1. IntroductionLeased line is a permanent telephone connection between two points set up by a telecommunications common carrier. Typically, leased lines are used by businesses to connect geographically distant offices. Charges are fixed and are not dependent on usage frequency. This will be very cost effective for constant bulk transmission of data and information. This guideline document will help the NOC engineers to understand to configure and trouble shoot Leased lines. It also gives details on the process to be followed for commissioning and de-commissioning leased lines.

1.1 PURPOSE OF LEASED LINEUnlike dial-up connections, a leased line is always active (i.e., a end to end dedicated connection). As the connection is a dedicated one, it will not carry anybody else's traffic. This will assure security and a given level of quality.

1.2 TYPES OF CIRCUITS 1.2.1 2-WIRE CircuitAs the name implies, this type of Leased lines will be built on a single pair of wire (i.e., 2 wires). It works in half duplex mode (at any point of time the traffic will be flowing in a single direction). It supports up to a maximum of 2Mbps.

1.2.2 4-WIRE CircuitAs the name implies, this type of Leased lines will be built on a two pairs of wires (i.e., 4 wires). It works in full duplex mode in which one pair is used for transmission and remaining pair is used for reception (at any point of time the traffic will be flowing in both directions). It supports up to a maximum of 2Mbps.

1.2.3 MLLN (Managed Leased Line Network)Managed Leased Line Network (MLLN) is the data network concept that provides fully secure data communication services over Managed Leased Lines which will be managed by the Basic Service Operators (BSO). It is a system by which corporate offices and business establishments can establish a   truly private communication network for the purpose of transmitting and receiving information from one location to another. The media available for use by MLLN are fiber-optic lines, microwave transmission systems, satellite transmission

systems, coaxial cable systems and copper lines. The specialty of MLLN is given below.

ü MLLN has greater quality and intensity.ü Need based bandwidth ü Time based bandwidth ü Confidentiality is assured

1.3 COMPONENTS INVOLVED

1.3.1 ModemA hardware device, which allows a computer to transmit and receive information over telephone lines. It converts digital signals into analog signals and vise versa. It is Data Communication Equipment (DCE).

There are three types of DCE’s available in leased linesü V.35 modemü G703 modemü Converter

1.3.1.1 V.35This modem will directly connect to the router’s interface, as it is having V.35 connector in it. It is also back-to-back connected to G703 modem. The pin out / signal description of V.35 is given below.

Name Pin Description Type

FG A Frame/Chassis Ground -

SG B Signal Ground -

SDA P Send Data A Differential

SDB S Send Data B Differential

RDA R Receive Data A Differential

RDB T Receive Data B Differential

RTS C Request To Send Unbalanced

CTS D Clear To Send Unbalanced

DSR E Data Set Ready Unbalanced

DTR H Data Terminal Ready Unbalanced

RLSD F Received Line Signal Detect Unbalanced

TCEA U Transmit Clock Ext A Differential

TCEB W Transmit Clock Ext B Differential

TCA Y Transmit Clock A Differential

TCB AA Transmit Clock B Differential

RCA V Receive Clock A Differential

RCB X Receive Clock B Differential

LL J Local Loop back Unbalanced

RLB BB Remote Loop back Unbalanced

TM K Test Mode Unbalanced

- L Test Pattern Unbalanced

1.3.1.2 G703G.703 is a CCITT standard for transmitting voice over digital carriers such as T1 and E1. G.703 provides the specifications for pulse code modulation (PCM) at data rates from 64 Kbps to 2.048 Mbps. G.703 service is typically used for interconnecting data communications equipment such as bridges, routers, and multiplexers. G.703 is transported over balanced (120 ohm twisted pair) or unbalanced (dual 75 ohm coax) cable. Whether the G.703 is balanced or unbalanced depends on your geographic location and the carrier that supplies the service.

At data rates of 64 kbps over balanced wire, there are three ways of transmission: co-directionalcentral directionalcontra-directional

Co-directional uses four twisted wires, two to transmit and two to receive. The data and timing are sent in the same direction over the same wires. Central directional is rarely used. Here the clock signals

are supplied on different wires from a centralized clock such as an atomic clock. Central directional can use six or eight wires to send a clock signal in both directions or in separate directions. The six-wire version uses two for the clock signals and four for the data signals, and the eight-wire version uses four for clock signals and the other four for data. The contra-directional is an eight-wire version that uses two wires each for transmitting and receiving and two pairs for the clock signals. (Clock signals originate at the Data Communications Equipment (DCE) and are sent to the Data Terminal Equipment (DTE).)

