GLOBAL PROFESSIONAL SERVICES

Transcription

1 GLOBAL PROFESSIONAL SERVICES Your Telecommunication Training Solutions Provider Contact: Samuel Kiser Office ( ) Mobile ( ) 5165)

2 TELEPHONY NETWORKS BASIC OVERVIEW GLOBAL PROFESSIONAL SERVICES

3 Introduction Network Overview - Course Module NW-1 Welcome to the ever- changing science of telephony! It is a major part of the world of Communications. This course is designed to provide a high level overview of the telephony network and how it operates.

4 Network Overview In the early days - The first telephone network consisted of telephone sets connected to a switchboard. You talked to an operator who manually inserted patch cords into jacks to connect the calling party to the called party. You even had to crank a handle to generate a ringing signal to alert the operator you wanted to place a call. A long distance call connection was timed by a precision clock called a Calculagraph. It was started and stopped by the operator and would print on a card for billing purposes, as the start/stop lever was pulled. Today, of course, it is all automatic using digital electronics. The telephone network transported local voice traffic over a two wire copper cable loop. The loop is a pair of conductors called a tip and ring, named for the metal connection points on the plug that was plugged into the cord board. Long distance calls used 4 wires (2 pairs), called trunks, between telephone office locations.

5 Network Overview In the early days - The network had a service area that was limited by the DC power plant used to power the telephone circuit and the resistance of the copper cable. The modern telephone network is: diversified and able to provide a variety of services including Video, Voice, Digital Data, Fax and Analog data transmission through the network. Carrier systems combine (multiplex) numerous circuits for transmission over copper cable pairs and in effect provide pair gain. Microwave and Satellite systems increase the distance signals can be transmitted. Fiber Optic systems have supplanted Microwave and Satellite as the medium of choice for local and long haul traffic. High bandwidth (large capacity) and complex digital signal processing / multiplexing are supported by fiber optics and allow the network to transport multiple services over the same facilities virtually error free.

6 Service Areas Central Office Switch Central Office Switch Copper pairs

7 OUTLINE LESSON 1 INTRODUCTION LOCAL NETWORK LOCAL WITH TANDEM OFFICE CALL ROUTING INTEREXCHANGE LOCAL CALLS LONG DISTANCE CALLS INTERLATA NETWORK INBAND VS OUT OF BAND SIGNALING SS7 OVERVIEW

8 Introduction- The early telephone network was designed to allow customers to talk to each other. At first, customers communicated over copper cable pairs connected to a manual switchboard. As stated earlier, an attendant would receive verbal requests to place a call. Plugging a cord into a board to connect the two phone lines together established the call. This system worked in small towns but as more and more subscribers attached to the network, this type of manual switching was not capable of handling the demand. Eventually, automatic switches were developed.

9 Local Network STA. A LOCAL LOOP CO SWITCH LOCAL LOOP STA.B The Local Network is an area where phone calls are toll free. Station A dials the number of station B. The CO Switch recognizes both numbers are on the same switch and connects the 2 phones together. Network Configuration: 1 - Central Office Switch 2 - Local Loops 2 - Telephone sets

10 Telephones located on different local switches CO A TRUNK CO B LOCAL LOOP STA A LOCAL LOOP STA B Station A dials Station B. Switch A recognizes the dialed number is not on the same switch and routes the call to Switch B. Switch B identifies the number belongs to station B and connects the two stations together. Note: A Trunk is a network Facility that is terminated by a Switch on both ends. Trunks are usually 4 wire facilities (This means separate transmit and receive paths). Network Components: 2 - Central Office Switches 1 - Trunk 2 - local loops 2 - Telephone Sets

11 Tandem Switching As network size increases direct trunks between switches may be busy. A Tandem Switch is added to handle trunk overflow and route calls between switches that do not have direct trunks or the direct trunk group is blocked (all trunks are busy). TRUNK CO A LOCAL LOOP LOCAL TANDEM DIRECT TRUNK TRUNK CO B LOCAL LOOP Call Flow: STA A STA B In this example, Station A dials the phone number of Station B. Switch A recognizes the number belongs to Station B, on Switch B and attempts to route the call over the direct trunk to Switch B. However, the direct trunk is busy. The switch then routes the call to the tandem and the tandem then routes the call to Switch B, then to Station B. The tandem handles overflow calls for local switches and also serves as a gateway to other local or long distance networks. Network Components: 3 - Trunks 2 - Local Switches 2 - Local Loops 2 - Telephone Sets 1 - Tandem Switch

12 Call Routing The central office switch routes calls based on the ten-digit telephone number. The telephone number is divided into three parts. These are known as Area Code (also called Numbering Plan Area or NPA), NNX, and Station number. NPA / AREA CODE - first 3 digits - identifies a large geographical area such as a state or part of a state. NNX - next 3 digits - identifies a local area, usually a city or part of a city. In large cities it may identify one particular switching machine (an exchange) within that area code. STATION NUMBER - last 4 digits - identifies a specific telephone line (customer). CALL ROUTING: The switch looks at the NPA/AREA CODE to decide if the call is local to its area. Next the NNX is checked to see if the called party is in the local switch or has to be routed to another switch via a trunk. Finally the last 4 digits identify the specific telephone subscriber. In this scenario all signaling is transmitted over the actual lines the subscriber will talk on. This type of signaling is called INBAND signaling.

