BROADBAND TRANSMISSION STANDARDS DSL, ADSL and other flavors

2000 Bijan Mobasseri 2 Background l For the longest time it was taught that analog phone lines are bandlimited to 4 KHz l Nyquist’s first criterion then states that maximum theoretical data rate on such lines is 8000 pulses/sec. l So what happened?

2000 Bijan Mobasseri 3 Historical trends l Modems started out at 300 bits/sec using binary FSK modulation l Over time speed grew to 1200, 2400, 9600 and finally 56K bits/sec. l This increase was made possible by using high density digital modulations l And yes, this was all done within the confines of 4 KHz bandwidth l 56K, V.90, was the first standard taking advantage of digital backbone at 64 Kbps

2000 Bijan Mobasseri 4 26 Mb/sec over ordinary phone lines? l DSL in its various flavors, allows for up to 26 MB/sec over twisted pair. How is that possible? l The answer is that it is the switch that has limited us not the phone lines l Solution: avoid the switch by extracting digital signals before they get to the switch and re-route them to a broadband network

2000 Bijan Mobasseri 5 Original objectives l Cover a serving distance of feet on a single twisted pair telephone subscriber loop at T1 rate of Mb/s. Motivation: video-on-demand (meets H.261 MPEG rate) l Other features POTS availability: 300 Hz-4KHz Upstream control channel: 16-64Kb/s in 10 KHz-50 KHz Downstream: Mb/s in KHz

2000 Bijan Mobasseri 6 DSL vs. voice band modems CO Local loop Narrowband network internet Old Way upstream downstream

2000 Bijan Mobasseri 7 DSL vs. voice band modems internet Broadband splitDSL split Local loop <1 mile 26 Mb/s 3-26 Narrowband network CO Broadband POTS/ISDN VDSL 120 KHz 300 KHz 30 MHz Splitter allows for the coexistence of POTS and DSL on the same line

2000 Bijan Mobasseri 8 Spectrum picture POTS UpstreamDownstream ADSL 1.5 Mb/s 8 Mb/s G.Lite

2000 Bijan Mobasseri 9 Why the “A” in ADSL? l Asymmetric DSL rates are Downstream: Mb/s for distances less than feet (to CO) up to Mb/s up to feet Upstream: 16 to 640 Kb/s l Asymmetry is due to the bundling of the twisted pair telephone wires. Symmetric rates interfere with each other l Symmetric rates must be much slower and cover shorter distances

2000 Bijan Mobasseri 10 DSL flavors l ADSL Modulation: CAP or DMT Data rate: 8 Mb/sec downstream (in 240KHz-2 MHz) and up to 1 Mb/sec upstream in the KHz simultaneously with POTS l HDSL Four wire access for achieving symmetrical rates at T1 (1.544 Mb/sec) or E1(2.048 Mb/sec). Modulation: CAP64(passband) or 2B1Q( baseband) Spectrum in copper loop:0-300 KHz or KHz Fractional T1 rates supported (i.e. nx64 Kb/sec) No simultaneous voice data, HDSL2 will do that

2000 Bijan Mobasseri 11 More DSL flavors l SDSL Single-pair (two wire) implementation of 2B1Q or CAP modem Echo cancellation and adaptive equalization Symmetric data rates of 384 Kb/s, 768 Kb/s, 1 Mb/s or 2 Mb/s. SDSL is expected to eclipse HDSL due to its single pair, symmetrical data rate property l VDSL Supports much shorter distances Very high-speed DSL: Mb/s downstream Mb/s upstream for asymmetrical services and Mb/s for symmetrical services(short <1000 ft, two-wire loops) Modulation: M-ary CAP, DMT

DSL Modems Discrete Multi Tone (DMT) or Orthogonal Frequency Division Multiplexing (OFDM)

2000 Bijan Mobasseri 13 The general problem:multipath l Transmitted signal arrives at the receiver through a number of paths

