Introduction to Communication Networks Spring Unit 4 Multiplexing, Framing, and some solutions...

Transcription

1 Introduction to Communication Networks Spring 2007 Unit 4 Multiplexing, Framing, and some solutions... EECS 122 SPRING 2007

2 Acknowledgements slides comming from: Data and Computer Communication by Wiliam Stallings (our supplementary textbook). Data Communications and Networking by B. Forouzan, Mc Graw Hill, 2004 ( a very nice-to-read book!) Some figures have been used form the earlier issues of the EECS 122 tought by Prof Jean Walrand. Introduction to Telephones & Telephone Systems by A. Michael Noll, Artech House, 1986 Megabit Data Communication, John T. Powers, Henry H. Stair II, Prentice Hall Digital Telephony by J. Bellamy:, J. Wiley & Sons, 2rd edition, of 63

3 MULTIPLEXING 3of 63

4 Multiplexing General Problem: Several - n- different channels (voice, TVchannels) should be supported between a pair of locations. We would like to avoid usage of n physical links (cables). Looking at the features of media you will easily see that the supported bandwidth exceeds by far the bandwidth needed for each channel... 4of 63

5 Variants of multiplexing The dimensions of multiplexing time (t) frequency (f) code (c) space (s i ) sometimes Care for separation: guard spaces, code orthognonality Multiplexing can be Synchronous (constant allocation) Statistical (variable allocation) 5of 63

6 Frequency Multiplex Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum (in the synchronous case for the whole time!) Note: guard zones in frequency are needed!! Special case: Wave division Mux 6of 63

7 Schema for FDM [Forouzan] mod 7of 63

8 FDM of Three Voiceband Signals 8of 63

9 Example - Community Antenna TV (CATV) Currently used systems require about 6MHz /TV Channel 9of 63

10 Time Multiplex The whole bandwidth is used all the time, but alternatively by different channels! 10 of 63

11 Time Multiplex: Interleaving of data segments [Forouzan] 11 of 63

12 Time and Frequency Multiplex [Schiller] Combination of both methods A channel gets a certain frequency band for a certain amount of time Example GSM cellular telephony: FDM with TDD (8 bidirectional channels per frequency band) is used... k 1 k 2 k 3 k 4 k 5 k 6 c f t of 63

13 Time Division Duplex (TDD) and FDD Similarly a Frequency Division Duplex - FDD with two frequency Channels: for up-link and down-link respectively, can be defined 13 of 63

14 Bursty Data Burstiness of data In many data communication applications, data occur in bursts separated by idle periods This type of data can often be transmitted more economically by statistical (or asynchronous) multiplexing of 63

15 Synchronous vs. Statistical TDM Note: Data slots must be addressed! 15 of 63

16 Statistical Multiplexing Gain [mod.from N.Mc Keown, Stanford] Comment: Synchronous Multiplexing would use 2C bits/s statistical uses R<2C. But: how to define R? The queue helps smooth the load but there might be losses!!! Rate A+B 2C R < 2C C A B R time Statistical multiplexing gain = 2C/R Other definitions of SMG: The ratio of rates that give rise to a particular queue occupancy, or particular loss probability. 16 of 63

17 Example of Statistical Multiplexer Performance 17 of 63

18 Probability of Overflow and Buffer Size ρ- is the ratio of the offered load to the nominal service rate of the system see Queuing (later) 18 of 63

19 FHSS (Frequency Hopping Spread Spectrum) I Discrete changes of carrier frequency sequence of frequency changes determined via pseudo random number sequence Two versions Fast Hopping: several frequencies per user bit Slow Hopping: several user bits per frequency Advantages ROBUSTNESS: impact of frequency selective fading and interference limited to short period! of 63

20 FHSS : Schema of the operation Fast hopping Slow hopping 20 of 63

21 FHSS System overview user data modulator narrowband signal modulator spread transmit signal transmitter frequency synthesizer hopping sequence received signal demodulator narrowband signal demodulator data hopping sequence frequency synthesizer receiver of 63