13 Call routing NPA NNX STATION FIRST 3 NEXT 3 LAST 4 DIGITS - DIGITS - DIGITS LARGE LOCAL AREA AN INDIVIDUAL GEOGRAPHIC TOWN, SMALL TELEPHONE LINE AREA CITY OR PART WITHIN THE OF A CITY NPA / NNX

14 Intralata Calls Tandem switches are also used to connect other local area switches that are not connected by direct trunks. In this example Station A calls Station B, but Station B is in a different area code. Station A dials the number for station B. The local switch recognizes the local number is not in its area and INTRALATA TANDEM 1 CO A LOCAL TRUNK LOCAL LOOP STA A INTRALATA TRUNK INTRALATA TANDEM 2 LOCAL TRUNK CO B LOCAL LOOP routes the call to Tandem 1. Tandem 1 knows that the area code is located on Tandem 2 and routes the call to it. Based on the NNX the call is then routed to Local switch B. Local switch B then terminates the call to station B. Network Components: 2 - Tandem Switches 2 - Local Switches 3 - Trunks 2 - Local Loops 2 - Telephone Sets STA B

15 Local Long Distance Local, Long distance or Intralata, Long distance calls are handled like the example on the previous slide. However, toll charges will be assessed for completing the call. A LATA (Local Access Transport Area) is the area designated by the PSC (Public Service Commission) for local and local long distance toll calls that can be handled by the local telephone carrier. Calls within the LATA are called INTRALATA calls and are billed by the local telephone carrier. This was a result of Divestiture in 1984, where AT&T and the Bell companies were split apart, as mandated by the U.S. government. INTERLATA calls traverse two or more LATA s. Switching calls between LATA s is the responsibility of the INTERLATA EXCHANGE CARRIERS (IEC s), AT&T, SPRINT, MCI, etc. These calls are usually billed by the IEC. Calls were processed according to these rules from 1984 until the last couple of years, when the FCC allowed the local Bell entities back into the Interlata long distance business once they met certain requirements. Today, customers specify whom they want to provide their telephone service, both local and long distance.

16 Interlata call CALL FLOW: Station A calls Station B. The local switch identifies the number as a call that must be routed over an INTERLATA EXCHANGE CARRIER (IEC) Network. The call is routed to the INTRALATA Tandem. The INTRA LATA Tandem routes the call to the IEC tandem. INTERLATA TANDEM INTRALATA TANDEM CO A LOCAL TRUNK LOCAL LOOP LONG LONG DISTANCE DISTANCE NETWORK NETWORK INTERLATA TANDEM INTRALATA TANDEM CO B LOCAL TRUNK LOCAL LOOP Depending on the distance between LATA s the call may be routed over STA. A STA. B (continued - next page) LATA A LATA B

17 Interlata call cont d several IEC Switches before delivery to an INTRALATA Tandem in LATA B. After the call reaches LATA B, the Local network routes the call to the subscriber at B. The IEC receives time based Toll charges for the duration of the call. The local carrier receives a fixed access fee per line for connection to the work. INTERLATA TANDEM INTRALATA TANDEM CO A STA. A LOCAL TRUNK LOCAL LOOP LONG LONG DISTANCE DISTANCE NETWORK NETWORK INTERLATA TANDEM INTRALATA TANDEM CO B STA. B LOCAL TRUNK LOCAL LOOP LATA A LATA B

18 Inband vs Out of Band Signaling Signaling is the process of sending alerting signals or information across the network. So far we have discussed call routing based on digits transmitted over the same path as the actual conversation. Transmitting signaling over the same path is called INBAND SIGNALING. Out of Band signaling is the transmission of signaling information over a separate circuit or path. Out of band signaling has the advantage of not tying up network equipment during the call setup phase. The Out of BAND system currently in use is called Signaling System 7 or SS7. SS7 uses at least two data link circuits (56 kbps or higher) to establish communications between a local or tandem switch and a Signal Transfer Point (STP). Call requests are converted into a SS7 packet of data and transmitted to the STP. The STP passes the packet to the SCP (Signal Control Point) for database lookup of the telephone number. The database contains information about the telephone numbers and services that are subscribed to that number. The database returns the required information needed to complete the call. Trunks are reserved through the network, but not actually used until verification that the far end telephone is not busy. The SS7 network is a separate network devoted to switching calls.

