Proposal: Option for in-band POTS and ISDN. Mikael Isaksson, Tomas Stefansson, Per Ödling, Frank Sjöberg, Kate Wilson

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1 1(5) Standards project: T1E1.4: VDSL Title : Proposal: Option for in-band POTS and ISDN Source : Contact: Telia Research AB Mikael Isaksson, Tomas Stefansson, Per Ödling, Frank Sjöberg, Kate Wilson Mikael Isaksson Telia Research AB, Aurorum 6, SE Luleå, Sweden Fax: Mikael.R.Isaksson@telia.se Distribution: T1E1.4 Technical subcommittee working group Status: Proposal Abstract: We propose an option allowing VDSL to use the spectrum now reserved for narrowband services, such as POTS and ISDN. The main motives for the proposal are: 1) This option will provide lower network complexity and maintenance costs by avoiding duplicate line cards. 2) We can increase the number of customers using VDSL for rates below 15 Mbps. This is because when VDSL can use the full spectrum, the reach can be significantly extended. For example, the reach increases by up to 1 meters for a 2 Mbps symmetric service on TP1 in a central office based scenario [1]. This contribution has been prepared to assist ANSI Standards Subcommittee T1E1.4. This document (T1E1.4/97-282) is offered as a basis for discussions and is not a binding proposal of Telia Research AB. Telia Research AB specifically reserves the right to add to, amend or withdraw the statements contained herein.

2 2(5) 1 Proposal We propose an option allowing VDSL to use the spectrum reserved for narrowband services such as POTS and ISDN. 2 Motivation VDSL has the potential to replace all major wired services currently available. For VDSL to supply today s and tomorrow s broadband and narrowband services, it will require: access to the low-frequency band currently reserved for narrowband services such as POTS and ISDN fulfilment of both latency requirements given in [2]. The success of VDSL is dependent on a stepwise introduction [1]. Coupled to this is a demand for long reach at lower bit rates. If we incorporate the narrowband services as part of the VDSL system, we can increase the number of customers and simplify the network. Because we can extend the reach by, for instance, 5 meters for bit rates below 15 Mbps with the broader spectrum, we can increase customer coverage. We can simplify the network and lower maintenance costs by reducing the number of line cards. 2.1 Network Complexity - Doubling of Line Cards An important issue for the success of VDSL is the network complexity. If a system supports a narrowband service and VDSL separately, it will require two separate line cards. This represents essentially a doubling in complexity. By making the narrowband services a feature of VDSL, we remove the need for a second line card. 2.2 Handling of service changes By putting narrowband services in-band, the network operator can make service changes without dispatching service personnel to the access node. Service changes can be performed just by replacing or upgrading the network termination equipment at the customer site, which in many cases can be done by the customer. This can, for example, be changing the number of narrowband services provided to a customer. The ability to remotely manage service changes is important in a situation where a high penetration of broadband services can be expected. If there is no need for a separate band for the narrow-band services, this spectrum can be released for VDSL use. This requires the proposed option in the VDSL standard. 2.3 Extended reach The part of the spectrum currently occupied by narrowband services has the lowest signal attenuation and FEXT. If VDSL can use this part of the spectrum, we can increase the capacity of long cables substantially. We have calculated achievable bit rates for a VDSL system using the full spectrum and compared that with a VDSL system that coexists with an analog splitter. The calculations are made on a DMT-based system with a duplex efficiency of about 9%. (Calculations on a CAP or MQAM system would give similar results.) We assume that the environment is VDSL self-fext dominated. The splitter effectively removes either the lowest 3 khz (for POTS) or the lowest 6 khz (for ISDN) of available spectrum for VDSL. We simulated the bit rate of a VDSL system using the entire spectrum and compared that to the bit rate of VDSL systems that do not use the lower 3 khz and the lower 6 khz. For the bit-rate calculations of the system using the entire spectrum, we subtract the 16 kbps reserved in both up- and down-stream direction for either POTS or ISDN.

