Call Detail Records CDR

How analysis of Call Detail Records (CDRs) provides valuable information?

Whenever a mobile phone call or transaction is made, a Call Detail Record (CDR) is automatically generated by the mobile network operator. CDRs are a digital record of the attributes of a certain instance of a telecommunication transaction (such as the start time or duration of a call), but not the content. If you pay a monthly bill for your mobile phone services, take a look at the itemized list of calls: these are essentially CDRs.

An additional piece of information that gets recorded in CDRs by a mobile network operator is to which cell towers the caller and recipient’s phones were connected at the time of the call. Because the mobile network operator knows the locations of their cell towers, it is possible to use CDRs to approximate the location of both parties. The spacing of cell towers, and thus the accuracy in determining the caller’s location, varies according to expected traffic and terrain. Cell towers are typically spaced 2-3km apart in rural areas and 400-800m apart in densely populated areas. This geospatial information is extremely useful for many commercial applications up to humanitarian and development solutions.

Call detail record contains data fields that describe a specific instance of a telecommunication transaction, but does not include the content of that transaction. By way of simplistic example, a call detail record describing a particular phone call might include the phone numbers of both the calling and receiving parties, the start time, and duration of that call. In actual modern practice, call detail records are much more detailed, and contain attributes such as:

· the phone number of the subscriber originating the call (calling party, A-party)

· the phone number receiving the call (called party, B-party)

· the starting time of the call (date and time)

· the call duration

· the billing phone number that is charged for the call

· the identification of the telephone exchange or equipment writing the record

· a unique sequence number identifying the record

· additional digits on the called number used to route or charge the call

· the disposition or the results of the call, indicating, for example, whether or not the call was connected

· the route by which the call entered the exchange

· the route by which the call left the exchange

· call type (voice, SMS, etc.)

· any fault condition encountered

Internally, mobile phone companies use CDRs as the basis for billing customers and maintenance of their business, but CDRs can also serve other functions. Social scientists, researchers and public sector organizations have begun to research additional applications of CDRs. CDRs stored by a carrier have the potential to reveal personal information, so the records need to be altered in several important ways before being shared with third parties for analysis. First, all personally identifiable information must be removed. Typically, this is accomplished by encrypting the phone numbers of both caller and recipient. In many cases, the data from multiple callers may be aggregated to reduce risk of re-identification. Finally, data is often processed to contain the latitude and longitude from the cell tower closest to where the calls were placed. Consequently, by the time CDRs are shared, they look something like this

#caller,#callee,start time,tower coordinates,

u1, v1,2009-12-01 00:44:13,(40.421377,-3.698631)

u2, v2,2009-09-15 23:33:19,(40.441566,-3.697189)

u3, v3,2009-12-10 14:33:29,(40.414634,-3.709735)

u4, v4,2009-12-10 19:53:09,(40.419974,-3.630850)

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While at first glance it is difficult to assess the value of this rather rudimentary data, remarkably useful information on human behavior may be derived from large sets of de-identified CDRs. There are at least three dimensions that can be measured:

1. MOBILITY: As mobile phone users send and receive calls and messages through different cell towers, it is possible to “connect the dots” and reconstruct the movement patterns of a community. This information may be used to visualize daily rhythms of commuting to and from home, work, school, markets or clinics, but also has applications in modeling everything from the spread of disease to the movements of a disaster-affected population.

2. SOCIAL INTERACTION: The geographic distribution of one’s social connections may be useful both for building demographic profiles of aggregated call traffic and understanding changes in behavior. Studies have shown that men and women tend to use their phones differently, as do different age groups. Frequently making and receiving calls with contacts outside of one’s immediate community is correlated with higher socio-economic class.

3. ECONOMIC ACTIVITY: Mobile network operators use monthly airtime expenses to estimate the household income of anonymous subscribers in order to target appropriate services to them through advertising. When people in developing economies have more money to spend, they tend to spend a significant portion of it on topping off their mobile airtime credit. Monitoring airtime expenses for trends and sudden changes could prove useful for detecting the early impact of an economic crisis, as well as for measuring the impact of programmes designed to improve livelihoods