19 Network with SS7 Signaling INTERLATA TRUNK LOCAL TANDEM LOCAL TRUNK LOCAL SWITCH A 56 KBP/S LINKS INTERLATA TANDEM STP SCP 56 KBP/S LINKS INTERLATA TRUNK LOCAL TANDEM LOCAL TRUNK LOCAL SWITCH B STA. A STA. B A call placed from station A to station B generates a call request to the STP from the local switch. The STP then accesses the SCP. The SCP replies with routing information to the STP. The STP contacts all switches involved in completing the call, checks if the far station is busy and for trunk availability. The trunks are reserved until a path is verified from end to end. If the line is busy or a path cannot be found, a busy signal is returned to the customer, and the reserved trunks are released for use on other calls.

20 Network with SS7 Signaling cont d INTERLATA TRUNK LOCAL TANDEM LOCAL TRUNK LOCAL SWITCH A 56 KBP/S LINKS INTERLATA TANDEM STP SCP 56 KBP/S LINKS INTERLATA TRUNK LOCAL TANDEM LOCAL TRUNK LOCAL SWITCH B STA. A STA. B The SS7 links, STP s, and SCP s are redundant. If a link fails, the backup takes over. Links are route, equipment and power diversified. The backup STP and SCP are located in different CO s. The Wireless network is also connected to the SS7 network. Caller ID information is supported by SS7. The SS7 network is very carefully monitored. Failures can result in towns being isolated.

21 The SS7 Network 1. A packet data network that overlays the traditional network and provides signaling control. 2. Transfers call signaling information from the voice/payload path into the separate control path conserving precious payload time. 3. Provides/shares the call signaling information to switches up and down the line to allow them to correctly set up the user path. 4. Prevents tying up switches and facilities in the event the call cannot be completed.

22 Summary The telephone network began simply and became more and more complex. The modern network is diverse and can transport a variety of signals. The early network was made up of telephones, a switch and cable pairs. The network grew and tandems and trunks were added. LATAs were introduced to identify local service areas. Interlata companies carry phone calls between LATAs. Signaling System 7 was developed to provide out of band signaling and more efficient processing of telephone calls.

23 Lesson 2 - The Telephone OUTLINE -- LESSON 2 INTRODUCTION OPERATION OF THE TELEPHONE DC SIGNALING REQUIREMENTS AC CONTROL SIGNALS ALERTING TONES

24 Introduction: This lesson explores the telephone and its operation. Telephone Loop Central Office Switch Central Office Switch Copper pairs Carrier system RT Remote Terminal

25 Operation of the Telephone Telephones convert sound waves into electrical energy. The electrical signal is transmitted thru the network. Human voice is contained in a 4khz wide channel (bandwidth). 300 Hz to 3300 Hz

26 Inside the phone The handset contains a transmitter and a receiver rec CO Switch trans Line card H T,R pair H The handset cord has 4 wires in it, 2 for transmit, 2 for receive. A device in the phone called a hybrid transformer combines the two directions into one pair of wires (the local loop) that is bidirectional. The hybrid was originally made up of transformers, but is now electronic in both phones and CO switches.

27 Inside the phone- cont d The hybrid is not perfect: this imperfection helps when using the phone, in that you can hear yourself talk. The reflection back of your own voice is called sidetone. rec CO Switch trans Line card H T,R pair H

28 Inside the phone- cont d The hybrid is not perfect: it hurts in the switch/network because it reflects the signal back to the talker. Because of the delay of the reflection back in a long distance call, it is annoying. This reflection is called echo. rec Switch line card trans To local and long distance trunks H T,R pair H Echo cancellers can eliminate echo in long distance networks. They are not usually required for distances less than about 500 miles.

