CPE G.fast is a state-of-the- art copper access DPU CPE Copper pairs technology that CPE Customer premises provides fiber-grade DPU NTU transmission speed L2+ L2+ over existing copper, processing processing In-premises minimizes energy 0 YR BB network G.9701 DRA G9701 consumption,reduce色 PSE FTU-R TCE TCE FTU-R maintenance cost, VCE FXO adapter ATA and provides great U-R- U-0 robustness and flexibility Copper pair for the customers. FX5 In-premises adapter winng To the CO Access network FXS adapter Figure 1.Example of G.fast deployment using RPF and derived POTS are fed by the PSE.To avoid G.fast performance packets from upper layers(L2+)are mapped loss,no other devices should be connected to the into data transmission units (DTUs)that are in-premises wiring conveyed transparently over the line.Reed-Solo- In other installations,the ATA may reside mon forward error correction [8]improves noise in the NTU,while derived voice service can be immunity:each DTU is assembled from multi- distributed throughout the premises via cordless ple Reed-Solomon codewords,and DTU bytes phone technology or by using smartphone con- are interleaved.The number of codewords and nection to in-premises WiFi;no particular option their size are configured to fit the throughput of for voice distribution is implied by G.fast. the line.Noise is further mitigated by retrans- G.9701 uses the frequency spectrum from mission of DTUs received in error:the number 2.2 to 106 MHz with full crosstalk cancellation of retransmissions for a DTU is limited by the between the lines sourced by a DPU:near-end latency bound.For retransmission and latency crosstalk (NEXT)is avoided by using synchro- control,each DTU contains a sequence number nized time-division duplexing (STDD),and and associated timestamp. far-end crosstalk(FEXT)is cancelled using vec- Discrete multi-tone (DMT)modulation [8] toring.No alien crosstalk cancellation is defined. is used for passing DTUs and management data Other G.fast innovations include dynamic allo- over the line.The advantages of DMT are well cation of resources between DPU sourced lines, known,especially its capability to operate on lines efficient energy-saving techniques,and dynam- with multiple bridged taps such as in-premises ic performance maintenance.Pair bonding is wiring.The specified tone spacing of 51.75 kHz defined to allow multiplication of customers'bit is 12 times that of very-high rate DSL 2(VDSL2) rate. [8]because G.fast loops are much shorter.Thus, G.fast customer installations vary by signal 2048-tone DMT is sufficient to cover the cur- attenuation and noise,and may be influenced by rent G.fast frequency spectrum,simplifying the other technologies.For reliable self-installation design.Each DMT symbol is cyclically extended provisioning,which reduces the operator's cost, using both prefix and suffix.The prefix mitigates G.fast defines a flexible and robust transmission inter-symbol interference and is configurable to protocol,online reconfiguration,and dynamic address a wide range of loop lengths.The suf- adaptation of bit rate,all maintained via robust fix is applied for transmit spectrum shaping and management channels.Zero-touch management overlaps with the following symbol to improve further reduces the operator's cost by avoiding efficiency:suffix size always fits the size of the truck rolls for future equipment upgrades and windowing [8].The default cyclic extension yields adding new subscribers. a symbol duration of 20.83 s.Up to 14 bits can G.fast is a state-of-the-art copper access tech- be loaded per tone.To further increase bit rates nology that offers fiber-grade transmission speed future versions of G.fast may extend the frequen- over existing copper,minimizes energy consump- cy spectrum to 211.968 MHz. tion,reduces maintenance cost,and provides The advantages of the G.fast duplex- great robustness and flexibility for customers. ing scheme (STDD)compared to FDD are FUNDAMENTALS OF G.9701 TECHNOLOGY described in [3].With STDD,the upstream and downstream sets of DMT tones can be selected TRANSMISSION METHOD independently,making STDD flexible in the fre G.9701 specifies the functionality of G.fast trans- quency domain.A particular selection depends ceivers (FTU-O and FTU-R)that establish a on the deployment scenario,and involves chan- high-speed transmission path between y-O and nel characteristics and spectrum compatibility Y-R reference points(Fig.1).The user's data issues (see below). IEEE Communications Magazine.March 2016 119IEEE Communications Magazine • March 2016 119 are fed by