Overview TOP

DSL stands for digital subscriber line, a somewhatnondescriptname for an exciting technology.
The "D" is historical since the original form of DSL was adigital service.
(Digital means that whatever is traveling over the line does so aseither a "1" with current or a "0" without current.)However, DSL has developed into a high-speed analog signal (usuallyrepresented by a sine wave) and is no longer digital. The "S"or subscriber refers to you or your company. You "subscribeto" or rent the DSL line from a telecommunications serviceprovider. "L" (line) means that this is an outside line (alsocalled a circuit) that comes into your premises on a telephone cablefrom a telecommunications service provider. This is the same type oftelephone cable used for your everyday telephone service.

The high-speedanalog transmissions for DSL have different signaling patterns dependingupon the type of DSL circuit and the type of hardware at either end. Twoof the more commonly used DSL signaling patterns are CAP (carrierlessamplitude phase modulated) and DMT (discrete multitone).

Themost common use of a DSL circuit is to physically and permanentlyconnect you to the Internet so that you are "always on". Itcan also enable you to connect to other locations (such as other officesof your company) through the Internet. With access to DSL, you do nothave to use a conventional modem with your computer and a regular phoneline to "dial in" (or "dial up") every time you wishto access the Internet. However, there is an additional piece ofequipment required- a DSL modem.

One of the reasons that DSL is in demand is that it offers substantialcapacity (also called speed or bandwidth) over a single pair of copperwires. Most home and office locations are already equipped with a sparepair of wires on the cable that delivers the regular telephone dialtone. ThereforeDSL does not require a new separate telephone cable. Since DSL isdesigned to use copper cable exclusively, it does so all the way fromyour premises back to the telecommunications service provider's centraloffice and a device called a DSLAM or Digital Subscriber Line AccessMultiplexor. (Note: A DSLAM can also be located onsite in a multitenantor campus environment.)

Thescenario described above using a separate pair of copper wires is knownas using "dry copper." It is also possible to combine thedelivery of your dialtone telephone line for voice communications withDSL on a single pair of copper wires. This is called delivering the DSLon "wet copper." You can have a telephone conversation and useyour DSL at the same time, sharing bandwidth. Once this transmissiongets to the DSLAM, the voice is separated and sent out over the publicswitched telephone network (PSTN), and the data on the DSL is sent tothe Internet service provider's site. There are some distancelimitations with DSL, so your distance from the telecommunicationsservice provider's central office will affect whether or not you can getDSL and it's speed. In general, the further the distance, the lower thespeed you'll get, although speed is also a function of the hardware. Ifthe DSLAM is onsite (such as in an office building), then distance isn'tan issue. When the DSLAM is onsite, it is connected to another circuitto continue the transmission back to the telecommunications serviceprovider; the DSL circuit runs only from the nearby DSLAM into your homeor office.

ADSL or Asymmetrical Digital Subscriber Line is most typically used foraccess to the Internet at home. The capacity of the circuit is greatercoming from the Internet into the home (called "downstream")than going the other direction (called "upstream"). This isbecause home users are more likely to receive more information(graphics, sound, and video) than they send (keystrokes and mouseclicks).

SDSL or Symmetrical Digital Subscriber Line is the type used by mostbusinesses who need to send as well as receive significant amounts ofinformation. The capacity of the circuit is the same in both directions.

The capacity affects how quickly the screen on your computer will displaythe information from the Internet or how quickly you can send a computerfile from your office to another location. If you are sending large fileswith pictures, you will need a higher-capacity DSL circuit than if you aresending regular text files or text email. The capacity of the circuit alsoneeds to be greater if several people will be using it at the same time.(You can connect to DSL through a computer server that is shared withother people in your office.)

Determining the capacity of the DSL circuit that's right for you mayrequire a bit of trial and error. You can pay the highest rate and orderthe greatest capacity or order a lower capacity and see if it'ssufficient. The capacity of the DSL can be changed by your serviceprovider, usually within a few days.

Capacity specifically refers to the volume of information that can travelover the circuit within a given time frame. Typical capacities are 128Kbps(kilobits per second - meaning that 128,000 bits can pass a point on thecircuit in one second. 256Kbps, 512Kbps, 768Kbps (a popular midrangespeed), 1Mbps (mega bits per second),1.5Mbps is the approximate capacityof T-1 telecommunications circuit that traditionally costs more than DSL.