29 DC Signaling Range Telephones need a minimum of 20 milliamps of DC current to function volts ( the amount of voltage supplied by the CO batteries to a typical telephone circuit) can provide 20 milliamps DC over a maximum resistance of 2400 ohms. The 2400 ohms contain certain fixed values. What determines the maximum loop resistance? 2400 ohms total for 20 ma 400 ohms CO battery feed resistance, 200 in tip, 200 in ring -48 vdc R 25 ohms for drop wire - telephone pole to house T 10 ohms for central office wiring 400 ohms for the telephone set = 1,565 ohms left for loop resistance of actual cable pair. ( KΩ ) Most Telcos engineer for something below the maximum of 1565 ohms, usually somewhere around 1465 ohms, to allow for equipment and cable variations. CO BATTERY

30 Loop limits Depending on cable size and resistance a telephone set can be located approximately 4 to 5 miles away from the CO and still operate satisfactorily without any additional voltage boost. Telephones located outside of these distances will require the addition of electronics called loop extenders (which boost the voltage) or they may be put on a carrier system.. Loaded or Nonloaded pairs 400 ohms resistance 1565 ohms cable resistance -48 volts 400 ohms resistance

31 VF (voice frequency) Loss The loop also introduces a loss of volume to the signal (voice conversation) as it travels down the cable pair. This loss is measured in decibels (db) named after Alexander Graham Bell, the inventor of the telephone. The term dbm refers to a db referenced to a milliwatt of power. ( 0 dbm =.775 volts into 600 ohms.) The dbm measurement is used to determine the loss in volume to the voice signal between the CO and the customer location. A long loop requiring a loop extender (DC) may also require amplification of the VF signal (AC). (Approximately 8 db loss - max.) A load coil, a small coil of wire like device, connected to the cable pair every 6000 feet may also be used as another way to reduce VF loss. However, while aiding low frequency voice signals, load coils prohibit the use of newer digital services, such as DSL. Load coils must be removed from the cable pair before DSL service can be installed.

32 AC CONTROL SIGNALS Various AC signals are used to alert or notify customers of call status: DIALTONE: A combination of 440 & 880 HZ tones. Dial tone is the sound you hear when you pick up the telephone. Dial tone is used to notify a customer that the Central Office switch is ready to receive a call request. 20 HZ RINGING : VAC is used to ring the bell in the telephone set. This alerts subscribers of incoming calls. This comes from a ring generator in the CO. 120 IPM: Fast busy - alerts customer called party line is busy. 160 IPM: RECEIVER OFF HOOK loud signal used to alert customer that telephone is left off hook. DUAL TONE MULTI FREQUENCIES : also know as TOUCHTONE (@TRADEMARK AT&T) DTMF is a combination of two frequencies. Each combination represents an individual number or symbol on the phone pad.

33 DIAL PULSING There are two methods for producing dialed digits. Dial Pulsing is the original method for making a phone call. When a subscriber picks up the phone to make a call, the loop is closed and dc current flows from the Central Office thru the handset. The rotary dial on the telephone is used to dial the telephone number. The rotary dial produces dial pulses by mechanically opening and closing the loop. Rotary dial telephones are equipped with cams to calibrate the dial pulse to ten pulses per second. The digit receiver in the CO cannot handle dial pulses that are transmitted faster than ten pps. DIALED DIGITS

34 DTMF Dialing Dual Tone Multi-Frequency (DTMF) are the tones produced by the touch pads on a telephone set. DTMF combines two tones to produce a dialed digit. DTMF can be transmitted and received faster than dial pulses. DTMF also has the advantage of portability across the network. A DTMF pulse can be transmitted across any analog circuit. Dial pulses can only be transmitted on circuits with DC continuity. Circuits that traverse carrier systems rarely maintain DC continuity. 1209~ 1336~ 1477~ DTMF is the preferred method of dialing digits. The diagram shows the DTMF tones. For example, dialing a 2 requires the combination of 1336 and 697 HZ. A receiver (or tone detector) in the CO recognizes the incoming tone as a 2 having been dialed by the subscriber * 0 # 697~ 770~ 852~ 941~

35 Summary Telephones convert sound waves into electrical energy. DC current is used to power the telephone. 20 milliamps of current are required to power the telephone. AC tones are used for Dial Tone, Busy, and Reorder. Dial pulses/ DTMF tones are used to transmit digits to the telephone switch.

36 Lesson 3 Outside Plant OUTLINE LESSON 3 INTRODUCTION THE EARLY NETWORK CABLE PAIRS NONLOADED CABLE PAIRS LOADED CABLE PAIRS BRIDGE TAP SUBSCRIBER LOOP CARRIER & D4 BANKS FIBER OPTICS

37 Introduction This lesson looks at cable make-up, the electrical properties of cable pairs, the differences between loaded and non-loaded pairs, bridge taps, Subscriber Loop Carrier (SLC), D4 banks and Fiber Optics.