22 CDMA: Code Division Multiple Access A Channel: a unique code in the same spectrum at the same time 22 of 63

23 Space division multiplexing...wireless... Assume a sectorized antenna Transmisison/receive in one of the sectors does not limit the usage of other sectors of 63

24 FRAMING 24 of 63

25 Framing WHY: The physical layer supports bit-synchronization. Data units bigger than a single bit must be recognized... HOW: Time gaps (not good - might be squeezed within the physical layer), Physical signaling - the physical layer has to support some control symbols, besides of a 0 and a 1, say a J and K. Example: the Manchester extension of the IEEE token ring. Field Lenght marker at the beginnig of the field: Whole notion of unit lost if this lenght marker would get corrupted! Specific symbols: Character oriented, bit oriented variants. Clock based 25 of 63

26 Framing - delimiting symbols Delimiting characters in character based transmission, i.e. the case when the transmitted information is composed of symbols - c.f.the ASCII code table. SYN SYN - used for the synchronization SOH...STX...ETX header and for the text. Transmission of binary information: the framing sequence for the DLE STX...binary information... DLE ETX What about a DLE inside the binary information?? input binary information:... DLE... transmitted binary information:... DLE DLE... extracting the information:... DLE... A) This scheme is closely tied to an 8 bit character representation. B) A single error can cause a misinterpretation. 26 of 63

27 Framing - delimiting symbols (2) Example (a) DLE STX A DLE B DLE ETX (b) DLE STX A DLE DLE B DLE ETX Stuffed DLE (c) DLE STX A DLE B DLE ETX (a) Data sent by the network layer. (b) Data after being character stuffed by the data link layer. (c) Data passed to the network layer on the receiving side. 27 of 63

28 Bit oriented transmission-delimiting flag Delimiting flags in bit oriented transmission, i.e. the case when the transmitted information is represented as a string of bits (the concept of octets is sometimes used to support the bit manipulation). The usual flag pattern: Transmission transparency is assured via bit stuffing: The transmitter always stuffs a 0 after The receiver removes a 0 following User data are transmitted as Combinations of solutions discussed above are frequently used to increase the power of framing - e.g. delimiting flags together with byte (symbol) count. See additional reading after bit error unit! 28 of 63

29 Bit-oriented Transmission The packet framed by two special bit patterns called flags. Bit stuffing is used to prevent a flag from occurring in the middle of a packet. The bit-stuffing procedure is illustrated in (b): a bit 0 is inserted after each pattern that appears in the packet. The reverse procedure (bit destuffing) is performed at the receiver to restore the original packet. 29 of 63

30 Clock based Idea: to have a sequence of markers in predefined positions here the sequence 101 in proper distance all the elements are assumed to have equal length! Problem: This sequence, with the proper spacing, MIGHT appear just by chance in the content!!! Solution: Look several times the random appearance will not be repeatable over numerous frame series! 30 of 63

31 Examples of transmission systems 31 of 63

32 Phone Backbone - FDM Carrier Standard OLD! [Forouzan] 32 of 63

33 FDM Carrier Standards - OLD some numbers 33 of 63

34 American Digital Hierarchy Each channel carries data (voice) digitized at a rate of 8000 samples per second with 8 bit per sample. A frame contains 24 channels plus one framing bit per frame. Thus, the required transmission rate for DS-1 is 8000 x (24 x 8 + 1) bits per second = Mbit/s. 34 of 63

35 American Digital Hierarchy synchronization [bellamy] T1/D4 Superframe and D4 Channel Slots. A super frame combines 12 frames of 193 bits each. The framing bits of these frames produce the T1/D4 superframe pattern of of 63

