FAST TUTORIALS FOR TIME-CHALLENGED TECHNICIANS

Transcription

1 Tech Notes from a Telephone Engineer FAST TUTORIALS FOR TIME-CHALLENGED TECHNICIANS POTS Dialtone Description: POTS Plain Old Telephone Service Long held industry acronym for standard dialtone service to a business or residence. Service initially intended for voice conversation only, later used to connect dial up modems for internet and remote system access, and FAX machines for document transfers. POTS is a 2 Wire service, requiring just a single cable pair to the customer site. The conductors are identified as Tip (return side of line often with ground potential) and Ring ( 50V DC from the serving equipment). Initially delivered from a Central Office based switch to customers over long runs of telephone cables, now also delivered by remote switching equipment cabinets located in the customer neighborhood over shorter copper or fiber optic cables. The remote switching equipment ties back to the Central Office switch over fiber optic or high speed copper digital links. When fiber optic cable extends all the way to home or business, electronic service delivery equipment (dialtone producing equipment) is located at customer premise. Power for the circuit is provided by the carrier s switching equipment and typically 48V DC to 52V DC. Voice signals and incoming call ringing signals are superimposed over the DC operating current. Call signaling has two parts: Call Control, which consists of DC open circuit termination (On Hook), and a DC resistance of 430 Ohms when Off Hook initiating or answering a call. This On Hook/Off Hook is also used for sending a Flash during a call to answer a second incoming call (Call Waiting), transfer call to another party, or conference a third party. Dialing dialing accomplished in Off Hook mode by a series of pulses (Rotary Dial), or tones (DTMF Dialing). DTMF= Dual Tone Multiple Frequency. Caller ID a third type of signaling occurs either On Hook (Caller ID) or Off Hook (Caller ID on Call Waiting). This is accomplished with a 1200 baud analog data transmission for the carrier serving equipment. Page 1 of 12

2 Associated Equipment: Central Office Switch the voice switch in the Central Office that provides dialtone and connects the customer with the rest of the telephone network worldwide on a dialed, switched connection basis. Line Card the circuit card in the switching equipment that terminates the customer loop. Some modern switches utilize a fabric matrix rather than a dedicated line card, meaning the customer loop can be connected to a variety of internal switching channels on demand. MDF Main Distribution Frame (in some cases, now called PDF Protected Distribution Frame) this is the mechanical connecting point in the Central Office or roadside cabinet where cable pairs from the customer are cross connected with jumper wiring to the Line Card or Line Location pins from the switch. In a typical office, hundreds of thousands of cable pairs are terminated into blocks which contain removable (pluggable) lightning protection modules. The MDF also provides a quick, convenient method for disconnecting the switch from the customer loop for troubleshooting processes. NID Network Interface Device: The regulated demarcation owned by the carrier and located at the customer site which provides lightning protection and connection points for customer wiring. Two versions are still in use: older versions offer screws and nuts only; newer versions (since 1985) offer an RJ11 jack/cable which may be unplugged easily for testing customer wiring. Station Wiring This is the customer owned wiring between the NID and the connected telephone instruments. EML Expected Measured Loss: This is the calculated loss of the circuit computed by cable records. The EML will be compared with the AML Actual Measured Loss. EML is expressed in db. AML Actual Measured Loss: This is the signal power measurement at the subscriber NID. If the AML varies by +/ 1dB of the EML, circuit should be investigated. If for example the EML was 3.5dB but the AML was 7.4dB, this would indicate potential significant bridgetap on the cable pair, the customer was significantly farther from the CO than shown in records, or the cable gauge is much finer than indicate in the records (26 gauge vs 22 gauge, for example). This is what is tricky EML and AML are expressed in db, but the actual measurement at the NID will be in dbm, indicating it is measured against a 0.0 dbm reference signal being transmitted from the CO. For example, a measured loss of 4.2 dbm would be expressed as an AML of 4.2 db, meaning the circuit lost 4.2 db of signal getting from the CO to the subscriber NID. Since the Page 2 of 12

3 reference signal was at 0.0 dbm, any loss measured loss would be less than zero (0.0), so would be a negative number. Full instructions for calculating Loop Loss (AML) may be found in the Loop Design Tutorial on the Home Page of this CD. Local Loop the Local Loop is the facility between the last serving network element (line Card, etc.) and the Customer premise. This is facility may be copper, fiber optic or a mix of both. However, at some point the POTS is converted back to electrical signals which traverse copper wiring at the Customer Premise. CONTINUE TO NEXT PAGE Page 3 of 12