38 Basic Network In the beginning the network consisted of a telephone set, a cable pair and a switch. Local Network Loaded or nonloaded cable pairs 400 ohms resistance 1565 ohms cable resistance 48 volts ohms resistance CO Switch

39 Cable Pairs A cable pair consists of a pair of copper wires. Each wire is covered by insulation. Cable pairs come in different gauges. The wire gauge (diameter) specifies the size of the wire. Cable pair gauges range from smallest, 26 gauge, to largest 19 gauge. 22 and 24 gauge are the two intermediate sizes. The two insulated copper wires are twisted together to form a pair. A group of pairs are twisted together to form a 25 pair binder group. Binder groups are combined to form the appropriate size cable, i.e., 100 pairs or 200 pairs or larger. PROTECTIVE COVERING BINDER GROUP Once a cable is formed, it is coated with a protective covering. Older cables were coated with lead. Newer cables are coated with Plastic. Depending on the application, metallic sheaths are added for strength, shielding or armor plating. The copper pairs are covered with paper (pulp) or plastic (PIC). A sticky gel fills the voids in PIC cables to keep water out. Air pressure is used on older lead cables to keep water out. Water in a cable can cause problems such as shorts, noise or crosstalk.

40 Non-loaded vs- loaded pairs RESISTANCE CAPACITANCE INDUCTANCE (Coil of wire) Cable pairs electrical characteristics consist of resistance, capacitance and inductance. Each of these components produces a negative effect on voice conversations or data traffic. Resistance limits the flow of electricity through the pair. Capacitance attenuates high frequencies, and Inductance acts as a low pass filter. Capacitance and resistance are the most detrimental to voice traffic. The effects of resistance can be reduced by boosting the 48 volt talk battery to a higher voltage. Range extenders, often called REG units, are used for this purpose. Inductance cancels out the effects of capacitance. Researchers discovered that by adding inductance, to reduce capacitance, the voice frequency response (how the other person s voice sounds) and VF loss, could be improved. 88 millihenries of inductance, added at an interval of every 6000 feet of cable to offset the capacitance, is sufficient. A device known as a load coil is added to the cable pairs. Cables that have load coils added are called loaded cables.

41 Non-loaded vs- loaded pairs Nonloaded cable pairs Advantages: Non-loaded pairs can carry high-speed data circuits (56 Kbps, ISDN, DSL and T1 carrier speeds). They are easier and less expensive to install and maintain. They are compatible with all voice and data services. Disadvantages: There are limited distances that voice frequencies can travel without special treatment.

42 Non-loaded loaded-vs-loaded pairs Loaded cable pairs 6 kilofeet Advantages: The addition of load coils provides improved performance in the voice frequency range (300 hz to about 3300 hz). Load coils are an inexpensive way to increase analog performance by decreasing VF loss. Disadvantages: Load coils act as a low pass filter. This means only frequencies below about 3500 Hz can be passed. This eliminates loaded cable from use on any high-speed data circuits. In fact, a great portion of the cable pairs that were loaded have had the load coils removed. Also, misplaced load coils cause transmission impairments. Extra effort is required when adding new cable to insure the cable is loaded properly.

43 Bridge tap Cable pairs were installed to provide telephone service to customers. The larger cables were installed in the most populated areas. Cable was expensive to install. Customers moving from place to place would order service, then later cancel that service, when they moved. To better handle service demands, pairs were bridged together and run down multiple streets. Thus, the same numbered pair would appear on several streets. A customer that ordered service would terminate his service on a pair that ran past his house. The cable that was bridged to his pair then became BRIDGE TAP. Bridge tap is a piece of cable that is attached to a working pair with the end of the bridged pair(s) left open. Excessive bridge tap can impair transmission quality. Example: Cable pair 12 is providing service to subscriber A. Pair 12 is also bridged down the streets of subs. B and C. Pair 12 CO EXCHANGE CABLE SUB C BRIDGE TAP SUB A SUB B

44 Subscriber loop carrier (SLC) Subscriber Loop Carrier systems first appeared on the scene during the late 1960s. SLC systems are an adjunct to the cables/pairs already in outside plant. SLC systems take the customers voice (analog) signal and converts it into a digital signal for transport to the CO, and back. Digital signals are easier to transmit, than analog signals. SLC systems are often referred to as Pair Gain systems because the equipment combines 24 customer s signals on to 2 pairs of copper wires. One pair is for transmitting to the CO and the other pair is for receiving from the CO. Since only 2 pairs of wires are needed to accommodate 24 separate conversations, we effectively gain 22 cable pairs. This is a huge cost savings, that is used to offset the initial expense of the electronics. SLC systems allow us to serve customers beyond the range of 4 5 miles, for copper pairs. The outside plant portion of the SLC system is known as a Remote Terminal (RT). The RT can be mounted on a pole, placed inside a hut, or in an environmentally controlled vault. RESIDENCE 1 RESIDENCE 24 CUSTOMER PAIRS (VF SIGNALS) SLC RT TRANMSMIT TO CO DIGITAL RECEIVE FROM CO

45 Subscriber loop carrier (SLC)-cont d There are 2 different types of SLC systems: stand alone (or traditional) and integrated. Inside the CO the traditional SLC system terminates the digital signals coming from the RT in the field into a COT (Central Office Terminal). The signals are converted back to analog and wired into the analog switch machine for routing to the called party, or to some other destination if it is a non-switched circuit of some type. CO SLC RT TRANSMIT TO CO DIGITAL RECEIVE FROM CO COT 24 ANALOG CONVERSATIONS TO THE SWITCH Integrated SLC systems do not utilize a COT. The switch machine that serves these RTs is a digital switch. Since the signals arriving from the RTs are already digital, they can be fed directly in to the switch itself for call processing.

