XDSL and DSLAM Access Technologies

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2 XDSL and DSLAM Access Technologies Abstract What are the differences between the different forms of xdsl technology, such as ADSL and HDSL? How are they implemented. What are the limitations? What are DSLAMS, and how are build up the protocol stacks in those topologies? Contents 1. Voice-band and Base-band XDSL Overview... 5 IDSL: ISDN Digital Subscriber Line... 5 HDSL: High-speed Digital Subscriber Line... 5 S-HDSL: Single-pair High-speed Digital Subscriber Line... 6 ADSL: Asymmetrical Digital Subscriber Line... 6 ADSL-Lite: Asymmetrical Digital Subscriber Line Lite... 7 VDSL: Very High-speed Digital Subscriber Line DSLAM topologies and their protocol stacks... 9 Overview... 9 Encapsulation protocol stacks for connectivity Bridging Routing (USB) (ATM) over Ethernet The Subscriber Management System (SMS) Other technologies Cable-modem technology Topology for cable modem fast backbone access Conclusion Glossary The first choice for the l@st mile

3 1. Voice-band and Base-band DSL (Digital Subscriber Line) based technologies are also often referred to as base-band technologies. This opposed to voice-band technologies. Both types of modem equipment produce an analog signal for transmission over a copper connection. The difference between base-band and voice-band is related to the used frequency spectrum. In the case of voice-band technology, this spectrum is limited to the frequency band between 300Hz and 3500 Hz. This band is referred to as the voice-band, as it is the frequency band which is used by the Plain Old Telephone Service (POTS). The advantage of Voice-band modem equipment is that it can be used in conjunction with the backbone telephony network, for direct transmission of the analog signals over the backbone infrastructure. The main disadvantage of voice-band technology is that it is limited in speed (33.6 kbps for leased-line applications, 56 kbps (theoretical) for Internet dial-up connections) Exchange Exchange M o d e m M o d e m Exchange Exchange Local Loop Local Loop Voice-band Modem connection The real history of voice-band technology started in 1964, when the V.21 modulation type was voted by ITU-T (previously CCITT). It offered the possibility to transport 300 bps over a 2-wire PSTN or analog Leased Line connection. The same wire can carry today up to 33.6 kbps (V.34), whereas V.90 and V.92 can support on digital backbone networks downstream speeds up to 56 kbps (theoretical), and upstream speeds up to 33.6 kbps and 48 kbps. 3 The first choice for the l@st mile

4 V V V.22bis 84 V V.32bis 91 T.FAST 92 V V V V History of voice-band technology xdsl or base-band technology is virtually imposing no restrictions on the used frequency spectrum. This means that the generated signal of an xdsl modem can NOT be transported over a classical telephone line. Therefore, the xdsl connection is restricted to a plain copper connection (galvanic connection). When used in carrier environments, the xdsl is used to connect an end-user to the backbone network of the carrier. It uses the twisted copper infrastructure (local loop), which is available between the end user and the carriers backbone. M o d e m backbone Exchange Exchange Local Loop M o d e m Local Loop 4 The first choice for the l@st mile

5 2. XDSL Overview Since the frequency restrictions are omitted for the xdsl or base-band modem technology, much higher speeds can be obtained. With the actual technology, it is possible to transport up to 8 Mbps over 3 or 4 km, depending on the diameter of the copperwire. The xdsl family includes several technologies, covering several speeds and distances. 144 kbps 2W ANSI T1.601 <8 km 1.5 Mbps/ 384kbps G W 2 Mbps 4W-6W ETR152 G km Mbps 2W 3..10km 6..8 Mbps/ 640 kbps 2W ANSI T1.413 G km +20 Mbps 2W <300 m IDSL ADSL HDSL S-HDSL ADSL VDSL Lite Different xdsl technologies IDSL: ISDN Digital Subscriber Line Well standardized technology, derived from the Basic Rate ISDN NT access equipment, using 2B1Q modulation Standards: ANSI T1.601 Typical supported speeds: 64, 128, 144 kbps Number of used copper pairs: 1 Typical distance (0.5 mm): 8 km HDSL: High-speed Digital Subscriber Line Standardized technology, usig 2B1Q or CAP modulation technology and fixed line speed of 2 Mbps Standards: ETSI ETR 152, ITU-T G911.1 Typical supported speeds: 2 Mbps (Nx64 k, depending on manufacturer) Number of used copper pairs: 1,2 or 3 (depending on the model) Typical distance (0.5 mm): 3..5 km (depending on the model: 2 or 3 pairs) 5 The first choice for the l@st mile

