The issue of call drops has been taking centrestage in all discussions related to the telecom industry of late. While the government is of the view that operators are not putting much effort into finding a solution, operators claim that they are constrained by the shortage of spectrum and towers. The latest development in this regard is the Telecom Regulatory Authority of India’s (TRAI) decision to implement a mechanism that makes it mandatory for operators to compensate consumers for call drops. However, operators say this will be ineffective and only lead to an erosion of revenue. The failure of the industry and the government to come to a mutually acceptable conclusion on the matter has meant that consumers continue to deal with a deteriorated quality of service (QoS).
In an attempt to make the discussion more informed, TRAI has now released a technical paper to analyse the call drop issue at length. The paper, which is based on drive tests conducted in Delhi, Mumbai, Kolkata, Pune, Bhubaneswar and Surat, discusses the possible reasons for the increase in call drops and suggests measures that operators can take to reduce them, and thus improve their QoS.
According to TRAI, the prime reason for the dip in QoS is the immense pressure on existing facilities as a result of the ever-increasing subscriber base without a commensurate increase in mobile telecom infrastructure. The growing demand for cellular connectivity is causing cell sizes in mobile wireless cellular networks to shrink, especially in urban areas. This has led to a greater number of handovers (the transfer of an ongoing call or data session from one channel connected to the core network to another channel) and an increased probability of call drops.
Call drops can also be attributed to spectrum issues. In Delhi, for instance, spectrum availability has been near-constant since 2009 for some operators while having decreased for others who have used a portion of spectrum for deploying new technology rather than expanding coverage, despite the growing number of subscribers. This has led to an increase in
subscriber density per MHz of spectrum. Although service providers have installed additional towers, the growth of base transceiver station (BTS) sites has not kept pace with the subscriber base growth. This spectrum deficiency or non-installation of BTS sites has compromised call quality.
Some operators in Delhi had undertaken major frequency changeovers in the 900 MHz and 1800 MHz bands on live networks after the spectrum auction, which necessitated proper network tuning. The lack of this tuning could have resulted in call drops. For new frequencies near the CDMA band, operators were required to install additional filters, which they may not have had in their networks prior to the auction. The non-installation or delayed installation of these filters could have resulted in calls being dropped due to interference.
Call drops could occur due to technical reasons, like in circumstances where a mobile user enters an area without adequate signal strength or one where the signal is interrupted, interfered with, or jammed. From the network’s perspective, this is similar to a user leaving the coverage area. Calls are also dropped upon handovers between cells, possibly because of a traffic imbalance between two cell sites, when the new site is at capacity and cannot accept additional traffic.
Co-channel and adjacent channel interference can also be responsible for calls being dropped in a wireless network. Neighbouring cells with the same frequencies can interfere with each other, resulting in deteriorated QoS and dropped calls. The addition or modification of network sites by operators on a regular basis requires networks to be constantly updated or reconfigured, and if this is done in an erroneous manner, the result can be dropped calls. They can also be caused by transmission problems due to a faulty transceiver within a BTS or faulty transmission media. Hardware issues like equipment failure and antenna-related problems like improperly connected cells, damaged feeder cables, and unsecured feeders and connectors are other possible reasons. The difference between uplink and downlink signals may vary and be another factor responsible for dropped calls.
Propagation factors on signal behaviour, such as reflections and multipath, diffraction and shadowing, building and vehicle penetration, propagation of signals over water, propagation of signals over vegetation (foliage loss), fading of signals, and interference could also lead to call failures. Irregular user behaviour like mobile equipment failure, phones getting switched off during a call, and the subscriber charging capacity being exceeded during a call are other reasons.
Call drops have also been increasing as a result of residents objecting to the installation of mobile towers due to fears of radiation. Protests in residential areas have resulted in towers being pulled down or the deployment of new towers being stalled, affecting mobile service quality. Every tower that is pulled down exerts additional load on adjacent cells, resulting in poor call quality. In fact, a reduced number of towers in an area actually increases the power levels of handsets as they will have to exert more power to make signals reach the BTS. Weak signal strengths in buildings or premises lead to users installing signal boosters that enhance the complete GSM band instead of a specific service provider’s signal, causing interference. Delhi alone has over 250 identified illegal boosters.
The rate at which calls are dropped is an important QoS parameter for cellular networks, which makes reducing their frequency a prime obligation of the telecom industry. In the technical paper, TRAI suggests the following measures that operators can take to address the issue.
- Dynamic channel allocation: Many service providers have opted for time division multiple access-based dynamic channel allocation in heavy load conditions to reduce call drops. The network’s performance is evaluated on the basis of the probability of call drops due to handovers in busy traffic conditions. A bandwidth window is applied, which changes size according to the network traffic condition. With this solution, higher priority and real-time handover calls (voice and multimedia calls) get the requested bandwidth while lower priority handover calls (data calls) get minimum bandwidth. Thus, the probability of dropping handover calls is reduced to a minimum and a larger number of users can be served with bandwidth usage being maximised.
- Half-rate traffic channel: When traffic volumes are high and capacity cannot be expanded, half-rate traffic channels can be used to serve more mobile switching centres, decreasing the congestion ratio.
- Allocating multiple backhaul for the same call: Service providers can allocate multiple routes for specific flows, thereby bypassing any congestion in a particular route. An example of this is the use of Contention Free Transmission Opportunities, where, if calls directed towards a specific destination are commuted through a gateway that is busy, they can be routed through another gateway to avoid disconnection.
- Hybrid channel allocation: This involves treating new calls as per the fixed channel allocation method and handover calls as per the dynamic channel allocation method to reduce call blocking and call drops. The application of this queuing technique increases the efficiency of cellular performance, especially in micro and pico cellular environments, effectively utilising the available allocated radio spectrum. This leads to a decrease in call blocking and call drops and an increased capacity for users in the available channels.
- Infrastructure improvements: Enhancements in towers and related infrastructure are elementary solutions that need to be in place for ensuring reliable telecom services. New technologies like in-building solutions, smart antennas and distributed antennas can also be considered for supplementing infrastructural developments. In-building solutions provide mobile coverage inside buildings where coverage, capacity or quality is not satisfactory because traditional wireless macro networks do not provide seamless and uniformly good quality service in an entire coverage area. Distributed antenna systems (DAS) are a network of spatially separated antenna nodes connected to a common source that provides wireless service within a geographic area or structure. DAS can serve many telecom service providers simultaneously as service providers can use such systems on a shared basis for providing better in-building and outdoor radio frequency coverage. Similarly, smart antennas can help reduce multipath and co-channel interference, thus enabling higher capacity in wireless networks.
- Using specifically tuned cell phone signal boosters for each service provider can help reduce and eliminate dropped calls. Other effective strategies are cell splitting, sectoring, and efficient handover management.
Having discussed the possible reasons and countermeasures for call drops, TRAI’s technical paper exhorts all stakeholders to realise that mobile towers do not have an unlimited capacity for handling network loads. Hence, there is an urgent need to increase the number of towers for catering to the growing subscriber base. Also problems like the removal of towers from certain areas should be adequately addressed. TRAI has also urged operators to invest in reinforcing the existing 2G infrastructure as well as expanding 3G in order to improve the country’s overall mobile coverage.