46 D4 Banks The DS1 system is similar in function to the SLC system, except it is used in the INTEROFFICE network. It requires 2 D4 channel banks, 1 in each CO, and is used to transport 24 digital signals between the COs. It requires 2 metallic cable pairs (one to transmit signals and one to receive them from the far end). DS1 systems were actually deployed in the interoffice network prior to the SLC systems in the customer loop. Central Office Central Office D4 Bank D4 Bank 2 metallic trunk pairs

47 Fiber Optics Placing fiber optic cable in outside plant has been on-going for the last 10+ years. The process is slow and expensive. Much of the Interoffice network has been completed, but only a very small percentage of customer loop plant is fiber optic. A fiber optic system requires a Fiber Optic Terminal (FOT) at each end. The FOT requires a digital input. The signal is first digitized and then fed into the FOT where the electrical signal is converted to a light signal for transmission to the far end. Central Office 24 analog inputs D4 Bank Electrical FOT OPTICAL FAR END CO The light signals have the advantage of being much faster and offer the customer a whole new array of services, because of the increased bandwidth available. Also, the fiber cables and optic signals are not bothered by electrical interferences, such as those created by overhead power cables and lightening storms. Many advantages, but again, it is expensive.

48 Summary Wire (cable pairs) come in different gauges. Cable pairs electrical characteristics include: resistance, capacitance, inductance. Cables can be loaded or non-loaded. Bridge tap can cause transmission impairments. Carrier systems carry 24 circuits over 2 cable pairs. Fiber optic systems are gradually replacing metallic based carrier systems.

49 Lesson 4 The Central Office OUTLINE LESSON 4 INTRODUCTION CABLE VAULT DC POWER PLANT DISTRIBUTING FRAMES CENTRAL OFFICE SWITCH TOLL AREA DSX1 DSX3

50 Introduction The CO is the building that houses the equipment required to provide telephone service. Almost all customer services pass through a CO at some point. The building: usually is hardened to withstand tornadoes and earthquakes. provides a controlled environment for the switching machine and other equipment. This lesson describes the CO and the equipment within.

51 Cable Vault The cable vault is the area where the copper and fiber cables enter the CO. The cable vault is usually located underground. Air pressure can be applied to underground cables to keep the cables dry. This is particularly important for paper insulated cables. Underground conduits enter and leave the building through the vault. Cables, known as Riser Cables connect to the outside plant cables entering the building and extend these pairs to the equipment and facilities upstairs. The riser cables usually terminate on a distributing frame in the equipment room. RISER CABLE OUTSIDE PLANT CABLE X CABLE SPLICE CABLE VAULT

52 DC Power Plant TO TELEPHONE EQUIPMENT POWER COMPANY AC DC POWER TELEPHONE CO. RECTIFIERS BATTERIES The DC Power plant provides DC power to telephone equipment in the CO. Rectifiers are used to convert commercial AC to DC current. This DC current is used to charge -48 volt batteries. Once charged, the -48 volt batteries provide power to the CO equipment. The batteries also act as filters in that they absorb voltage spikes in the commercial source, thereby keeping a steady -48 volts applied to the equipment.

53 DC Power Plant-cont d. TO TELEPHONE EQUIPMENT POWER COMPANY AC DC POWER TELEPHONE CO. RECTIFIERS BATTERIES The central office will deplete the batteries in a few hours if commercial AC power is not restored. For this reason central offices are equipped with diesel or turbine engines of sufficient size to provide AC power until the commercial AC is restored. Fuel tanks are located on premises. The DC power plant, combined with the engines for backup, allows the CO to provide provide uninterrupted phone service during storms or long commercial power outages.

