**2.1 xDSL attainable rate**

The maximum attainable rate and allocated bandwidth per ISP plan (Mbps) will affect the femtocell performance. While the bandwidth allocation depends on subscription profile, maximum attainable rate are determined by corresponding xDSL technologies and physical copper cable quality (mainly characterized based on its attenuation and SNR).

In order to discuss about attainable rate and to derive xDSL quality model as femtocell backhaul, TELKOM conducted a study on its copper cable performance in supporting several xDSL technologies including ADSL2, ADSL2+, and VDSL2. Even though the study initially performed to assess IPTV implementation, the study result is relevant to support femtocell implementation.

The study of xDSL performance could not be separated by FTTx technologies implementation, since both are complementary to each other. Performance of transmission technologies over copper have been evaluated for the following reference architectures:


The studies are performed by using simulation under Telecom Italy supervision. The cable models have been defined according to TELKOM's installed cables. Only FTTE and FTTC configuration will be described in this paper. While FTTE can provide internet service for subscriber located 3-5 away from Local Exchange, the FTTC can provide adequate and reasonable performance for residential in order to support IPTV service over xDSL broadband.

Structure of Telkom Indonesia cables can be summarized as following:


First level of aggregation:

140 Mobile Networks

ADSL and ADSL+ are deployed using the existing PSTN infrastructure. The DSLAM node is located in the central office and defined as aggregation nodes. The existing copper cable is used so that the broadband service can be evenly distributed from Local Exchange, Street Cabinet and household's area. The size of access network zone is determined by the maximum copper cable length [5]. At DSLAM side, traffic is multiplexed and transmitted

An ISP may implement xDSL technology using different approach including DSLAM in the local exchange (fiber to the exchange, FTTE), DSLAM/MSAN in the cabinet (fiber to the cabinet, FTTC) and DSLAM/MSAN in the building/house (fiber to the house/building, FTTH/FTTB). For this study, we focus on FTTE and FTTC deployment where MSANs are located in the central exchange (FTTE) and street cabinet respectively to reach customer

The maximum attainable rate and allocated bandwidth per ISP plan (Mbps) will affect the femtocell performance. While the bandwidth allocation depends on subscription profile, maximum attainable rate are determined by corresponding xDSL technologies and physical

In order to discuss about attainable rate and to derive xDSL quality model as femtocell backhaul, TELKOM conducted a study on its copper cable performance in supporting several xDSL technologies including ADSL2, ADSL2+, and VDSL2. Even though the study initially performed to assess IPTV implementation, the study result is relevant to support

The study of xDSL performance could not be separated by FTTx technologies implementation, since both are complementary to each other. Performance of transmission technologies over copper have been evaluated for the following reference architectures:

a. FTTE (DSLAM or MSAN in Exchange) which use technology: ADSL2/2+ technology as

b. FTTC (MSAN in street cabinet) which uses technology: ADSL2/2+, VDSL2 (profile 8b,

c. FTTB (ONU or MSAN at building) which uses technology: VDSL2 (profiles 17a and

The studies are performed by using simulation under Telecom Italy supervision. The cable models have been defined according to TELKOM's installed cables. Only FTTE and FTTC configuration will be described in this paper. While FTTE can provide internet service for subscriber located 3-5 away from Local Exchange, the FTTC can provide adequate and reasonable performance for residential in order to support IPTV service over xDSL

Structure of Telkom Indonesia cables can be summarized as following:

copper cable quality (mainly characterized based on its attenuation and SNR).

over fiber based transmission to the IP backbone.

residential with the cable length less than 4 km.

**2.1 xDSL attainable rate** 

femtocell implementation.

the last mile access

30a)

broadband.

use the same Tx level as ADSL2/2+)

Diameter of conductors: 0.6mm-0.8mm

 Insulation: polyethylene Basic structure: quad (2 pairs)


Based on those information, cable model were constructed based on the following parameter:

	- Primary cable: 50 pairs (1 super unit, 25 quads)
	- Secondary cable: 50 pairs (5 units, 5x5 quads)
	- Drop cable: 20 pairs

Crosstalk model is based on following assumptions:

	- KNEXT (@ 1 MHz) = -52.2 dB,
	- KFEXT (@ 1 MHz @ 1 km) = -45.0 dB

Noise mix follows the assumption;

	- Mix 30%BB: 1 SHDSL (@2.3Mbit/s), 14 ADSL2/2+, 35 POTS (or vacant)
	- Mix 50%BB: 1 SHDSL (@2.3Mbit/s), 24 ADSL2/2+ or VDSL2, 25 POTS (or vacant)

xDSL Physical layer setting for performance evaluation follows


Based on the xDSL modeling, the simulation can provide the following results:


Based on above data and the xDSL modeling, the performance of ADSL2/ADSL2+ and VDSL2 can be seen in Figure 2 and Figure 3.