4 Service Delivery Configurations: Page 4 of 12

5 Service Demarcation at Customer Location (Demarc): The regulated carrier provides electrical/lightning protection at the customer s building edge, or in some cases, into the customer s controlled Space in a multitenant building. The regulated Demarc is one of several types depending on number of lines delivered as follows: SINGLE LINE CONTINUE TO NEXT PAGE Page 5 of 12

6 SINGLE & MULTI LINE Page 6 of 12

7 SINGLE LINE for APARTMENTS Page 7 of 12

8 MULTI LINE BUSINESS NOTE For complete list of Regulated Jacks and pin outs, see sidebar on Home Page of this CD. CONTINUE TO NEXT PAGE Page 8 of 12

9 Service Delivery Parameters: POTS service delivery limits are as follows: Loop Voltage: There is no requirement for a specific voltage on a POTS line. For over 100 years, the Central Office provided 48V DC to 52V DC on most POTS lines, but as loops shortened and equipment was moved closer to subscribers (roadside cabinets, etc.), line voltage varied from 26V DC to 52V DC. In fact, many PBX utilize less than 20V DC. The important parameter is Loop Current, regardless of line voltage. Loop Current: =/>20mA 430 Ohms (equal to 20mA DC or greater). Corrective Actions Loop 1,004 Hz: =/< 8.5dBm (equal to 8.5dBm or less [from 0.0dBm to 8.5dBm]; readings over 8.5dBm require analysis [ 8.6dBm to 12.0dBm for example]). Corrective Actions Circuit 600 Ohms Terminated: 0.0dBrnC to 20dBrnC acceptable; 21.0 dbrnc to 30dBrnC marginal; Readings over 30dBrnC require analysis. Corrective Actions Power Influence (Noise to Ground): 65dBrnC to 80dBrnC acceptable; 81dBrnC to 90dBrnC marginal; Readings over 90dBrnC require analysis. Corrective Actions Circuit Balance (Power Influence minus Circuit Noise): Target is 60dB or higher. Balance below 60dB requires analysis. Corrective Actions CONTINUE TO NEXT PAGE Page 9 of 12

10 Commissioning & Troubleshooting Processes: NOTE The following processes detailed in step x step instructions for specific test HOME Page of this CD 1. Initial turn up testing or after clearing a trouble from the line: a. Loop Current test at NID w/no CPE connected This is a simple measure of current through a resistance of 430 Ohms which simulates the telephone instrument resistance. Reading should comply as described in the Service Delivery Parameters section above. Here is a drawing of how the measurement is achieved: b. Loop 1,004 Hz at NID w/no CPE connected This is a dialed test to the local Central Office based responder capable of transmitting a 1,004 Hz 0.0 dbm from the CO to the customer site, and conform as described in the Service Delivery Parameters section above. The 1,004 Hz power level is measured at the NID with a special test set configured as shown below: NOTE Central Office Switch Resistance omitted for clarity. Page 10 of 12

11 c. Circuit Noise at NID w/no CPE connected This is a dialed test to the local Central Office based responder capable of providing a balanced, 600 Ohm Quiet termination to the line. The measured readings should conform as described in the Service Delivery Parameters section above. Circuit Noise is measured at the NID with a special test set configured as shown below: d. Power Influence (Noise to Ground) at NID w/no CPE connected This is a dialed test to the local Central Office based responder capable of providing a balanced, 600 Ohm Quiet termination to the line. The measured readings should conform as described in the Service Delivery Parameters section above. Power Influence (Noise to Ground) is measured at the NID with a special test set configured as shown below: Page 11 of 12

12 e. Calculate Loop Balance at NID w/no CPE connected This is done by subtracting Circuit Noise Reading from Power Influence reading. For example, PI of 80dBrnC minus CN of 20dBrnC = 60dB. Target for POTS is balance of 60dB or greater. f. Complete in/out calls w/no CPE connected be sure to make at least one outbound and one in bound call g. Connect CPE and place test call into subscriber (if authorized). Test Equipment Required: Butt Set (to connect to line and monitor or place test calls) Subscriber TIMS (Transmission Impairment Measuring Set) or equiv. to measure: o Loop Current o Insertion Loss at specific frequencies (1,004 Hz initially) o Circuit Noise o Power Influence Optional Test Equipment for Analyzing Copper Pairs: Loop Test Set o VOM function to analyze resistive faults and loop resistance o Open meter function to analyze cable length o TDR (Time Domain Reflectometer) to determine load coils and/or faults This completes the POTS Dialtone Tutorial For Step x Step instructions for testing POTS Dialtone circuits, goto Home page of this CD and select the desired test location Page 12 of 12