You may hear about other types of DSL which are all variations on thetheme. HDSL (high bit-rate DSL) has a speed of 1.544Mbps and uses eithertwo or three pairs of copper wires instead of one pair. IDSL (ISDN DSL)has the same speed - 128Kbps or 144Kbps - as another service called ISDN(Integrated Services Digital Network) which requires dialing HDL into the Interneton a special type of line delivered on a pair of copper wires. DSL Lite(also called G-Lite) is a lower-speed version of ADSL. RADSL is RateAdaptive DSL that adjusts the speed of the transmission based signalquality. VDSL is a "Very High Speed DSL" (12.9 to 52.8Mbpsdownstream and 1.5 to 2.3 Mbpsupstream).

What Is x-DSL?

One of themajor problems facing the ILECs was the ability tomaintain and preserve their installed base. Ever since the TelecommunicationsAct of 1996, the ILECs have faced mounting pressure to provide faster and moreaccurate Internet access. Therefore, a new form of communications was needed towork over the existing copper cable plant. One of the technologies selected wasthe use of DSL. The DSL family includes several variations of what is known as digital subscriberline (DSL). The lowercase x in front ofDSL stands for the many variations. These will include

• Asymmetric digital subscriber line (ADSL)

• ISDN-like digital subscriber line (IDSL)

• High-bit-rate digital subscriber line (HDSL)

• Consumer digital subscriber line (CDSL)

• Rate-adaptive digital subscriber line (RADSL)

• Very high-speed digital subscriber line (VDSL)

• Symmetric or single digital subscriber line (SDSL)

• Single-pair high-speed digital subscriber line (SHDSL)

One can see that many variations are available. Eachdigital subscriber line (DSL) capability carries with it differences in speed,throughput, and facilities used. The most popular of this family under today'stechnology is asymmetrical digital subscriberline (ADSL). ADSL is a technology primarily provided by the ILECs because theexisting cable plant can support the speeds, which can vary depending on thequality of the copper. The Competitive Local Exchange Carriers (CLECs) are quickly becoming involved in providing xDSL now. However, the most important and critical factor in dealing with ADSLtechnology is the capability to support speeds from 1.5 up to 8.192 Mbps. TheILECs can also support POTS for voice or fax communications on the same line.This means that the ILECs do not have to install all-new cabling to supporthigh-speed communications access to the Internet, which is burning up thewires today.

Modem Technologies TOP

Before proceeding too far inthis discussion, a quick review of modem technology is probably in line. Modems,or modulator/demodulators, were designed to movedata across the voice communications network. Users still struggle to transmitdata across the voice networks at speeds up to 33,600 bps. Even with the newer modems called the V.90, which are supposed to operate at 56 Kbps, we still see significant reductions inspeed. Although this may seem like high-speed communication, our demands andneeds for faster communications have quickly outstripped the capabilities of ourcurrent modem services, making the demand for newer services more evident.High-speed modems could be produced, but the economics and variations on thewiring system prove this somewhat impractical. Instead, the providers looked fora better way to provide data communications that mimic the digital transmissionspeeds we readily accept.

Using the telephone company'svoice services, the end user installs a modem on the local loop. This modem isthe data circuit-terminating equipment (DCE) forthe link. A modem is used to communicate across the wide-area networks. The ILEC installs a voice-grade line on the copper cable plant and allows the end user toconnect the modem. The modem then converts the data into an analog signal. Thereis no magic in modem communications today, but in the early days of datacommunications, this was considered voodoo science. The miracle of datacompression and other multibit modulationtechniques quickly expanded the data rates from 300 bps to today's33.6 Kbps. Newer modems are touted to handle data at speeds of up to 56 Kbps,but few come close to these rates. So, the reality of all the pieces combinedstill has the consumer operating at approximately 33.6 Kbps.