6 Amplitude khz 2B1Q HDSL (2 pair) Frequency Amplitude 0 4 khz 10 khz 263 khz CAP HDSL (2 pair) Frequency Frequency spectrum of HDSL technology S-HDSL: Single-pair High-speed Digital Subscriber Line Technology still under standardization at ETSI, using TC-PAM (Trellis Coded Pulse Amplitude Modulation) modulation technology and adaptive line speeds up to 2 Mbps. The Modulation is derived from HDSL-2, an American standard for 1.5 Mbps transmission over 1 pair, using TC-PAM. Typical supported speeds: Nx64 kbps Number of used copper pairs: 1 Typical distance (0.5 mm): km (depending on the speed) ADSL: Asymmetrical Digital Subscriber Line Technology using DMT (Discrete Multi Tone) modulation technology and adaptive downstream speeds up to 6 or 8 Mbps. The uplink speed is limited for most implementations to 640 kbps. The modulation is mainly used for residential internet access, where it can be combined with POTS (Plain Old Telephone Service) on the same copper pair. In order to support this, the use of POTS splitters (active or passive filters) is necessary. Standards: ANSI T1.413, ITU-T G922.1 Typical supported speeds: Downstream up to 6 or 8 Mbps, Upstream up to 640 kbps Number of used copper pairs: 1 6 The first choice for the l@st mile

7 Typical distance (0.5 mm): 2..8 km (depending on the speed) Amplitude 0 4 khz 25 khz 138 khz 200 khz Category 1 ADSL 1.1 Mhz Frequency Amplitude 0 4 khz 25 khz 138 khz 200 khz Category 2 ADSL 1.1 Mhz Frequency Frequency spectrum of ADSL technology ADSL-Lite: Asymmetrical Digital Subscriber Line Lite Technology using DMT (Discrete Multi Tone) modulation technology and adaptive downstream speeds up to 1.5 Mbps. The uplink speed is limited for most implementations to 384 kbps. As for full ADSL, the modulation is mainly used for residential internet access, where it can be combined with POTS (Plain Old Telephone Service) on the same copper pair. In order to support this, the use of POTS splitters (active or passive filters) is necessary only at the CO (Central Office). For the CPE (Customer Premises Equipment) side, no POTS-splitter is required. ADSL Lite is a strip down version of full ADSL, and is mainly targeting the low-cost residential PCmarket. Standards: ITU-T G922.2 Typical supported speeds: Downstream up to 1.5 Mbps, Upstream up to 384 kbps Number of used copper pairs: 1 Typical distance (0.5 mm): 3..8 km (depending on the speed) VDSL: Very High-speed Digital Subscriber Line Technology in research phase, offering adaptive downstream speeds up to 52 Mbps over very limited distance (300 m). This technology will mainly be used in 7 The first choice for the l@st mile

8 asymmetrical configurations (lower upstream speed), and in combination with fibre to the curb (FTTC). Main application domains will be in VoD (Video on Demand) and other multimedia applications. Standards: not yet available Typical supported speeds: Up to 52 Mbps (in asymmetrical mode), up to 26 Mbps (in symmetrical mode) Number of used copper pairs: 1 Typical distance (maximum speed, 0.5 mm): 300m 8 The first choice for the l@st mile

