FAST TUTORIALS FOR TIME-CHALLENGED TECHNICIANS

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Tech Notes from a Telephone Engineer®

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Description:

DDS‐ Digital Data Service Long‐held industry acronym for transporting digital data at rates of: 2.4kbps, 4.8kbps, 9.6kbps, 19.2kbps, 38.4kbps, 56kbps, 64kbps, N x 56kbps or N x 64kbps. The last two “N x” mean the circuit can be multiples of 56kbps or 64kbps such as 256kbps (4 x 64kbps). The DDS subscriber hand‐off is a 4‐Wire interface with Transmit & Receive leads.

For rates below N x 56/64, circuit usually provisioned on local copper pair or special channel card if subscriber served by a channel bank or fiber optic system. Rate multipliers above 56/64 require a T1 circuit to the subscriber which may limit access only to the channels required to achieve the subscribed rate. In the case of 256kbps, the T1 would provide bandwidth of only 4 DS0. This arrangement is call a “Fractional T1” or “FT1” for short. The remainder of the T1 bandwidth (20 channels) would be unused if not subscribed to other services.

In telecommunications transport hierarchy, the lowest channel level is the DS0 (dee‐ess zero, also pronounced dee‐ess oh). A DS1 (T1) is made up of 24 DS0 channels plus framing bits totaling 1.544mbps (mega‐bit per second). When the T1 framing is set for Super Frame (SF, also called D4), DS0 channels are 56kbps. When the T1 framing is set for Extended Super Frame (ESF‐ the most modern configuration), DS0 channels are 64kbps.

DDS

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When provisioned direct to copper cable, pairs must be:  Free of all Resistive and Capacitive faults (see section Identifying Basic Faults on Cable Pairs & Station Wiring on Home Page of this CD)  No Load Coils  No Bridgetap  Cable Pair Loss must be within Loss Budget for subscribed data rate (see Qualifying Cable Pairs for Services on Home Page of this CD)

When provisioned over DSL (Adtran Total Reach®, etc.), pair must be:  Free of all Resistive and Capacitive faults (see section Identifying Basic Faults on Cable Pairs & Station Wiring on Home Page of this CD)  No Load Coils  Some Bridgetap allowed  Cable Pair Loss must be within Loss Budget for subscribed data rate (see Qualifying Cable Pairs for Services on Home Page of this CD) Associated Equipment:  4‐Wire Interface‐ This is the transmit and receive interface to a copper loop. The 4‐Wire interface features bi‐polar signaling scheme for transmission on a cable pair.

 OCU‐DP‐ Office Channel Unit‐Datapath‐ this is the channel unit residing in the transmission equipment that converts the 4‐Wire copper loop interface (bi‐ polar) to T1/DS0 interface required for multiplexing up to higher‐level T1 for transmission to distant locations. When the multiplexer handles 24 DS0’s, it is called a “Channel Bank”. The OCU‐DP contains a “sealing current generator” and usually located in a Central Office or roadside cabinet near the subscriber. Sealing current is a simplex powering scheme to power the service providers Smartjack demarcation unit located at the subscriber location. The smartjack is also called a Network Interface Device (NID).

 DS0‐DP‐ This is the channel unit that typically is cross‐connected to another similar unit in Central Office to extend the DDS circuit from one area to another. The use of Digital Cross‐Connect Systems (DACS) has greatly reduced the use of DS0‐DP’s, as DS0 channels are mapped between T1’s electronically without the need for a Channel Bank.

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 Composite Clock (CC)‐ DDS must be synchronized by connection to a BITS clocking system (Building Integrated Timing Supply‐ a standard synchronization system in carrier offices). In most cases, the channel bank in the local Central Office contains an Office Interface Unit (OIU) or similar card that accepts a Composite Clock (CC) signal which is made up of a 64kb signal with a purposeful 8kb error rate. The channel bank converts this to a 64kHz [bit] and 8kHz [byte] clock used by the channel bank and DDS card for framing. If the DDS serving equipment is not provisioned with synchronization signals, the DDS will be have continual errors due to frame slips.

 DS0 DSX‐ The DS0 DSX is a panel of jacks which terminate the DDS circuit, providing access for testing and monitoring. There are typically two jack positions for each DDS circuit; one facing the OCU‐DP, and one facing the subscriber loop. The DS0 DSX has three jacks per position: Monitor (non‐ intrusive when used), Transmit (intrusive when used) & Receive (intrusive when used).