DSL refers to a pair of modems installed on the last mile of line, which facilitatehigh data speeds. Network providers use the existing wires and add the DSL modems to increase the throughput. DSL modems offer duplex operations. The speed of a DSL modem may be 128 Kbps oncopper at distances up to 18,000 feet using the twisted pair wires. Thebandwidth used is from 0 to 80 kHz. IDSL uses the 128-Kbps full-duplex basic rate interface (BRI). The IDSL technique is all digital, operating at two channels of64 Kbps for voice or nonvoice operation and a 16-Kbps data channel for signaling, control, and data packets. ISDN was very slow tocatch on, but the movement to the Internet created a whole new set of demands.Now more ILECs and the CLECs offer ISDN services. As the deployment of IDSL was speeding up on the localloop, the providers developed a new twist, called always on ISDN, mimicking a leased set of channels that are always connected. Bonding thechannels together, users can surf the Net at speeds of 128 Kbps. Note that thisis a symmetrical digital subscriber line.

In 1958, Bell developed a voicemultiplexing system that uses the 64-Kbps pulsecode modulation (PCM). Using the PCM techniques, voice calls were sampled 8,000 times per second and coded using an8-bit encoding. These samples were then organized into a framed format, using 24time slots to bundle and multiplex 24 simultaneous conversations onto a singlefour-wire circuit. Each frame carries 24 samples of 8 bits and 1 framing bit, 8,000 times a second. Thisproduces a data rate of 1.544 Mbps. We now refer to this as a Digital SignalLevel 1 (DS-1) at the framed data rate. This rateof data transfer is used in the United States, Canada, and Japan.

Throughout the rest of the world, standards were set tooperate using an El with a signaling rate of 2.048 Mbps. The differences betweenthe two services (Tl and El) are significantenough to prevent their seamless integration.

However, in the digital arena, Tl required that the providerinstall the circuits to the customer's premises. The local provider installs afour-wire circuit. Repeaters are spaced at every 5,000 to 6,000 feet. Wheninstalling the Tl on the local loop, limitations of the delivery mechanism getin the way. Alternate mark inversion (AMI) consumes all the bandwidth andcorrupts the surrounding cable spectrum quickly. Consequently, the providers canonly use a single Tl in a 50-pair cable. Thisinefficient use of the wiring makes it impractical to install T1s to small office and residential locations. Further limitations require theproviders to remove bridge taps, clean up splices, and remove load coils fromthe wires to get the Tl to work.

To circumvent these cabling problems, HDSL was developed. HDSL does not require the repeaterson a local loop of up to 12,000 feet. Bridge taps will not bother the service,and the splices are left in place. This means that the provider can offer HDSLmore efficiently for 1.544 Mbps. The modulation rate on the HDSL service is moreadvanced. Sending 768 Kbps on one pair and another 768 Kbps on the second pairof wires splits the Tl.

Originally, HDSL used two pairs at distances of up to 15,000feet. HDSL at 2.048 Mbps uses three pairs of wire for the same distances. Themost recent version, HDSL-2, uses only one pair of wire and is more acceptable to the providers. Nearly allproviders today deliver Tl capabilities on some formof HDSL.

The goal of the DSL family wasto continue to support and use the local cable plant. Therefore, providinghigh-speed communications on a single cable pair became paramount. Most localloops already employ single cable pair today; thus, it is only natural to assume that providers would want this capability. SDSL wasdeveloped to provide high-speed communications on that single cable pair, but atdistances no greater than 10,000 feet. Despite this distance limitation, SDSLwas designed to deliver 1.544 Mbps on the single cable pair. Typically, however,the providers provision SDSL at 768 Kbps. This creates a dilemma for thecarriers because HDSL can do the same things asSDSL.

SDSL uses only one pair of wires to provide duplex high-speedcommunications, but it is limited in distance. Not all users require symmetricalspeeds at the same time. ADSL was therefore designed to support different speedsin each direction at distances of up to 18,000 feet. Because the speedsrequested are typically to access the Internet, most users look for higherdownload speeds and lower upload speeds. Therefore, the asymmetrical nature of this service meets those needs.

Typically, when equipment is installed, assumptions are madebased on minimum performance characteristics and speeds. In some cases, special equipment is used to condition the circuit to achieve those speeds.However, if the line conditions vary, the speed will be dependent on thesensitivity of the equipment. In order to achieve variations in throughput andbe sensitive to the line conditions, rate-adaptive DSL was developed. Thisenables the flexibility to adapt to changing conditions and adjust the speeds ineach direction to potentially maximize the throughput on each line. In addition,as line conditions change, one could see the speeds changing in each directionduring the transmission. Many of the ILECs haveinstalled RADSL as their choice given the localloop conditions. Speeds of up to 768 Kbps are the preferred rates offered by theincumbent providers.