9 3. DSLAM topologies and their protocol stacks Overview DSLAM (Digital Subscriber Line Access Multiplexer) is a multiplexing technology that is used for multiplexing several users onto a higher speed link. The technology allows reducing the number of physical connections between the Central Office modem equipment and the backbone. The multiplexing can be based on a TDM (Time Division Multiplexing) scheme, or can be based on switch based schemes like Frame-Relay, or ATM. The switch-based schemes have the advantage that they allow limiting the speed of the uplink to the backbone to a value lower than the sum of the access speeds of all the end-users. This ratio is called the contention ratio and is a measure for the quality of the service. DSLAM implementations based on switch based technologies are mainly used for Internet access services, while DSLAM implementations based on TDM are more common for leased line services and interfacing to legacy backbone infrastructures. This first picture shows a DSLAM with ADSL modems and POTS splitters for Internet Access. Internet Access POTS Telephony Backbone ATU-R Internet Transport Infrastructure ATM, Fr-R,,.. DSLAM ATU-C ATU-C ATU-C ATU-R CO ATU-R DSLAM: Digital Subscriber Line Access Multiplexer CO: Central Office (Incumbent Operator) ATU: ADSL Termination Unit POTS: Plain Old Telephone Service DSLAM technology used with ADSL for Internet ACcess 9 The first choice for the l@st mile

10 Encapsulation protocol stacks for connectivity The way the data are encapsulated, is relevant for the offered functionality (router, VPN support, etc.), and for the efficiency with which the data are transported between the end user and the backbone (protocol overhead, filtering) Although in most implementations the encapsulation today is based on ATM, also Frame-Relay or pure encapsulation is possible. The examples in the next paragraphs describe the ATM based access protocol stacks. Bridging The most basic form of delivering connectivity is based on the direct encapsulation of Ethernet frames into ATM. ISP PVC TU-C DSLAM TU-R RFC 1483 STM-1 RFC1483 ADSL 10Base-T Subscriber 1483 Bridged At the end-users side, the Ethernet frames are directly encapsulated into an ATM PVC. For doing this, one uses RFC 1483 (Multiprotocol encapsulation over ATM adaption layer 5). This system allows setting up a single ATM PVC, to one destination (most commonly the Internet). Integrated in the TU-R (Termination Unit Remote), integrated bridging (filtering on MAC layer) limits the used bandwidth on the DSL link. Advantages are its simplicity and compatibility with the large installed base of Ethernet equipment. Routing Instead of bridging, it is also possible to encapsulate directly the layer in ATM, adding routing of the data as well. This allows for a better bandwidth usage, and the set-up of multiple ATM PVC s to different destinations. This encapsulation is described in RFC 2225 (previously RFC 1577), Classical and ARP over ATM. 10 The first choice for the l@st mile

11 Coporate ISP PVC1 PVC2 TU-C DSLAM TU-R RFC 2225 STM-1 RFC 2225 DSL 10Base-T Subscriber RFC 2225 Routed Advantage is the compatibility with the installed base of Ethernet equipment. Disadvantage is many PVC s which may to have set-up for all the different users going to different services (in the example Internet and corporate office connectivity), and the rather complex configuration which is needed in the ADSL TU (Termination Unit) is the protocol, which is used today for most dial-up internet connections. By using also in fixed xdsl connections, it is possible for both the service provider and the end-user to maintain practically the same network configuration. In the end user equipment, can be carried over USB, Ethernet or ATM. Below, those possibilities are described. (USB) ISP PVC TU-C DSLAM TU-R RFC 2364 STM-1. RFC 2364 ADSL USB Simple USB based connection RFC 2364 describes the transport of over ATM. As with dial-up connections, features such as dynamic assignment of addresses, default routers and DNS servers help simplify remote management. 11 The first choice for the l@st mile

12 (ATM) When the end-user PC is equipped with an ATM NIC (Network Interface Card), one can carry the over ATM up to the end-user equipment ATM NIC ISP PVC TU-C DSLAM TU-R RFC 2364 STM-1. RFC 2364 DSL. RFC 2364 ATM25 The big advantage from this system is that end-to-end ATM traffic management becomes possible (requires Winsock2) over Ethernet ATM based connection In the two previous topologies, the connections are terminated on a single PC platform at the end-user. Since most PC s today are equipped with Ethernet connection, it makes sense to terminate the as a over Ethernet connection. This allows multiple PC s sharing the same DSL connection. Ethernet ISP PVC TU-C DSLAM TU-R RFC 2516 RFC 1483 STM-1 RFC 2516 RFC 1483 DSL RFC Base-T oe oe ( over Ethernet) was standardised as RFC The first choice for the l@st mile