Femtocell Performance Over Non-SLA xDSL Access Network 143

mode, since it will offer a higher efficiency (more data, less error correction redundancy code in each packet). The fast channel mode will allow users to have faster and smaller ping times. However as the real time applications such as IPTV was introduced to the market, interleave channel mode is required. In video applications, there is no time to retransmit data if errors are detected. In order to limit the impact of long burst errors, an interleaver device is used to spread the data out or shuffles the data after encoded by the Reed-Solomon code [6]. By using Reed Solomon and interleaver as in ADSL and VDSL technology, long error bursts will be equally distributed, so that the errors can be corrected more easily using forward error correction. Since there are bits used for codeword, bits number for data in the interleave mode will be less, hence it will affect the

ADSL2/ADSL2+ has a limitation in the upstream bandwidth which is up to 1.1 Mbps, when FAST mode is used or up to 900 kbps when INTERLEAVED is used. However this relatively high upstream bandwidth is only available if the cable length is less than 600 meter from MSAN location. To ensure the stability, 512 kbps bandwidth should be considered for both modes, since the bandwidth is available even when the cable length 6 km away from MSAN

For higher upstream bandwidth, VDSL2 should be considered under FTTC configuration. With VDSL2, the upstream data rate can reach around 5 Mbps at the range up to 1 Km.

In order to characterize xDSL as femtocell backhaul, we also test transmission delay of ADSL and ADSL2+ from modem to the DSLAM. The experiment has been done in TELKOM R&D Centre Test Bed (called OASIS). Due to the fact that the measurement conducted in the testbed, it is impossible to varying the cable length to have exact transmission delay observation. For the reason that delay is the inverse of frequency carrier, we measured the delay transmission by varying bandwidth profile. By limiting bandwidth profile, the frequency carrier is set and affects the transmission delay. We did not consider

delay due to lost in cable as in the commercial DSL deployment.

The network configuration for this observation can be seen in Figure 4.

Fig. 4. Transmission Delay Measurement in TELKOM R&D Centre's Testbed

The transmission delay of xDSL over various bandwidth profiles can be seen in Figure 5.

total data rate.

location.

**2.2 xDSL transmission delay** 

Fig. 2. Performance of ADSL2 and ADSL2+ using FTTE (MSAN at Local Exchange)

Fig. 3. Performance of ADSL2, ADSL2+ and VDSL in FTTC (MSAN at street cabinet)

In general, attainable data rate depend on xDSL technology. VDSL2 deliver higher data rate since it uses wider frequency plan up to 12 MHz compared to ADSL2+ which uses 2.2 MHz and ADSL2 uses 1.1 MHz. As the cable length increases the total impedance will also increase, it will add more attenuation to the signal. As a result the data rate will decrease as the cable length increase. Higher frequency will experience more sever attenuation as a function of distance, so that the higher data rate can only be maintained in shorter distance.

As can be seen from the graph, the maximum attainable rate also depends on xDSL channel mode. DSL is designed to deliver internet access. By default it uses fast channel 142 Mobile Networks

Fig. 2. Performance of ADSL2 and ADSL2+ using FTTE (MSAN at Local Exchange)

Fig. 3. Performance of ADSL2, ADSL2+ and VDSL in FTTC (MSAN at street cabinet)

In general, attainable data rate depend on xDSL technology. VDSL2 deliver higher data rate since it uses wider frequency plan up to 12 MHz compared to ADSL2+ which uses 2.2 MHz and ADSL2 uses 1.1 MHz. As the cable length increases the total impedance will also increase, it will add more attenuation to the signal. As a result the data rate will decrease as the cable length increase. Higher frequency will experience more sever attenuation as a function of distance, so that the higher data rate can only be maintained in shorter distance. As can be seen from the graph, the maximum attainable rate also depends on xDSL channel mode. DSL is designed to deliver internet access. By default it uses fast channel mode, since it will offer a higher efficiency (more data, less error correction redundancy code in each packet). The fast channel mode will allow users to have faster and smaller ping times. However as the real time applications such as IPTV was introduced to the market, interleave channel mode is required. In video applications, there is no time to retransmit data if errors are detected. In order to limit the impact of long burst errors, an interleaver device is used to spread the data out or shuffles the data after encoded by the Reed-Solomon code [6]. By using Reed Solomon and interleaver as in ADSL and VDSL technology, long error bursts will be equally distributed, so that the errors can be corrected more easily using forward error correction. Since there are bits used for codeword, bits number for data in the interleave mode will be less, hence it will affect the total data rate.

ADSL2/ADSL2+ has a limitation in the upstream bandwidth which is up to 1.1 Mbps, when FAST mode is used or up to 900 kbps when INTERLEAVED is used. However this relatively high upstream bandwidth is only available if the cable length is less than 600 meter from MSAN location. To ensure the stability, 512 kbps bandwidth should be considered for both modes, since the bandwidth is available even when the cable length 6 km away from MSAN location.

For higher upstream bandwidth, VDSL2 should be considered under FTTC configuration. With VDSL2, the upstream data rate can reach around 5 Mbps at the range up to 1 Km.