Not all consumers need symmetrical high-speed communicationto access the Internet. Furthermore, ADSL speedsare more than the average consumer may be looking for. Lower speedcommunications capability was developed using CDSL. With other forms of DSL, splitters are used on the line. CDSL was designed to eliminate the splitter onthe line. Moreover, speeds of up to 1 Mbps in the download direction and 160 to384 Kbps in the upload direction are provided. It is expected that the speedsand CDSL will meet needs of the average consumer for some time to come. Auniversal ADSL working group developed what is called ADSL-lite, also called G.lite. This specification wasratified in late 1998, using the working group's specifications for servicedelivery to the average consumer. An example of this DSL-lite service is provided by the Nortel Networks' 1-Mbmodem.

It was only matter of time until some users demanded higherspeed communications than was offered by the current DSL technologies. VDSL was introduced to achieve the higher speeds. In fact, speedsranging from 13 to 52 Mbps are available, but the distance limitations of thelocal cable will be a big factor. In order to achieve the speeds, one can expectthat a fiber feed will be used to deliver VDSL. This technique will most likelycarry ATM (cells) as its primary payload. We can expect some hybrid arrangements to deliver this speed to the door forhigh-speed data at up to 52 Mbps downward and 1.5 to 6 Mbps upward.

These are the typical installation and operationalcharacteristics; others will certainly exist in variations of installation andimplementation.

Single-Pair High-Speed Digital Subscriber Line (SHDSL) TOP

SHDSL is the newest member of the DSL family being currentlystandardized at ITU (G.shdsl) and ETSI (TM6) in 2001. G.shdsl targets the small business market. Multiple telephone and data channels,videoconferencing, remote LAN Access, and leased lines with customer-specificdata rates are among its many exciting characteristics. Spectrally friendly withother xDSLs, it supports symmetric data ratesvarying from 192 Kbps to 2.321 Mbps across greater distances than othertechnologies. SHDSL uses a Trellis Coded Pulse Amplitude Modulation (TC PAM)-based modulation scheme, which is different from the other forms ofxDSL. It will operate over asingle pair of wires at ranges from 6,000 feet to 20,000 feet (2.3 Mbps to 192Kbps rates, respectively) on 26 AWG copper. Thiswill produce a 35 to 50 percent improvement in rate at a given range overtraditional symmetric DSL and a 15 to 20 percentimprovement in distance at a given rate over traditional symmetric DSL. G.shdsl is also known as G.991.2 standard.

The local providers are extremely excited at the possibilityof installing higher speed communications and preserving their local cableplants. No one wants to abandon the local copper loop, but getting more datareliably across the local loop is imperative. Therefore, the ability to breathenew life into the cable plant is an extension of the facilities in place. Thisalso means that they can create a new form of revenue streams from the oldcopper. Consumers are looking for high-speed access (primarily to access theInternet) for whatever the application. Yet, at the same time, consumers arelooking for a bargain. They do not want to spend a lot of money on theircommunications services.

The providers are trying to bump up their revenues, withoutmajor new investments. They would like to launch as many new service offeringson their existing cable plant and increase the costs to the end user. This is abusiness decision, not a means of trying to rake the consumer over the coals.Yet, a happy medium must be found of providing services and generating revenueswith limits on expenses. To do this, the xDSL family offers the opportunity to meet the demands while holding down investmentcosts. The key ingredient for success is to minimize costs and satisfy theconsumer. Make no mistake—if the local provider does not offer high-speedservices, someone else will.

Many approaches were developed as a means of encoding dataonto xDSL circuits. The more common are carrierless amplitude phase (CAP) modulation and discreet multitone (DMT) modulation. Quadrature phase modulation (QAM) has also been used, but the important part is the standardization. The industryas a rule selected DMT, but several developers and providers have used CAP. Itis therefore appropriate to summarize both of these techniques.

Discreet Multitone (DMT) Modulation

DMT uses multiple narrowband carriers, all transmitting simultaneously in a parallel transmission mode. Eachof these carriers carries a portion of the information being transmitted. Thesemultiple discrete bands—or in the world of frequency-division multiplexing, subchannels—are modulated independently of each other, using a carrierfrequency located in the center of the frequency being used. These carriers arethen processed in parallel form.