the PSE. To avoid G.fast performance loss, no other devices should be connected to the in-premises wiring. In other installations, the ATA may reside in the NTU, while derived voice service can be distributed throughout the premises via cordless phone technology or by using smartphone con￾nection to in-premises WiFi; no particular option for voice distribution is implied by G.fast. G.9701 uses the frequency spectrum from 2.2 to 106 MHz with full crosstalk cancellation between the lines sourced by a DPU: near-end crosstalk (NEXT) is avoided by using synchro￾nized time-division duplexing (STDD), and far-end crosstalk (FEXT) is cancelled using vec￾toring. No alien crosstalk cancellation is defi ned. Other G.fast innovations include dynamic allo￾cation of resources between DPU sourced lines, efficient energy-saving techniques, and dynam￾ic performance maintenance. Pair bonding is defi ned to allow multiplication of customers’ bit rate. G.fast customer installations vary by signal attenuation and noise, and may be infl uenced by other technologies. For reliable self-installation provisioning, which reduces the operator’s cost, G.fast defi nes a fl exible and robust transmission protocol, online reconfiguration, and dynamic adaptation of bit rate, all maintained via robust management channels. Zero-touch management further reduces the operator’s cost by avoiding truck rolls for future equipment upgrades and adding new subscribers. G.fast is a state-of-the-art copper access tech￾nology that offers fi ber-grade transmission speed over existing copper, minimizes energy consump￾tion, reduces maintenance cost, and provides great robustness and fl exibility for customers. fundAmentAls of G.9701 technoloGy trAnsmIssIon method G.9701 specifi es the functionality of G.fast trans￾ceivers (FTU-O and FTU-R) that establish a high-speed transmission path between -O and -R reference points (Fig. 1). The user’s data packets from upper layers (L2+) are mapped into data transmission units (DTUs) that are conveyed transparently over the line. Reed-Solo￾mon forward error correction [8] improves noise immunity: each DTU is assembled from multi￾ple Reed-Solomon codewords, and DTU bytes are interleaved. The number of codewords and their size are confi gured to fi t the throughput of the line. Noise is further mitigated by retrans￾mission of DTUs received in error; the number of retransmissions for a DTU is limited by the latency bound. For retransmission and latency control, each DTU contains a sequence number and associated timestamp. Discrete multi-tone (DMT) modulation [8] is used for passing DTUs and management data over the line. The advantages of DMT are well known, especially its capability to operate on lines with multiple bridged taps such as in-premises wiring. The specifi ed tone spacing of 51.75 kHz is 12 times that of very-high rate DSL 2 (VDSL2) [8] because G.fast loops are much shorter. Thus, 2048-tone DMT is sufficient to cover the cur￾rent G.fast frequency spectrum, simplifying the design. Each DMT symbol is cyclically extended using both prefi x and suffi x. The prefi x mitigates inter-symbol interference and is confi gurable to address a wide range of loop lengths. The suf￾fi x is applied for transmit spectrum shaping and overlaps with the following symbol to improve efficiency; suffix size always fits the size of the windowing [8]. The default cyclic extension yields a symbol duration of 20.83 s. Up to 14 bits can be loaded per tone. To further increase bit rates, future versions of G.fast may extend the frequen￾cy spectrum to 211.968 MHz. The advantages of the G.fast duplex￾ing scheme (STDD) compared to FDD are described in [3]. With STDD, the upstream and downstream sets of DMT tones can be selected independently, making STDD fl exible in the fre￾quency domain. A particular selection depends on the deployment scenario, and involves chan￾nel characteristics and spectrum compatibility issues (see below). G.fast is a state-of-the￾art copper access technology that provides fi ber-grade transmission speed over existing copper, minimizes energy consumption, reduces maintenance cost, and provides great robustness and fl exibility for the customers. Figure 1. Example of G.fast deployment using RPF and derived POTS. NTU CPE Copper pairs CPE •••••• ••• CPE DPU To the CO PON feeder Customer premises In-premises BB network In-premises wiring FXO U-R adapter U-O Copper pair Access network ATA FXS adapter L2+ processing γ-O γ-R TCE PE DRA TCE VCE PSU G.9701 FTU-R L2+ processing G.9701 FTU-R PSEHN-PHY PON-PHY FXS adapter DPU