In the concept of leased line, there must be always even number of G703 modems. G703 modems not only transmit data to a large distance compared to V.35 but also encrypt data.

1.3.1.3 ConverterIt will do both functions of V.35 and G.703 in the same equipment. This will be used in places where the local loop is less than 50 meters.

1.4 RouterIn packet-switched networks such as the Internet, a router is a device or, in some cases, software in a computer, that determines the next network point to which a packet should be forwarded toward its destination. The router is connected to at least two networks and decides which way to send each information packet based on its current understanding of the state of the networks it is connected to. A router is located at any gateway (where one network meets another), including each point-of-presence on the Internet.

A router may create or maintain a table of the available routes and their conditions and use this information along with distance and cost algorithms to determine the best route for a given packet. Typically, a packet may travel through a number of network points with routers before arriving at its destination. Routing is a function associated with the Network layer (layer 3) in the standard model of network programming, the Open Systems Interconnection (OSI) model.

A router connects networks together, controlling the routing of packets from source to destination and providing alternate paths when necessary.

The symbol of router as used in network diagrams is given below.

1.5 MUXIn communication transmission systems, mux (pronounce muks, sometimes spelled "MUX") is an abbreviation for multiplexing, a device that sends multiple signals on a carrier channel at the same time in the form of a single, complex signal to another device that recovers the separate signals at the receiving end. The receiver is sometimes called a demux (or "DEMUX").

1.6 Basic Service Operators (BSO)At Sify, there are four Basic Service Operators who provide connectivity services. They are,

ü Bharat Sanchar Nigam Limited (BSNL)ü Bharti Telecom Nigam Limited (BTNL)ü Bharti Telesonic Limited (BTSOL)ü Tata

1.7 Sample Connectivity of Leased Line

1.8 FEATURES OF LEASED LINESü Works in Synchronous modeü Available in multiples of 64 Kbps. ü Uses Time Division Multiplexing (TDM)

2. MODEM CONFIGURATIONTo configure a modem, there are five major components to be configured in it. They are,

ü Framingü Clock Sourceü Mapping of timeslotü Speed

ü Timeslot start position

2.1 FramingThere are 2 modes of framing available to configure the modem. They are,

ü Framedü Unframed

2.1.1 Framed

When the modem is set to FRAMED, the user can select the required time slots and the speed. You can get a maximum of 1984 Kbps because; only 30x64 Kbps channels will be used for data transfer. Rest of 128 kbps will be used for signaling.

2.1.2 Unframed

When the modem is set to UNFRAMED, all the 32x64 Kbps channels are used for transmitting and receiving data. Hence you will get the entire 2.048 Mbps

2.2 Clock SourceClock is a reference signal to synchronize both ends of a leased line. There are three ways to provide clock to a leased circuit. They are,

ü Generating the clock internal to the DCE and sending it to DTE. ü Getting clock from the service provider equipment.ü Both ends of the Leased line will synchronies with an external

clock such as atomic clock.

Getting clock from the service provider equipment is the most preferred way of getting clock source. Based on the above, there are three types of settings. They are,

ü Internalü Lineü External

If a modem is set to Internal clock, it will produce the clock signals and will send it to the DTE.

If it is set to Line, the DCE equipment will receive the clock from the service provider end.

If it is set to External, the DCE equipment will receive the clock either from Service provider equipment or from an external clock source like atomic clocks.

2.3 Mapping of timeslotWhen a modem is set to frame, it is required that the time slots need to be configured as per the required bandwidth. Bandwidth of each time slot is 64 Kbps. So if 256 Kbps of bandwidth is required, then you need to select 4 time slots.

Important thing to remember here is, if your selecting certain time slots, the same time slot has to be set in all the modems in the circuit.

For example, you are selecting TS1 to TS3 and TS5 for a 256 Kbps circuit, and then the same time slots have to be selected in all the modems in that circuit.

For selecting the time slot, there are two methods available. They are,

ü USERü SEQ

If User option is selected, the engineer who configures the modem has to select the time slots manually.

If SEQ option (short for sequential) is selected, then the sequence of time slots will be assigned starting from the time slot selected by the user.

2.4 SpeedSpeed is the bandwidth requirement of the user. Starting from 64 Kbps to 2048 Kbps can be configured.

If a modem is set to unframe, then all the time slots of the circuit will be used. So, you will get a bandwidth of 2048 Kbps.

If it is set to Framed, the maximum bandwidth it can scale is based on the number of time slots selected. It can be a maximum of 1984 Kbps.

