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Managed Leased Line Network

Dr Rajiv Kumar Singh. He is working with Bharat Sanchar Nigam Limited & holds a Ph.D. degree in electronics engineering from Indian Institute of Technology (BHU), Varanasi. His current research interests include wired and wireless technologies for high-speed Internet access


Salient features of MLLN

Managed leased line network or MLLN service is specially designed for having effective control and monitoring on the leased line so that the downtime is minimised and the circuit efficiency is increased.

MLLN mainly deals with data circuits ranging from 64kbps to 2048kbps (n×64kbps). One of the major attractions of MLLN is its ability to provide differential time-dependent bandwidth on demand basis to the customers. For example, it is possible to provision 1024kbps MLLN circuit for 16 hours a day and 512kbps for remaining 8 hours of the day, as per the requirement of the customer.

Another great thing about MLLN is its very efficient NMS that can proactively maintain the circuit without waiting for customers to book a complaint. NMS provides features such as bandwidth management, alternate or back-up transmission routing, powerful diagnostics and maintenance tools and self-repair tools.

Fig. 2: Managed leased line network architecture
Fig. 2: Managed leased line network architecture

MLLN-NMS also provides periodic performance report which is useful in providing high-speed leased lines with improved QoS, high availability and reliability to the business and good service to existing customers.

The network management system also supports service provisioning, network optimisation and planning and service monitoring. The system offers features such as end-to-end circuit creation and monitoring, software loop test to check connectivity of various network elements and fault isolation and software programmability of customer end equipment.

MLLNs offer great amount of security because media is not shared and is exclusively dedicated for a particular subscriber. With MLLN, lead time is very low for provisioning of a new leased line. Its modular system and new application can be implemented very fast by simply adding or plugging the units.

Managed leased line network architecture

Managed leased line network (MLLN) is a three-tier structure and comprises network elements such as digital cross connects (DXC), versatile multiplexer (VMUX), network termination units (NTU) and NMS (Fig. 2).

Stage 1 comprises NMS, regional DXC (RDXC) and/or sub-regional DXC (SRDXC), billing servers, database servers, etc. At this stage, all the network management functions are done from the central location. This stage provides connectivity to second-stage nodes and provides traffic aggregation.

Second stage comprises SRDXC, SSDXC (secondary switching DXC) and VMUX, and is located in major cities where demand for leased line is high. This stage provides connectivity to third stage and performs leased line traffic aggregation.

Third stage comprises VMUX and NTU and is located in small cities/towns where leased line demand is lower (approximately 20). This stage provides leased line traffic aggregation.

The media for interconnecting various network elements and extending line up to customer end may be optical fibre, copper wire, radio, microwave transmission or a combination of these.

MLLN has to provide high reliability service and is supposed to obtain efficiency greater that 99.5 per cent. Therefore all the interconnections of different network elements are provided as rings, wherever available, so that an alternate circuit path can be automatically used for routing the traffic in case of main route failure. In long-distance network, links between the same stations can be split into alternate physical path of rings to the extent feasible. This also saves on-port capacity required for providing alternate path within MLLN.

With these network elements, the MLLN is able to provide functions such as fault, configuration, accounting, performance and security management.

Network management system (NMS)

It is built on open architecture and relational database system and manages all the network elements and their functions centrally. It is able to configure, provision, manage and monitor all aspects and parameters of the remote elements of the MLLN network centrally without the need of any local intervention.

On-demand bandwidth configuration is performed at NMS and priority can also be set for a particular leased line. This enables the high-priority customer lines to be routed first to the standby route in case of failure of the main route.

Any change of configuration of any network element is auto-recognised by the NMS. NMS is also capable of re-initialisation of the network element in the event of software/hardware failure. A predefined routing schedule is used by NMS to perform fast re-establishment of circuits within the network across alternative paths totally automatically, in the event of failure.

A detailed fault report is generated in order to identify the exact problem so that immediate corrective measures can be taken in order to restore the services.

The fault information provided by NMS contains type of faulty network element, the time at which fault occurred, time when it corrected, etc. The NMS keeps polling all the network elements after some predetermined interval and generates alarms so that proactive measures can be taken.

Digital cross connect (DXC)

It is a large-capacity cross-connect device that separates channels coming from other devices and rearranges them into new channels for output. A digital cross-connection allows lower-level time division multiplexing (TDM) bit streams to be rearranged and interconnected amongst higher-level TDM signals. The signal is first de-multiplexed into a lower level after which it is cross-connected and then multiplexed again.

Versatile multiplexer (VMUX)

It is a small capacity cross connect device with several channel interfaces which is installed at different sites for providing user connectivity. VMUX multiplexes all tributaries coming from other devices and forms a higher hierarchy level output at the specified port. The VMUX is provided with different types of interfaces to connect SDSL and HDSL product family modems. The number of interfaces (such as 64/128kbps, n x 64kbps, E1 or hotline) depends on the type of VMUX configuration (VMUX type I, VMUX type II, VMUX type III/DC operation and VMUX type III AC operation).

Network terminating unit (NTU)

It is simply a base band modem and which is located at the customer’s premises. The NTU works on normal AC supply. NTUs of various capacities (64/128kbps and n×64kbps) are available with several interfaces (V.35, G.703, Ethernet). NTU also allows for the use of existing telecom copper cables (twisted pair) for digital traffic with medium distances (~5 km) and high speeds.


In the changing economic environment, dependence of organisational and industrial activities on leased circuits is increasing. In such a scenario, high QoS, high efficiency, highly secured network, customer-oriented tariffs along with desired bandwidth, time-dependent bandwidth provisioning, no congestion, centralised control and monitoring, lower lead time for new installations and proactive fault maintenance prove MLLN to be a commercial boon for corporate and individual customers.

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