54 Distributing Frames A distributing frame s function is to connect metallic cable pairs to the wiring from the switch and other equipment. In that manner, a cable pair can be cross-connected (wired) to a switching machine location to provide dial tone and access to the network of other subscribers. This is known as the MDF, or Main Distributing frame. The MDF provides: Equipment and personnel protection from foreign voltages. Termination of equipment and facilities. A flexible means of connecting components to build a circuit (via jumper wires). CROSS-CONNECT WIRING TO SWITCH METALLIC LOOP TO CUSTOMER MDF

55 Distributing Frames-cont d There are several types of MDFs: CROSS-CONNECT The Conventional frame is manually assigned and administered. The COSMIC frame is assigned and administered by computer software. METALLIC LOOP TO CUSTOMER WIRING TO SWITCH MDF Another type of frame is known as the Toll frame. This type of frame is used for Special Services. Special equipment, used for special applications, can be found on this frame. You will find D channel banks, amplifiers, data bridges, etc., located on this Frame.

56 CO Switch The central office switch: Terminates customer lines and switches calls between them. Terminates trunks. (A trunk is a facility between 2 switches.) Provides dial tone to alert customers the switch is ready to receive digits. Provides ringing to alert customers to incoming calls. Provides custom calling features, egs., call waiting, three way calling, call forwarding, etc. SWITCH TYPES: STEP BY STEP CROSSBAR ANALOG ESS DIGITAL ESS

57 CO Switch-cont d. The first fully automatic switching machines were the Step By Step and Crossbar switches. They were electromechanical. A group of individual relays and switches provided dial tone, and routed the call through the office in response to the digits dialed. They were labor intensive and parts wore out rapidly. The electromechanical switches were eventually replaced by the ESS (Electronic Switching Systems) machines. The first ESS machines were analog inside and used analog interface circuit packs to handle customer traffic. As more and more of the outside plant converted to digital equipment (D4 banks, SLC, etc.) it made sense to convert the ESS machine to a digital switch. Many, but not all, switches are digital today. Many calls, whether local or long distance, are handled digitally from end to end. ANALOG LINE DIGITAL ANALOG LINE SLC RT DIGITAL SWITCH DIGITAL SWITCH SLC RT

58 CO switch-cont d. It has been forecasted that, eventually, nearly every telephone line will be switched by a digital switching machine of some kind. The digital switch is more efficient than any of it s predecessors: It has all electronic components that are more reliable. It is not as expensive as comparable switches. It is faster than the electromechanical components used in other switches. It accommodates more complex signaling data. It is modular and can be easily grown. It requires conversion of analog to digital only at the end locations.

59 Toll Area Toll equipment is any equipment that is used to provide Special Services. Telephone services can be divided into 2 categories: POTS and Special Services. POTS stands for Plain Old Telephone Service and includes basic dial tone services (not Long Distance) and whatever equipment is required to provide it. Special Services, simply, are any services that do not fit into the POTS category. WATS lines, data services, PBX trunks, megalink services, optical rings, etc., are examples of Special Services. The equipment used to provide some of these services is found in the Toll area of the CO. Some examples are: metallic digital systems, test systems, Optical terminals, DSX jacks, etc.

60 Central Office bays Central office equipment is mounted on shelves in bays. These bays are 23 inches wide; height varies by CO. Below is an example of a multiple bay configuration in a CO. SONET D4 DLC SMAS SONET D4 DLC SMAS SONET D4 DLC SMAS D4 DLC SONET D4 DLC T3AS SONET D4 DLC RMS DS1

61 Digital Signal Crossconnect (DSX) Individual wires are used for wiring individual circuits on a frame. Once multiple circuits have been combined (multiplexed) together on a carrier system for transport to a foreign destination, wiring is done at the DSX level. (Circuits are wired onto the carrier system individually and carriers are tied together at DSX jack level.) Additionally, the jacks provide local test access. The Digital Signal Cross connect (DSX) is a jack field that terminates digital signals. DSX jacks are mounted in bays in the CO. Signals are terminated on the jacks and cross connected together using wires. All signals in the DSX area conform to a standard level. This allows any signal in a DSX to be connected to any other signal in the bay. There are two main types of DSX bays used in a CO. They are the DSX-1 and DSX-3. DSX-1 jacks terminate T1 Carrier systems. Twisted pair jumpers are used to connect T1s together. The jacks are mounted in a 23 inch wide rack. The number of jacks in a shelf varies from 24 to 84 jacks in a bay, based on the manufacturer. DSX-3 jacks terminate T3 carrier systems. Coaxial cable is used to connect T3 signals together.

62 Digital Signal Crossconnect (DSX)-cont d. TL MON OUT IN Monitor jack Activity indicator lamp Transmission path jack (plugging a patch cord into these jacks interrupts the circuit path)

63 Summary The CO is the building that house the telephone equipment. It contains the cable vault, the DC power plant, distributing frames, switching machine and toll equipment. The cable vault is usually underground. The various cables enter the CO at this point. The DC power plant charges the -48 volt batteries which supply power to telephone equipment. Distributing frames are used to wire together circuit components. The CO switch connects to customer lines and switches calls. The toll area contains all the equipment for non POTS services. DSX jacks are used for wiring carrier systems together and for monitoring, testing and patching.