2.5 Timeslot start positionTime slot start position is a configuration, which tell the user from which time slot, the modem has to take for bandwidth calculations. This option is available only for frame option.

3. TROUBLESHOOTING

3.1 LOOP TESTLoop test is performed to check the physical connectivity of the link.Two types of loop test are available they are

ü Hard loopü Soft loop

3.1.1 Hard LoopThis loop is given using physical media. This can be classified into two typesü 2 Wire Loop

ü 4 Wire Loop

3.1.1.1 2 WIRE LOOP This loop is used to test a single pair of wire.

This test can be performed to check both towards router end as well as near end exchange. When given towards router end the interface will in looped status.

Figure 1

4 WIRE LOOP This loop is used to test a four-wire circuit.

Wire 1

Wire 2

Wire 1 – TX +

Wire 1 – TX -

Wire 1 – RX +

Wire 1 – RX -

This test can be performed to check both towards router end as well as far end exchange till customer end router. When given towards router end the interface will in looped status, Refer Figure 1.

SOFT LOOPThis loop is given using active equipments namely V.35 modem and G703 modems.

V.35 MODEM LOOP There are three types

ü Local ü Remoteü Digital

LOCAL LOOPUsed to check the connectivity between local DTE interface and local V.35 Modem

REMOTE LOOPUsed to check the circuitry till near end G703

DIGITAL LOOPUsed to check end-to-end connectivity till far end router interface.

LOCAL V.35 LOCAL G.703

REMOTE G.703

REMOTE V.35

TX

RX

TX TX

RX RX

TX TX TX

RX RX RX

LOCAL DTE REMOTE DTE

LOCAL V.35 LOCAL G.703

REMOTE G.703

REMOTE V.35

TX

RX

TX TX

RX RX

TX TX TX

RX RX RX

LOCAL DTE REMOTE DTE

G703 MODEM LOOPThere are two loops

ü Localü Remote

LOCAL LOOPTo check the connectivity towards far end router interface.

LOCAL V.35 LOCAL G.703

REMOTE G.703

REMOTE V.35

TX

RX

TX TX

RX RX

TX TX TX

RX RX RXLOCAL DTE REMOTE

DTE

LOCAL V.35 LOCAL G.703

REMOTE G.703

REMOTE V.35

TX

RX

TX TX

RX RX

TX TX TX

RX RX RX

LOCAL DTE REMOTE DTE

REMOTE LOOPTo check the connectivity between local G703 and near end router interface

TROUBLESHOOTINGThere are many parameters that need to be considered when troubleshooting a leased line. They are

ü CRC Error Checkü Reliabilityü Collisionü Interface Resetü Other parameters

CRC ERROR CHECKThe CRC(Cyclic Redundancy Check) is a very powerful but easily implemented technique to obtain data reliability. The CRC technique is used to protect blocks of data called Frames. Using this technique, the transmitter appends an extra n- bit sequence to every frame called Frame Check Sequence (FCS). At the receiving end, validity of data is checked using the same n- bit sequence CRC combines three advantages:

· Extreme error detection capabilities. · Little overhead.

LOCAL V.35

LOCAL G.703

REMOTE G.703

REMOTE V.35

TX

RX

TX TX

RX RX

TX TX TX

RX RX RX

LOCAL DTE

· Ease of implementation.

In a router interface, CRC errors denote the number of invalid bits being transferred. CRC errors occur when CRC calculation does not pass-indicating the data has been corrupted due to one of these following reasons:

Ø Noisy serial line.Ø Serial cable is too long.Ø CSU line clock is incorrectly configured.

The solution to CRC error problem can be any one of the following:Ø Ensure that the line is clean enough for transmission

requirements. Shield the cable if necessary.Ø Make sure that the cable is within the recommended

length.Ø Ensure that all the devices are properly configured for a

common line clockCRC will be 0 in a link that is up and working well. If there is a problem in a link, CRC will have a positive value.

RELIABILITYReliability is the probability that a link will perform its prescribed duty without failure for a given time when operated correctly in a specified environment. Reliability means

Ø Maximum availability - works first time, every time

Ø Predictable performance any time of the day

In a router interface, it is calculated as an exponential average over 5 minutes

COLLISIONCollision is the result of two network nodes transmitting on the same channel at the same time. The transmitted data is not usable. Collision value will be 0 when the line is up and working fine. When collision has a positive value, it implies there is some problem in the link.

