19. Multiservice subscriber access networks based on dslam (ban). Review of wired multiservice access technologies.

Broadband subscriber access

Broadband subscriber access subsystem allows to connect xDSL (high bit rate) digital subscriber lines to the broadband subscriber modules of DSS. xDSL technology enables asymmetric and symmetric digital subscriber lines (of high bit rate) over analog 2-wire subscriber line. Asymmetric digital subscriber line provides different bit rates for upstream and downstream communication. Symmetric digital subscriber line provides the same bit rates for upstream and downstream communication.

Frequency division (shown in the figure 4) allows a single subscriber line to be used for both computer data exchange and voice calls at the same time.

Figure 4 – Frequency plan for ADSL

At the telephone exchange the line generally terminates at a DSLAM (Digital Subscriber Line Access Module) where a frequency splitter separates the voice band signal for the conventional phone network. Data carried by the ADSL is typically routed over the telephone company's data network and eventually reaches a conventional internet network. The 0.3…3.4 kHz is the frequency range used by normal voice telephony (PSTN). The band from 26 kHz to 138 kHz is used for upstream communication, while 138 kHz – 2208 kHz is used for downstream communication. ADSL provides 1 Mbit/s for upstream communication and up to 8 Mbit/s for downstream communication.

There are different types of ADSL available. They are listed below.

Technology	Bit rate, Mbit/s		Length of a SL, km
ADSL	0.064–0.8	1.554–8.448	2.5–5
ADSL lite	0.44–0.512	1.536	5
R-ADSL	0.542	7	3
ADSL2	0.8	8	0.18
ADSL2+	1	24.5	1.5

Digital Subscriber Line Access Module (DSLAM) is a module for broadband xDSL access. It multiplexes broadband traffic and directs it to the Ethernet/ATM network.

Figure 5 – ADSL broadband access

High bit rate SHDSL subscriber line provides 2 Mbit/s for upstream and downstream. Symmetric digital subscriber lines (over copper wires) are quite popular with busyness subscribers. It enables videoconferences, high bit rate e-mail services, date exchange, and other services. Frequency spectrum is given in the figure below.

f, kHz

Figure 6 – SHDSL frequency spectrum

SHDSL subscriber line requires SHDSL modems (one at customer premises and one at a TE). Characteristics of different technologies are given in the table below.

Technology	Bit rate, Mbit/s		Quantity of pairs	Length of a SL, km
SHDSL	0.192-4.624		1(2)	2-6
G.SHDSL	0.192-4.624		1(2)	2-6
VDSL	3.2-6.5	13-52	1	1.2
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Narrowband subscriber module is a hardware that supports analogue subscriber lines and ISDN subscriber lines.

Broadband subscriber module is a hardware that supports xDSL and optical subscriber lines.

An analogue narrowband subscriber module supports analogue subscriber lines, concentrates subscriber traffic, and provides subscriber and inner-system signaling. Structure of the analogue narrowband subscriber module is shown in the figure 7 (SM of “Kvant-E” is given as an example).

The subscriber module contains a time and space switch. The switch handles 8 E1(32 time channels each) trunks. The 0 trunk is assigned to signaling and synchronization unit (SSU). 1-4 trunks are assigned to 128 subscriber lines (every customer unit is assigned to a single time channel). The 5-th trunk is assigned to a testing unit (TU). The 6-th trunk is assigned to the digital attenuator (DA). The 7-th trunk is shared by a digital received and a digital generator of tone signals (DGTS).

R

CU

CU

PLM

PLM

T-0

T-1…4

T-6

T-1…4

T-7

T-5

SSU

DA

DR

DGTS

TU

TSS

8×8

CS-8A

TC-16

TC-16

ASL

ASL

E1 Trunk

A

B

Figure 7 – Structure of ASM

Signaling and synchronization unit (SSU) connects a subscriber module to a switching control unit (SCU) of host equipment. The SSU retransmits (in TC-0) synchronization signals received from the module of synchronization (MS). The SSU creates inner-system signaling channel.

CS-8A controller controls the whole subscriber module. It receives and processes address information form a phone or SCU’s controller. It interacts with a controller of SCU of HE via inner-system signaling channel (ISSC). It controls operation of TSS and other units of the SM.

Generator of ringing signal (R) generates 25 Hz AC signal for a called subscriber.

Testing unit (TU) measures parameters of subscriber lines and customer units.

Digital attenuator (DA) provides attenuation of 6 dB for a voice transmission path. The DA is needed for autonomous operation of the module. If a call transits HE then SCU provides attenuation.

Digital receiver (DR) receives DTMF dialing.

Digital generator of tone signals (DGTS) generates 32 tone signals that are necessary for any peripheral unit operation. Moreover, it supports one voice message (up to 4 seconds).

Narrowband digital subscriber module supports 2B+D₁₆ and 30B + D₆₄ digital subscriber lines. Structure of such a module is given below (SM of “Kvant-E” is given as an example).

DR

DGTS

TSS

8×8

SSU

CS-8A

DSM

DCU

DCU

DCU

T-0

T-7

TC-16

Figure 8 – Structure of DSM

Broadband subscriber module supports: 1) symmetric and asymmetric digital subscriber lines based on xDSL, 2) symmetric V5.2 lines, 3) optical lines towards transport network (ATM, Ethernet). Broadband subscriber module is also known as DSLAM. DSLAM multiplexes subscriber traffic, splits it into narrowband (telephone conversations) and broadband (broadband services) flows. DSLAM directs narrowband traffic to a TDM switch of a DSS while broadband traffic goes to a transport network. DSLAM has ATM/Ethernet (SDH) and optical (STM-1) interfaces to access the transport network.

Figure 9 – Structure of DSLAM

Let us look at BAN (DSLAM) of SI-2000 V.5. It supports 240 xDSL lines and contains 16 cards interconnected by an ATM bus (2x1.2 GBit/s). The DSAM may contain the following cards.

– Up to 15 ADSL/G.SHDSL cards (16 subscriber lines per each card).

– Splitter cards (16 splitters per each card).

1. BB CPU card for control.

2. STM-1 card containing ATM (155 MBit/s) network interface; or Fast Ethernet card; or 8 E1/IMA card with the ATM (8 MBit/s) interface.

Construction of BAN is given below.

ATM bus

System bus

Figure 10 – MBA section containing BAN

Broadband subscriber modules require narrowband analogue subscriber modules to direct narrowband subscriber traffic to them. In other words, a splitter splits narrowband traffic and it goes to an analogue subscriber module (to its customer unit).