In order to process the multicarrier frequencies at the same time, a lot of digital processing is required. In thepast, this was not economically feasible, but integrated circuitry has made thismore feasible.

The American National Standards Institute (ANSI) selected DMTwith the use of 256 subcarriers, each with thestandard 4.3125-kHz bandwidth. These subcarrierscan be independently modulated with a maximum of 15 bps/Hz. This allows up to 60 Kbps per tone used. The frequency spectrum for thecombination of voice and two-way data transmission. In this representation,voice is used in the normal 0- to 4-kHz band onthe lower end of the spectrum (although the lower 20 kHz are provided).Separation is allowed between the voice channel and the upstream datacommunications, which operates between 20 and 130 kHz. Then a separation isallowed between the upstream and the downstream channels. The downstream flowuses between 140 kHz and 1 MHz. The separation allows for the simultaneous upand down streams and the concurrent voice channel. It is on this spectrum thatthe data rates are sustained. Each of the subchannels operates at approximately4.3125 kHz, and a separation of 4.3125 kHz between channels is allocated.

Provisions for the high-speed data rates of full ADSL aregood, but not every consumer is looking for the high data rates afforded onADSL. Therefore, the Universal ADSL Working Group decided to reevaluate the need for the end user. They determined that many consumers need downloads of1 to 1.5 Mbps and uploads of 160 to 640 Kbps. Consequently, ADSL-lite specification was designed with thesespeeds for the future. Initially introduced in early 1998, the specification wasratified in late 1998 to facilitate the lower throughput needs of the averageconsumer. DMT is the preferred method ofdelivering G.lite service. There is no way to knowif the network providers can support hundreds of multimegabit ADSL up- and download speeds on their existinginfrastructure. However, using the G.lite specification can support low-demandusers more efficiently. Similar to the DMT used in the ANSI specification, thecarriers are divided is shown in the picture below. Note that in this case, the highend of the frequency spectrum tops out at approximately 550 kHz instead of the1-MHz range with ADSL.

Carrierless Amplitude Phase (CAP) Modulation

CAP is closely aligned to quadrature amplitude modulation (QAM). QAM as a technique is widely understood in theindustry and well deployed in older modems. Both CAP and QAM are single-carriersignal techniques. The data rate is divided in two and modulated onto twodifferent orthogonal carriers before being combined and transmitted. The maindifference between CAP and QAM is in the way they are implemented. QAM generatestwo signals with a sine/cosine mixer and combines them onto the analog domain.

CAP was one of the original proposals for use with ADSL technology. Unfortunately, this was a proprietary solution offered by a singlevendor, which turned heads away from acceptance. Most industry vendors agree that CAP hassome benefits over DMT, but also that DMT has morebenefits over CAP. The point is that two differing technologies were initiallyrolled out for ADSL (and the other family members), which contradict each otherin their implementation.

CAP uses the entire loop bandwidth (excluding the 4-kHz baseband analog voice channel) to send the bits all at once. The DMT techniquehas no subchannels. The lack of subchannels removes the concern and problemswith individual channel transmission. Most of the RBOCs started using CAP but have since moved on to DMT.

Comments on Deployment TOP

ADSL service is catching on. However, the ILECs and CLECs are dragging their feet. As of late1998, only about 150,000 ADSL modem pairs were installed in the United States.In contrast, over 800,000 cable modems were installed in residences andbusinesses across the country. The local owners of the copper loop have to takea more aggressive approach to delivering high-speed services or consumers willgo somewhere else. As the market continues to mature and standards continue todevelop, the local providers must preserve their infrastructure.

Whereas consumers are reluctant to proceed with ADSL, the HDSL and SDSL services are still very attractivealternatives, offering 1.544- to 2.048-Mbps symmetrical speeds or some variation, as already discussed.

In the future, when high-speed media are installed to thedoor or to the curb, the logical stepping stone will become the VDSL service, perhaps sometime in 2002 to 2003. Although trials are already underway, too much time passes until the results are complied and analyzed.Therefore, the reality of VDSL for the masses is still a long way off.

In general, a Voice over DSL (VoDSL) system functions as an overlay solution to a DSL broadband access network,enabling a CLEC to extend multiline localtelephone service off of a centralized voice switch. For example, Jetstream's VoDSL solution allows up to 16 telephone lines and high-speed continuous dataservice to be provided over a single DSL connection. AVoDSL solution typically consists of three components.