INTERFACE RESETIt is the number of times an interface has been completely reset. This can happen if packets queued for transmission were not sent within several seconds time. On a serial line, this can be caused by a malfunctioning modem that is not supplying a transmit clock signal, or by a cable problem. If the system notices that the carrier detect line of a serial interface is up but the line protocol is down, it periodically resets the interface in an effort to restart it. Interface resets can also occur when an interface is looped back or shut down. This parameter is used to indicate the fluctuations in the leased line link. It has a positive value set in it in case of any fluctuations, else has a value zero when the link is working fine.

OTHER PARAMETERSThe other parameters that can be looked for when solving a problem in a leased line are:

v RTS and CTSv DCDv DSRv DTR

RTS AND CTS When two stations need to communicate with each other, the sender of the message sends a Request To Send (RTS) message. Only when the receiver sends a Clear To Send (CTS) message back to the sender, actual communication starts.For eg: If a station, say A wants to send a data frame to station B, station A first sends a request-to-send (RTS) frame. If station B receives the RTS frame, then B issues a clear-to-send (CTS) frame. All stations within range of B receive the CTS frame and are aware that A has been given permission to send, so they remain quiet while station A proceeds with its data frame transmission. If the data frame arrives without error, station B responds with an ACK. This is illustrated in the following diagram

Station ARTS

A requests to send

a)

CTS

B announces A OK to send

b)

Data

A sends

c)

Station ARTS

a)

CTS

FrameC remains quiet

Station A

Station A

Station B Station C

Station B Station C

Station B Station C

DATA CARRIER DETECT (DCD)Data Carrier detect (DCD) refers to the number of times the carrier detect signal of a serial interface has changed state. For example, if data carrier detect (DCD) goes down and comes up, the carrier transition counter will increment two times. It also indicates modem or line problems if the carrier detect line is changing state often.

Process for COMMISSIONING AND DECOMMISSIONING OF LINKS

Objective

To understand the process for commissioning of backbone, Gateway and customer leased Circuits.

Inputs

Advice Note from BSOEquipments (Modem, Power card, box)Equipment installation form (only applicable for customer equipment)Outputs

Commissioned links

Procedure

Backbone/Gateway Links:

ü The SIFYIIC will get the advice note for commissioning of new link from BSO and intimate SIFYNOE. SIFYNOE will start commissioning of the link as mentioned below.

ü SIFYNOE will get the modem from SIFY stores as per link requirement.

ü SIFYNOE will configure the modem as per Link requirement.ü SIFYNOE will get the serial interface IP address from SIFYCPE by

raising an IP request in SIFY info desk.ü SIFYNOE will raise Change management request with the

following detailsü Originating and terminating location, IP address details, Router

Port ü Details.ü Once the CM is approved, the Configured modem will be placed

in the Communication Room Rack.ü SIFYNOE will do the Local wiring and test the same.ü Once the link is wired and tested, it will be terminated to the

corresponding Router interface.ü SIFYNOE will raise an info desk TAC to SIFYNETOPS for

configuring the router interface.ü SIFYNOE will coordinate with BSO for Channel allocation and

wiring (All BSO contact details are available in dcinfo.sify.net)ü SIFYNOE will test the end-to-end link using loop test.ü Once loop test is completed, the remote router serial interface ip

reach ability will be tested using ping.ü SIFYNOE will check the reliability, CRC, collision of the link.ü SIFYNOE will label the modem.ü SIFYNOE will send a mail to configure the link in NMS.

Customer Links:

ü The SIFYPMG will get the advice note for commissioning of new

link from BSO and intimate SIFYNOE. SIFYNOE will start commissioning of the link as mentioned below.

ü SIFYNOE will get the modem from SIFY stores as per link requirement.

ü SIFYNOE will configure the modem as per Link requirement.ü SIFYNOE will get the serial interface IP address from SIFYCPE by

raising an IP request in SIFY info desk.ü SIFYNOE will raise Change management request with the

following details§ Originating and terminating location.§ IP address details.§ Router Port Details.

ü Once the CM is approved, the Configured modem will be placed in the Communication Room Rack.

ü SIFYNOE will do the Local wiring and test the same.ü Once the link is wired and tested, it will be terminated to the

corresponding Router interface.ü SIFYNOE will raise an info desk TAC to SIFYNETOPS for

configuring the router interface.ü SIFYNOE will coordinate with BSO for Channel allocation and

wiring (All BSO contact details are available in dcinfo.sify.net)ü SIFYNOE will test the end-to-end link using loop test.ü Once loop test is completed, the remote router serial interface ip

reach ability will be tested using ping.ü SIFYNOE will check the reliability, CRC, collision of the link.ü SIFYNOE will label the modem.ü SIFYNOE will send a mail to configure the link in NMS.