For the Defense - Vol. 5, Issue 2 - 34

call detail records. Billing records will only contain
data for when a call was made or received and
when a text message was sent or received. It will
not contain the details such as tower location,
sector, or the base station ID. These records are
not sufficient for plotting CDRs.
Now, in order to understand further what a
CDR is and how it can be helpful in a criminal
case, we must understand the basics of how cell
phones communicate with one another and with
networks.
How a Cell Phone Tower Works - The Basics
The basic way a cell phone functions is by
communicating with a "base station" or "cell
site" to send and receive data. Cell phone
towers, with which you are probably familiar,
are examples of base stations. They can be found
atop most tall buildings in a city or "disguised" as
trees on the side of a highway, but are most often
seen as large metal towers with radios attached
high enough in the air to blanket the area far
and wide with radio waves. Cell service works
best with direct line-of-sight ("LOS") from the
base station to the cell phone, which is why cell
towers are typically found at the highest point in
the area.
A modern cell phone can perform a multitude
of functions from taking photos and videos
to playing video games; however, without the
Internet and the interconnectivity that comes
along with it, the device becomes painfully
useless while on the go. Your phone is constantly
listening to the airwaves around it to find a
compatible cell phone signal. When a base
station candidate is found, a "handshake" is
performed between the cell phone and the
tower, and once successful the phone is tethered
to this tower, and it will use it as its primary
voice and data source. While not as common
at present due to an increasing number of
cell towers, "roaming" may occur during the
connection process. If a Verizon customer is only
in an area covered by T-Mobile towers, the phone
could "roam" out of its network and utilize a
competitor's tower, assuming the underlying
technology is compatible with the device.
A phone decides which cell tower to connect
to based on various factors, with signal strength

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For The Defense l Vol. 5, Issue 2

being primary. As a general rule of thumb, a
cell tower in a dense city environment with
many lines of sight obstructions (tall buildings)
can "reach" about a quarter to a half of a mile
from its location. In the suburbs where a tower
could be placed on a nearby hilltop overlooking
a valley, it can easily project waves within a 5
to 15-mile radius with little obstruction. Once
connected to a tower, your phone is provided
a list of "neighboring cell sites," giving it quick
access to those others when moving away from
the tower's radius.
With the rollout of the next generation of cell
phone technology and the shift to "5G", it is
becoming common practice to build dozens of
smaller cell sites known as "microcells" to cover
an area as opposed to one large tower. When the
Pope visited Philadelphia in 2015, the expected
influx in connected devices along the Benjamin
Franklin Parkway forced innovation from the cell
phone providers. To increase capacity, small cell
sites were installed into the individual flag poles
along the Parkway. All of the cell providers share
these cell sites and as a result, cell service and
capacity are now much better along the Parkway,
as is location information recovered from Call
Detail Records in this area.
As you move around, your phone is constantly
looking to connect with other towers that may
offer a better-quality signal. When your phone
moves from one tower to another, it is called a
"handoff." When a handoff works properly, it
goes unnoticed. If you have ever been driving in
your car while talking on the phone and the call
drops or cuts out for a few seconds, it is likely the
result of a failed handoff. Handoffs occur all the
time, even when stationary. For example, there
may be a cell tower 300 feet from your current
location serving 5,000 devices. While this may
have the best signal strength, another tower a
half mile away that is currently only serving 100
devices may be able to offer a significant boost
in signal quality and speed, with the tradeoff of
only a slightly higher latency. These calculations
occur discreetly on your device dozens of times
per second, and cell providers monitor every
single handoff, data connection, call, and text
message you send/receive in order to optimize
their network and technology to get a leg up on
competitors.
For the Defense - Vol. 5, Issue 2

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