3 3(5) Some of the parameters used in the simulations are listed in the table below. System parameter AWGN -14 dbm/hz Number of FEXT disturbers 49 System margin 6 db Coding gain 3 db SNR-gap db Usable bandwidth /.3/.6-12 MHz HAM-bands removed , , , MHz Cable type TP1 Figure 1 displays achievable bit rates for the three VDSL systems (full spectrum, full spectrum minus the lower 3 khz, full spectrum minus the lower 6 khz). These bit rates are given for increasing wire lengths. The fullspectrum VDSL has a much higher bit rate for longer wires as shown in Figure 1. This is because as the wire length increases, the lower frequencies supply a larger portion of the capacity. 6 5 VDSL bit rates on TP1 (asymmetrical 8:1) Inband POTS / ISDN analog splitter (3 khz) analog splitter (6 khz) 3 25 VDSL bit rates on TP1 (symmetrical 1:1) Inband POTS / ISDN analog splitter (3 khz) analog splitter (6 khz) length (m) length (m) (a) (b) Figure 1 - Downstream bit rates for VDSL with in-band POTS / ISDN and an analog splitter, (a) asymmetrical bit rates (8:1), (b) symmetrical bit rates. Figure 2 displays the extra reach attainable by the full-spectrum VDSL. (Note that Figure 2 can be derived from Figure 1.) For example, the simulations indicate that wires 2 to 4 meter longer can be used for the 1 Mbps asymmetrical (8:1) bit rate case. 1 An SNR-gap of 9.8 db is used to achieve a BER of approximately The HAM-bands are all excluded and are not used for transmission.

4 4(5) 7 Extended reach on TP1 (asymmetrical 8:1) 7 Extended reach on TP1 (symmetrical 1:1) 6 Inband vs. Analog (6 khz) Inband vs. Analog (3 khz) 6 Inband vs. Analog (6 khz) Inband vs. Analog (3 khz) 5 5 extra reach (m) 4 3 extra reach (m) (a) (b) Figure 2 - Extension of the reach if in-band POTS / ISDN is used instead of analog splitter. (a) asymmetrical rates (8:1), (b) symmetrical rates. In the table below values for reach and extended reach is shown for different desired bit rates and for both TP1 and TP2 cables. Bit rates (Mbps) Reach (m) Extra reach (m) TP1 In-band POTS / ISDN 3 khz 6 khz 3 khz 6 khz 6.5/ / / / / / TP2 6.5/ / / /2. (ca 37 m) (ca 8 m) (ca 14 m) 6.5/ / Support of leased line services and primary rate accesses In addition to POTS customers, most network operators normally have a large number of PBX s connected to the PSTN / ISDN network. Many of these PBX s are connected using interfaces based on either E1 or T1. Leased-line services in the range of Mbps are in widespread use in most networks. Even if there are special transmission schemes (for example HDSL) available for the support of these services, the support of E1- and T1-based services should not be excluded from the VDSL specification. With the option of not reserving the lowest frequency band for out-of-band POTS / ISDN, the range of a VDSL system supporting this kind of service can be significantly extended. Correspondingly, the number of these leased-line systems that can be replaced with a VDSL system is significantly larger.

5 5(5) 3 Discussion There are many advantages of not having analog splitters that are not addressed here. For instance, it can be expected that eliminating the out-of-band POTS / ISDN would reduce the impulsive noise. This is because there would be no ring signals, polarity switching of DC voltage, dialing or other electro-mechanical switching with extensive contact bouncing. Further, with the longer reach and increased capacity offered by a full-spectrum VDSL system, the leasedline services can be replaced by VDSL. Without the current leased-line systems, interference situations would be improved. The option of releasing the narrow-band service spectrum for VDSL use will provide better coverage and lower maintenance costs. 4 References [ 1] Hans Öhman et al.,central Office Based VDSL, ANSI T1E1.4/97-141, May [ 2] Gavin Young, VDSL Dual Latency Issue, ANSI T1E1.4/97-132, May 1997.