• First, a carrier-class voice gateway resides in theregional switching center (RSC) and serves asa bridge between the circuit-based voice switch and the packet-based DSLaccess network.
  

• Second, an Integrated Access Device (IAD)resides at each subscriber premises and connects to a DSL circuit. It alsoserves as a circuit/packet gateway andprovides the subscriber with standard telephone service via up to 16 analogPOTS ports and Internet service via an Ethernet connection.
  

• The third component is the management system.

With VoDSL solutions, DSL broadband access networks now have the coverage, capacity, and cost attributesto enable CLECs to deliver local telephoneservices as well as data services to the small and mid-size business markets. Ithas already been established that DSL access networks have the right bandwidthto serve the data needs of small and mid-size businesses. With VoDSL accesssolutions, this is true for serving the local telephone service needs of thosesubscribers as well.

Some VoDSL solutions are capable of delivering 16 telephonelines over a DSL circuit along with standard data traffic. Because 95 percent ofsmall businesses use 12 or fewer telephone lines, a single DSL circuit providessufficient bandwidth to serve the voice needs of the vast majority of themarket. In addition, if more than 16 lines are required, most VoDSL solutionsenable a provider to scale service by provisioning additional DSL connections.In addition to providing the right capacity for providing local telephoneservice, DSL broadband access networks are very efficient in the way theydeliver service. TDM-based transport services,such as a Tl line, require the bandwidth of theline to be channelized and portions dedicated to certain services, such as atelephone line. Even if a call is not active on that line, the bandwidthallocated to that line cannot be used for other purposes. DSL access networksare packet-based, enabling VoDSL solutions to use the bandwidth of a DSL connection dynamically. VoDSL solutions onlyconsume bandwidth on a DSL connection when a callis active on a line. If a call is not active, then that bandwidth is availablefor other services, such as Internet access. This dynamic bandwidth usageenables providers to maximize the potential of each DSL connection, deliveringto subscribers the greatest number of telephone lines and highest possible dataspeeds.

Because telephony traffic is more sensitive to latency thandata traffic, VoDSL solutions guarantee the quality of telephone service bygiving telephony packets priority over data packets onto a DSL connection. Inother words, telephony traffic always receives the bandwidth it requires anddata traffic uses the remaining bandwidth. Fortunately, telephony traffic tendsto be very busy over the course of a typical business day, so the average amountof bandwidth consumed is minimal. For example, over a single 768-Kbps symmetric DSL connection, a CLEG could provideeight telephone lines (serving a PBX/KTS with 32extensions) and still deliver data service with an average speed of 550 Kbps.

VoDSL Network Infrastructure

Today’s voicenetworks rely on analog lines and trunks as well as digital T1 circuits toconnect to SMB subscribers. Asshown in the figure below, delivering voice and data to SMB customers requiresmultiple multiple voicelines and end-user data services.

The centralizedvoice networks in use today are highly reliant on expensive and complex legacyClass 5 TDM circuitswitches. Located at every subscriber end office, these switches supply localand long-distance phone service tosubscribers. Class 5 switches provide the following features and functionality:

Lines connect subscribers to network resources and are typically analogcopper loops or POTS lines.For POTS service, lines provide power to the user telephony equipment anddeliver call progressinformation from the switch, such as dial tone, ringing, and ringback.

Trunks are used to interconnect network resources such as Class 4 and 5switches. Class 5 switches ina regional area may have trunks between them for local calling, and Class 5switches may be connectedto ILEC and IXC Class 4 tandems to offer long-distance and toll calling.

On the side of the Class 5 switch that faces subscribers, the switch providesline-side signaling andfeatures such as callback (*69), call waiting, caller ID, and others. On thetrunk side of the switch,network signaling for toll call routing and PSTN database access is obtainedby accessing the SS7network over dedicated digital links, sometimes referred to as ‘A’- or‘F’-links. The trunk side Class 4tandem features, like feature group D (1+ dialing for long-distance carrieraccess), local numberportability (LNP), 800 number dialing, and directory services, are accessedover the A-link using SS7protocols, such as ISDN User Part (ISUP) and Transaction CapabilitiesApplication Part (TCAP).

The Class 5 switching matrix switches local calls from line to line, andnon-local and toll calls fromline to trunk and from trunk to trunk.