36 Extended Superframe Format Framing (1) Extension of the super frame from 12 repetitions to 24 repetitions. Framing bit positions take new functions and meanings (24 bits). Framing (6 bits) Error Checking (6 bits) Maintenance Communications (12 bits) 193rd Bit 125 µs 193rd Bit Bipolar format ,2 µs Binary code Basic North American PCM framing and signaling format. 36 of 63

37 Some specific ways of using it... [Stallings] 37 of 63

38 American TDM Carrier Standard [Forouzan] 38 of 63

39 TDM Carrier Standards North American and International TDM Carrier Standards 39 of 63

40 Just for info the International Frame of 63

41 Trunks 41 of 63

42 Mhmm... Not quite unified of 63

43 Instability of the timing.. Cyclic changes of the data rate of sending Systematic difference in timing between the sender and receiver. What can we do? cyclic- or fluctuating differences removed by elastic buffer... Systematic difference has to be dealt with Systematic differences. How? Plesiochronous operation (T-hierarchy) Synchronous operation (SONET) 43 of 63

44 Synchronization - Definition of terms: Single discrete signal may be either Isochronous : Constant frequency of signal changes (e.g. 8 khz of PCM-coded voice) Anisochronous Two discrete signals may be either Synchronous or Asynchronous Mesochronous (meso = middle, greek) Same center frequency, limited phase difference (e.g. signals from a single oscillator being processed in different stages) Plesiochronous (plesio = near, greek) Nominal same center frequency (e.g. two independent oscillators) Heterochronous (hetero = different, greek) 44 of 63

45 Changes in Length of Transmission Media Path length changes as a result of thermal expansion or contraction of guided media, or as a result of atmospheric bending of a radio path. While a path is increasing in length, the effective bit rate at the receiver is reduced because more and more bits are being stored in the medium. Example km long T2 transmission link using 22-gauge copper wires which have a velocity of propagation of km/s: Thermal expansion coefficient 16.5 ppm/ C The temperature of the wire increases by 20 C in one hour: Change in path length: Change in number of bits: Change of data rate: Relative change in received data rate: 6 Δd = = km ΔB = = 5.49 bits ΔR = = bps 3600 ΔR R = = of 63

46 Synchronization Elastic Buffer Can deal with fluctuating differences, introduces a delay of 63

47 Elastic buffer - implementation Elastic store operation with a one-frame memory. 47 of 63

48 Pulse Stuffing Concept (1) Two channel multiplexer showing equal data rates for each input No pulse stuffing needed since both streams have exactly the same rate 48 of 63

49 Pulse Stuffing Concept (2) Simplified pulse stuffing example Assume that stream 1 is slightly faster than 2 but within legally (standards!) defined upper bound. Additional information needed to allow adjustments of the information flow within each sub-channel. C i bit specifies if the following S i bit carries tributary data (C i = 0) or just stuffing. The output data rate should be equal to DOUBLE the maximum LEGALLY Permitted data rate of inputs. 49 of 63

50 American Digital Hierarchy Every multiplexing stage adds overhead. Digital Level Signal Type Rate in Mbits/s Number of Channels Channels of the type Number of kbits in overhead 0 DS DS-0-1 DS DS DS DS DS DS-2 DS of 63

51 Pulse Stuffing Concept (3) Example of a higher level multiplexing format for 6.312Mbps DS2 signal in North American digital hierarchy. A DS2 signal is derived by bit interleaving of four DS1 signals and adding the appropriate overhead bits. The C 1 C 4 bits are repeated 3 times each, in order to allow for majority voting. 51 of 63

52 A real multiplexer 52 of 63

53 Problems of PDH The Plesiochronous Digital Hierarchy had a number of problems: Each part of the world has its own transmission hierarchy (expensive interconnection equipment) Justification (bit stuffing) spreads data over the frame add-drop-multiplexers are hard to build extract a single voice call -> demultiplex all steps down switching of bundles of calls (n * 64 kbit/s) is difficult (every switch has to demultiplex down to DS0 level) The management and monitoring functions were not sufficient in PDH PDH did not define a standard format on the transmission link Every vendor used its own line coding, optical interfaces etc. Very hard to interoperate 53 of 63

