM Average aggregate input rate < multiplexed line capacity However, input may exceed capacity in peak periods Solution: –add buffer to hold temporary excess input –trade-off between size of buffer and line rate, or –trade-off between system response time and line rate

41. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 41 Trade-off between size of buffer and line rate: When utilization rises, so does the buffer requirement Utilization > 0.8 is usually not acceptable

42. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 42 Trade-off between delay and line rate: When utilization rises, so does the average delay experienced Utilization > 0.8: delay usually not acceptable For constant utilization, increased link speed (M) gives smaller average delay

43. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 43 So far we have considered only the average buffer size required. The actual buffer size being used will affect the probability of overflow. The following figure also shows that U>0.8 is undesirable.

44. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 44 Asymmetrical Digital Subscriber Line ADSL Link between subscriber and network –Local loop Uses currently installed twisted pair cable –Can carry broader spectrum –1 MHz or more

45. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 45 ADSL Design Asymmetric –Greater capacity downstream than upstream Frequency division multiplexing –Lowest 25kHz for voice Plain old telephone service (POTS) –Use echo cancellation or FDM to give two bands, a smaller upstream & a larger downstream –Use FDM within bands (a single bit stream is split into multiple parallel bit streams and each portion is carried in a separate frequency band) Range 5.5km (cover about 95% of all U.S. Subscriber lines)

46. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 46 ADSL Channel Configuration

47. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 47 Discrete Multitone DMT Multiple carrier signals at different frequencies Some bits on each channel 4kHz subchannels Send test signal and use subchannels with better signal to noise ratio 256 downstream subchannels at 4kHz (60kbps) –15.36MHz –Impairments bring this down to 1.5Mbps to 9Mbps (depending on line distance and quality)

48. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 48 DMT Transmitter

49. Department of Electronic Engineering City University of Hong Kong EE3900 Computer Networks Data Communication Interface Slide 49 xDSL High data rate DSL (HDSL) Single line DSL (SDSL) Very high data rate DSL (VDSL)