13 4. The Subscriber Management System (SMS) The main tasks of a SMS (also called BAS: Broadband Access Server), are the following: QOS management Authentication Creating tunnels to different services Aggregation of large number of subscribers in those tunnels When using, a point-to-point connection is set-up. In order to accommodate efficiently multiple connections to a variety of services (in the example ISP and corporate network), a tunneling mechanisme, based on L2TP can be used. The SMS will set-up a tunnel to each of the supported services. This tunnel will accommodate the multiple connections coming from all the connected users. The advantage is that for each tunnel, only one ATM PVC is necessary. The SMS can select the right tunnel, upon the setup of the connection of the end-user. The selection can be based on the user login-name. Coporate SMS TU-C DSLAM TU-R ISP L2TP RFC 2225 STM-1 RFC 2516 RFC 1483 STM-1 RFC 2516 RFC 1483 DSL RFC Base-T L2TP oe The example above is based on the over Ethernet connectivity. The same is valid for other based topologies ( overt ATM, over USB) 13 The first choice for the l@st mile

14 5. Other technologies Other New fast access technologies today include: Wireless Access Cable Modems Cable-modem technology Cable-modem technology uses the coax which is used for TV distribution. It allows downstream speeds up to 30 Mbps. Upstream speeds generally (depending on the used technology) are much lower. Head-end Coax Amplifier Fiber Cable Modem HFC: Hybrid Fiber Coax Topology for cable modem fast backbone access As seen on the picture, cable modem technology is based on an HFC (Hybrid Fibre Coax) network. Compared to ADSL for Internet Access, it may have following advantages and disadvantages: Advantages: Possible Access speed up to 30 Mbps Possible good price/speed ratio Disadvantages Shared medium (no guaranteed bandwidth) May require adaptations to the cable network for making it bidrectional 14 The first choice for the mile

15 6. Conclusion The high speed access methods for the near future are listed below. The table also reflects their main application domains. Max Speed Applications ADSL 640 kbps 8 Mbps Internet access POTS Cable Modem 30 Mbps (shared) Internet access Telephony (S)HDSL 2.3 Mbps Internet (professional) POTS (Cap based only) Frame-Relay access Leased Line access Campus networks Private networks (railways..) ADSL and Cable-Modems can offer asymmetrical internet access Cable Modems are most appropriate for residential use ADSL can address residential users and SME s (S)HDSL is more oriented towards medium size and bigger users (symmetrical high-speed), and can also be used for Frame-Relay and Leased Line services 15 The first choice for the l@st mile

16 7. Glossary 2B1Q ADSL ATM ATU BAS CAP CO DSL DSLAM FITL FTTC HDSL HDSL2 IAD IDSL ISDN L2TP NIC POTS oe PSTN SDSL SHDSL SMS TCPAM TDM TU TU-C TU-R VDSL VoD VPN Modulation technology used for IDSL and certain types of HDSL equipment Asymmetrical Digital Subscriber Line Asynchronous Transfer Mode ADSL Termination Unit Broadband Access Server (see also SMS) Carrierless Amplitude Phase modulation, modulation technology used for certain types of HDSL equipment Central Office Digital Subscriber Line Digital Subscriber Line Access Multiplexer Fibre In The Loop Fibre To The Curb High Speed Digital Subscriber Line American counterpart of the European SHDSL technology. Also uses the TCPAM modulation Integrated Access Device ISDN Digital Subscriber Line Internet Protocol Integrated Services Digital Network Layer 2 Tunneling Protocol Network Interface Card Plain Old Telephony Service Point to Point Protocol over Ethernet Public Switched Telephony Network Symmetrical Digital Subscriber Line Single pair High-speed Digital Subscriber Line Subscriber Management System (see also BAS) Trellis Coded Pulse Amplitude Modulation. New modulation technology used for G.SHDSL and HDSL2 Time Division Multiplexing Termination Unit Termination Unit Central Termination Unit - Remote Very High Speed Digital Subscriber Line Video On Demand Virtual Private Network Patrick De Boeck was born in Brussels and is an engineer in electronics with a post-academic degree in telecommunications. He started his career in 1990 for the R&D department of Telindus, where he was active in the development and implementation of new modem technologies. Since 1995, he s active as a product manager for the Telindus high-speed xdsl and fibre-optic product range patrick.deboeck@telindus.be 16 The first choice for the l@st mile

17 17 The first choice for the mile