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55 55 of 63

56 SONET / SDH (1) Synchronous Optical NETwork (SONET) and the Synchronous Digital Hierarchy (SDH) Started by Bellcore in 1985 as standardization effort for the US telephone carriers (after AT&T was broken up in 1984), later joined by CCITT, which formed SDH in 1987 Three major goals: Avoid the problems of PDH Achieve higher bit rates (Gbit/s) Better means for Operation, Administration, and Maintenance (OA&M) SDH is THE standard in telecommunication networks now Originally designed to transport voice - used for everything 56 of 63

57 SONET / SDH (2) SONET / SDH - Basic concepts SONET / SDH system consists of switches, multiplexers and repeaters (and the fiber in between) PATH is the connection between source and destination LINE runs between two multiplexers (possibly through repeaters) SECTION is the connection of any two devices (point-to point) Source Multiplexer Repeater Multiplexer Repeater Multiplexer Section Section Section Section Line Path Line 57 of 63

58 SONET / SDH (3) Level US Europe, Data rate Data rate Data rate Japan (gross) (SPE) (user) 1 OC OC-3 STM OC-9 STM OC-12 STM OC-18 STM OC-24 STM OC-36 STM OC-48 STM OC-192 STM No overhead bits needed for justification higher speed link is formed by byte-interleaving data from lower speed links exact multiples of lower speed data rates so e.g. OC-12 contains 12 byte interleaved OC-1 frames 58 of 63

59 SONET Clock-based [PD] each frame is 125us long e.g., SONET: Synchronous Optical Network STS-n (STS-1 = Mbps) STS-1 = OC-1 Overhead Payload Hdr STS-1 Hdr STS-1 Hdr STS-1 9 rows Hdr STS-3c 90 columns 59 of 63

60 SDH - Clocking All network elements are totally synchronous Still, there are delays in the network Hierarchy of clocks, lower levels synchronize to higher levels Stratum Min. Accuracy Min. Stability Pull-In Range 1 ±1 in Master Reference Master Reference 2 ±1.6 in ±4.6 in ±32 in 10-6 ±0.025 * 1 in ±7.0 * ±3.5 in 10-9 (some conditions) Should synchronize with a clock accurate to ±1.6 in 10-8 Should synchronize with a clock accurate to ±4.6 in 10-6 ±50 * accurate to ± 32 in 10-6 N/A Should synchronize with a clock * = Minimum accuracy relative to 1,544,000 bits/s. 60 of 63

61 What about tributary speed differences [PD] - Frames appear synchronously, and have always the header of fixed lenght (9 rows in STS-1) and position (at the begining of the frame!) - The Payload does NOT have to begin directly after the header it is fixed by a pointer (part of th eheader). - The Payload has always a constant length thus might overflow into the next frame - If there are excessive bytes, those are stored in the header and the moves to the left. If bytes are missing the empty bytes are marked and the pointer move to the right. Frame 0 87 columns 9 rows Frame 1 61 of 63

62 High Reliability 60 ms for reconfiguration [LUCENT] 62 of 63

63 What SONET/SDH does better (conclusions) SONET (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) are almost identical Interconnection is easy (exists, works) Justification, if still needed, is performed by pointers Data from each input is placed in a payload container (Administrative Unit- AU) it spans multiple SONET/SDH frames a pointer in the header of the SONET/SDH frame signals the start of the payload container in the frame (in 3-byte increment for SDH) positive and negative justification through this pointer slip buffer delay reduces from 193 bit for a T1 signal down to 24 bit Single 64 kbit/s lines (1 byte in the SONET/SDH frame) can be found and extracted in the frame HIGH RELIABILITY!!!! 63 of 63