64 Lesson 5 - Services OUTLINE LESSON 5 INTRODUCTION POTS POTS CUSTOM CALLING FEATURES SWITCHED SPECIAL SERVICES NON-SWITCHED SPECIAL SERVICES

65 Introduction This lesson discusses POTS and some of the features that enhance POTS service. Also included in the discussion are several of the more frequently requested switched special services and non-switched special services. The latter are often referred to as private lines. Specials are telecommunication services provided exclusively to a requesting customer, via a dedicated line or trunk. They may, or may not, be connected to a switching machine.

66 POTS POTS stands for Plain Old Telephone Service. POTS is the basic telephone service provided by the telephone company. It is the telecommunications service provided to customers which use one telephone line and routes a call through a central location (switch) to the other end. POTS is often referred to as basic service. POTS is considered a universal service that is to be available to just about everyone regardless of location. Remote locations may use radio systems or other expensive means to get the service there.

67 Custom Calling Features Custom Calling features are additional services that can be added to a POTS line. These features are usually added via switch translations. A customer who has existing phone service can add one or more features for a small nominal fee. Since these services are add-ons to existing service, they are very profitable for service providers. Some examples are listed below: DISTINCTIVE RINGING: provides a different ringing tone to identify an important call, eg., a spouse or child calling. CALL RETURN: allows redialing of your last incoming call. THREE WAY CALLING: allows customer to bridge 3 lines together during a call. CALL WAITING: CO switch beeps or clicks during an existing call to alert the customer of an incoming call. Customer can then flash the switch hook to answer the caller or ignore the call. CALLER ID: provides customer with phone number of incoming caller. The phone may have a display screen built in, or a separate display device may be added on.

68 Switched Special Services Switched Special Services are those telephone services that are switched via a Central Office switch but do not fall into the category of POTS. These services usually require special translations or equipment. A few examples are listed below: FOREIGN EXCHANGE LINES (FX) are telephone lines that terminate on a foreign switch. For example, a customer can purchase a line that terminates on a CO switch in a distant city or town. The line appears as a local number in that distant city and all calls made to local numbers in that city are toll free. FX lines are more expensive than POTS, but customers who have a high calling volume into a certain city can save money by purchasing an FX line and paying a flat monthly fee for the service. WIDE AREA TELECOMMUNICATIONS SERVICE (WATS) WATS is a switched service that allows customers to make calls to large geographic areas at a reduced bulk rate. There are 2 billing options: OUTGOING WATS AND INCOMING WATS. 800 SERVICE this is a type of incoming WATS service that bills calls with an 800 area code to the line receiving the call. If the call is Interlata, the call recipient (bill payer) selects their preferred Interlata exchange carrier.

69 Switched Special Services-cont d. cont d. PBX TO CO TRUNKS PBX stands for Private Branch Exchange. A PBX is a switch that is owned by a customer. Some customers have large numbers of phone lines on their premises. A PBX offers the advantage of local switching between the on premises lines. Also, the PBX can concentrate outgoing calls onto a fewer number of trunks connected to the CO. TIE LINES are a dedicated service that connects two PBXs together. The use of tie lines can reduce the cost of long distance calls between two locations.

70 Non-switched Special Services Non-switched special services (private lines) are an exclusive service between two locations of a specific customer. Since the service does not go over the switched network, the customer has access to a private, non-blocking connection at all times. This service almost always requires special equipment and engineering. There are hundreds of different non-switched special services. A few of the most common are listed below. RING DOWN a ring down is a dedicated telephone circuit between two points. The caller picks up the telephone and automatically rings the called party. MEGALINK a Megalink is a dedicated facility (T1 carrier system) between two points. The full bandwidth is available between the locations for sending and receiving information, usually data. There is one circuit, which operates at MB/S. CHANNELIZED MEGALINK similar to Megalink service, except there are 24 individual 64 KB/S communications circuits. DS3 SERVICE the bandwidth for this service is 45 MB/S. Once the customer jumps to DS3 bandwidth, they have a cost effective means to implement a host of new technologies, Including: video conferencing, workstation- based graphics, etc.

71 Summary POTS is the basic telecommunications service. Enhanced POTS, generally, are features added to switch translations and include: caller ID, call return, 3 way calling, etc. Switched special services require special translations and oftentimes special equipment. Included are: WATS lines, FX lines, Tie lines, etc. Non-switched special services are usually located between 2 customer locations. They include: Megalinks, Ring Downs, DS 3 service, etc.