 DSL equipment‐ (Adtran Total Reach®, etc.) DDS is a distance‐limited service with a line code that interferes with DSL transmissions. Therefore, many DDS circuits are now converted to a DSL‐type line code between the OCU‐DP and NID/Smartjack. In fact, the far end DSL capability is now integrated into the NID/Smartjack for many installations, reducing the number of network elements at the subscriber location. Adtran Total Reach® requires only a single cable pair to transport DDS to the subscriber location. While the DDS is DSL on the copper cable pair, it is converted back to standard 4‐Wire interface at the NID/Smartjack for handoff to the subscriber.

 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 subscriber are cross‐connected with jumper wiring to the OCU‐DP or DSL‐based transmission equipment. In a typical office, hundreds of thousands of cable pairs are terminated into blocks which contain removable (pluggable) lightning‐protection modules.

 Local Loop‐ the Local Loop is the facility between the last serving network element (OCU‐DP) and the subscriber location. This is facility may be copper, fiber optic or a mix of both. However, at some point the DDS circuit is converted back to electrical signals which traverse copper wiring at the customer demarc.  NID/Smartjack‐ 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. The NID/Smartjack contains a loopback circuit which may be remotely activated for circuit testing or

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sectionalization of troubles. The NID/Smartjack also contains history of circuit performance such as errors and loss of signal.

 Station Wiring‐ This is the customer‐owned wiring between the NID/Smartjack and the CSU/DSU.

 CSU/DSU‐ Channel Service Unit/Data Service Unit‐ Historically a Card/Shelf arrangement, modern CSU/DSU’s are usually a desktop unit resembling an external dialup modem. The CSU/DSU converts the 4‐Wire interface into whatever is required by the subscriber equipment, such as RS232, V.35, etc. The CSU/DSU also responds to remote loopback commands, allowing end‐to‐end testing or sectionalization of the circuit as we will examine below, and maintains history of quality measurements such as data error rates, loss of signal, etc.  DCE/DTE‐ Data Communications Equipment/Data Terminating Equipment‐

Rather than a specific device, DCE/DTE is a connection description and jack/plug pinout for mating communications and data equipment. For example, the customer equipment facing side of a CSU/DSU presents a DCE port (RS232, V.35, RS530, etc.). The subscriber equipment could be a dumb terminal, server, or other digital data communications device, and must be configured as a DTE.

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Service Delivery Configurations:

M D F Central Office NID/ SJ 2 Pair 2 Pairs Copper Cable Remote Channel Bank Fiber Optic Cable

Or T1 Business Business DDS over Copper Business 1 1 1 1

1 = 4-Wire DDS TEST POINT

DDS over DSL CSU/ DSU DTE M D F NID/ SJ 1 Pair Copper Cable 1 1 CSU/ DSU DTE NID/ SJ DDS over Channel Bank 1 1 CSU/ DSU DTE COT 2 Pair 1 Pair 2 Pair D A C S 2 Pair 2 Pair 2 Pair DS0 DSX’s DS0 DSX’s DS0 DSX’s DSL 1 1 1 1 1 1 1

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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 multi‐

tenant building. The regulated Demarc is one of several types depending on

number of lines delivered as follows:

SINGLE DDS

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MULTI‐LINE

NOTE‐ For complete list of Regulated Jacks and pin‐outs, see sidebar on Home

Page of this CD.

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Service Delivery Parameters:

DDS service delivery limits are as follows:

Signal Power Level:

Generally delivered at demarc at 0dB level but can also be set at ‐10dB

Signal Error Rate:

Circuit should perform at 99.999% error‐free on a yearly basis. However, most

turn‐up and repair tests should achieve 0 (zero) errors in 15 minutes.

Commissioning & Troubleshooting Processes:

NOTE‐ The following processes detailed in step x step instructions for specific test sets @ HOME Page of this CD

Initial installations require impairment testing if served by copper cables. These

tests will assure the cable pairs will support DDS or the DSL (Adtran Total Reach®,

etc.). The required tests are fully outlined in the Qualifying Cable Pairs for

Services section of this CD.