2000 Bijan Mobasseri 14 Effect on pulse transmission l Multiple copies of the signal interfere with each other(ISI). Let T be bit duration. Pulses are delayed differently forming a delay spread l Largest delay is called maximum delay spread  max T A received symbol can be influenced by  max /T previous symbols We want max delay spread to be less than T, i.e. bit duration

2000 Bijan Mobasseri 15 What are the choices? Single carrier approach l Either reduce delay spread or increase T l Increasing T means reducing bitrate. Don’t like that. l Example (DVB) Transmission rate: R=1/T=7.4 Msymbols/sec Maximum channel delay:  max =224  sec l For a single carrier modulation  max /T=1600 l The complexity of removing this much interference is enormous

2000 Bijan Mobasseri 16 Multi-carrier approach l Split data at rate R into N parallel stream of rate R mc =1/T mc =R/N l Each slower data stream is modulated by a different carrier frequency and transmitted in the same band l ISI is reduced by a factor of N  max /T mc =  max /NT For DVB, we have N=8192 streams resulting in  max /T mc =0.2

2000 Bijan Mobasseri 17 What are we talking about? Multicarrier modulation l Modulation method used in DSL is of a frequency division multiplexing flavor. l Channel is divided into many subchannels l This approach bundles bits and transmits them over different frequency bands to counter channel characteristics frequency Bits per hertz frequency line gain Bits per hertz

2000 Bijan Mobasseri 18 Channel allocation l DMT in current ADSL creates 256 downstream subbands, 4 kHz each l Each channel can be modulated with QAM at up to 15 bits/sec/hz. l Theoretical transmission rate is then Mb/sec over a zero length line. How? 15(b/s/Hz) x 4000(Hz/channel) x 256(channels)=15.36 Mb/sec

2000 Bijan Mobasseri 19 Bit allocation l Instead of using equalizers to correct channel response, DMT spreads data over all channels according to the SNR in each one. frequency Bits per hertz frequency line gain Bits per hertz Sweet spot (up to 15 b/s/hertz) Ideal channel

2000 Bijan Mobasseri 20 Orthogonal Frequency Division Multiplexing:first step l Take a bitstream running and divide it into N parallel channels each at a reduced rate … … N On each line, group Bits into a symbol. The symbols are much wider than the original bitstream thus defeating possible ISI

2000 Bijan Mobasseri 21 Modulation in OFDM l Each symbol is mapped to a QAM constellation. Channels are modulated by orthogonal frequencies lines 16 frequencies f1f1 f2f2 f3f3 f 16

2000 Bijan Mobasseri 22 OFDM signal model l The OFDM signal can be written as the sum of N pulses modulating N orthogonal carriers l Fixed k means summing symbols from different channels, each modulating a different frequency Symbol coming from a M-ary QAM const. M=sqrt(N)

2000 Bijan Mobasseri 23 OFDM epoch l Let’s look at “epoch” k=0. This is taking N symbols one from each channel and adding them up f1f1 f2f2 f3f3 f 16 k=0 k=1 T Assigned to One of these

2000 Bijan Mobasseri 24 Generating OFDM by IFFT l Sample v(t) at t=kT s l OFDM can be generated by an inverse FFT X 0,0 X 0,1 X 0,N-1 IFFT V 0 V 1 V N-1

2000 Bijan Mobasseri 25 Variable bit loading l Input data at Mf s bits /sec grouped into blocks of M bits at a block (or symbol) rate of f s then... Serial to parallel conversion-M bits at a time Mf s bits /sec m1m1 m2m2 m3m3 m n bits

2000 Bijan Mobasseri 26 Bringing in the multitones l Each group of bits then modulate a separate carrier: the m n th group modulates a carrier with carrier frequency f c,n l There are N c carriers spaced ∆f Hz apart. The total number of carriers where N c =n. l For the previous slide, we need 5 carriers because we broke up 8 bits into 5 groups of bits f cn f c1  1 bit 2 bits 1 bit 3 bits

2000 Bijan Mobasseri 27 Selecting modulations 3 bits 2 bits When a block contains 2 bits, they represent 4 states. The 4 states come from a 4-QAM modulation. Same goes for 3 bits