Installation (commissioning) tests include connection of DDS test set to customer

side of demarc (NID/Smartjack) to perform Bit Error Rate Test (BERT) to a distant

loopback, automated remote test system (CART, etc.) or with another technician.

Tests should achieve:

 Circuit sync at subscribed rate (19.2kbps, 64kbps, etc)

 15 minute error‐free BERT

 Test set should respond with CSU loopback when commanded from

far end

Maintenance testing may include the above when technician dispatched to field,

but carrier can sectionalize circuit and perform loopbacks and BERT from two

locations:

 Central Office‐based test system (Spirent/Hekimian, etc.) if equipped

 DS0 DSX panels

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When testing from the carrier Central Office at the DS0 DSX, the CSU may be

looped toward the carrier to perform BERT. If this test passes, all network

elements and cabling between the DS0 DSX and CSU are operating properly. If

this BERT fails, the CSU is looped down and the NID/Smartjack is looped toward

the carrier for BERT testing. If this BERT passes, fault is between the

NID/Smartjack and CSU.

CSU/DSU (DCE) DTE NID/ Smartjack Channel Bank _DSXDS0 DS0_DSX

T1 4-Wire Local Loop

M D F T T T T _T T = Test Points T

Carrier can loopback red and blue Customer can loopback red and green

Note the subscriber technician does not have access to the NID/Smartjack loopback, but can loop the distant CSU to perform BERT end‐to‐end.

The example above illustrates a complete, end‐to‐end DDS circuit. Both locations may be in the same town or could be across the country. For testing purposes, it makes no difference. A customer technician at the location with data terminal could use a DDS test set to loop the CSU located at the CSU

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located at the far end of the circuit, performing BERT across the entire circuit if desired. Carriers, on the other hand have a multitude of network elements that can be accessed for loopback testing, and also have the option of looping the customer CSU. These methods examined in details on Home Page location testing on this CD.

Special Note on BERT testing methods

While the primary method for locating faults is BERT testing, there are fault types which require more stringent methods for isolating and locating the failing circuit element.

For example, as DDS network elements become faulty due to age and deterioration or damage, one of the first functions to suffer is “clock recovery”. Clock recovery is the ability of the element to lock onto the incoming data stream and quickly synchronize, passing the data bits error‐free. A failing clock recovery circuit requires a longer series of pulses to sync than one that is functioning properly. This condition can be attributed to any network element such as channel cards, channel banks, OIS clocking units and CSU’s.

Normal customer traffic on DDS circuits is bursty and periodic. However, BERT test signals are one long, continuous flood of traffic patterns. Therefore, the test method does not emulate the actual customer traffic. Customer Traffic‐ bursty in nature BERT traffic‐ continuous flood of test pattern

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The problem with the above BERT signal is that the faulty channel card will recover clock after several pulses, then remain in sync for the remainder of the test sequence which may be 15 minutes. Since it is normal to see a few data errors when first establishing the BERT test, technicians reset the error counters on the test set to zero, and the circuit runs flawlessly until the BERT is completed. Satisfied that the circuit is running clean, it is handed back to the end user as a good circuit. However, once the end user connects the bursty data equipment, the circuit immediately fails again, as data bursts are of too short duration for the clock recovery circuit to capture and lock. The carrier technician will again perform BERT testing, which passes again with flying colors. The best method for detecting and locating network elements with failing clock recovery circuits is to use a BERT pattern that is also bursty in nature. That is, a BERT pattern that can be adjusted so that there is a short duration of test pattern (2047, 511, etc.), followed by Carrier Mode Idle (no BERT pattern generated), followed by another burst of BERT pattern as shown below. In other words, turn the selected test pattern on and off so it is now also bursty. BERT pattern in “Burst Mode” This method has been used to solve many DDS chronically‐failing circuits. When selecting a DDS test set, be sure to use one with “Burst‐Mode BERT”. Test Equipment Required:  DDS Test Set w/BERT and built‐in CSU function  VT100 Emulation to review performance info in network elements (NID/CSU) Optional Test Equipment for Analyzing Copper Pairs:  TIMS (Transmission Impairment Measuring Set) or equiv. to measure: o Insertion Loss at specific frequencies o Circuit Noise o Impulse Noise  Loop Test Set o VOM function to analyze resistive faults and loop resistance

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o Open meter function to analyze cable length o TDR (Time Domain Reflectometer) to determine load coils and/or bridgetaps