Surveillance or Security?: The Risks Posed by New Wiretapping Technologies by Susan Landau Read Free Book Online Page A
Authors: Susan Landau
device connects to the network. This is true even
if the device is connecting to the Internet from the same location; it
happens because the local network through which the device is connecting
is likely to have multiple addresses available to it.
While IP addressing can handle portability, it has some trouble with
mobility. Being connected is a heavy-duty operation-it sends signals
down the line that the IP address, which designates the physical location
of the device, is such-and-such. An IP address, say for routing a piece of
email or downloading a movie, needs to be stable for at least the duration of the data being transferred to it. That stability is quite complicated to
achieve when the user is mobile, and the problem has not been fully solved
yet. (What appears to be IP connectivity on a moving vehicle-for example,
the web browsing now available on some planes-is actually a local IP
network connecting to the Internet via a cell link. The IP address does not
change as the device moves.)
If Internet communications have not fully sorted out how to handle
highly mobile communications, another community had that problemand solution-as its raison d'etre. Cell phones allow users to communicate
while moving. This is not always easy; for example, if the speaker is on a
high-speed train, special technology is needed so that the call is not
dropped.52 There are two parts to enabling the mobility of cell phone users.
Cell phones (called mobile phones in Europe) rely on a network of radio
towers to transmit the communication. But a cell phone is not fixed in
space and so the system also needs a way to determine that the user has
paid for services. Thus each user is registered in a Home Location Register
(HLR), a very large database that stores subscriber information (including
to what services users are entitled) about all users in that home location.
Users are assigned to a particular HLR based on their phone number.53 That
solves part of the problem, but, of course, cell phone subscribers are
mobile. So as a subscriber moves about the network, information about
the user's privileges on the network must also travel. Otherwise there is
simply too much delay in completing calls while the network checks back
with the HLR. In addition to the HLR there is a local, smaller database
called the Visitor Location Register (VLR) that contains a portion of the
subscriber's information.
Calls are handed from tower to tower as the phone subscriber moves
during a conversation. These towers, or base stations, are divided into
groups called Location Areas. When a cell phone is turned on, the phone
identifies itself through its phone number to the nearest tower (note that
unlike a telephone on the PSTN, a cell telephone number is definitely not
a program used to define the phone's location). By checking in with the
appropriate HLR (easy to determine from the phone number), the base
station grants the phone service.
When a roaming phone is initially turned on, and maybe every thirty
minutes after that, a signaling message is sent from the phone to the HLR.54
This enables the HLR to route calls to the subscriber. The first time a
roaming subscriber tries to make a call, the HLR is queried: to what services
is the subscriber entitled? After that, the VLR is established in the new
Location Area; all queries go to it, rather than the HLR. (This is why the initial call made while roaming may take longer to connect than subsequent ones.) A new VLR is created whenever a subscriber crosses into a
new Location Area.
2.6 Voice on the Data Network
In the Internet, voice communications systems did not happen until
several decades after written applications (such as email). There was one
exception: in 1973 an engineering team led by Danny Cohen of the Information Sciences Institute (ISI) of the University of Southern California used
the ARPANET to make calls between ISI and MIT Lincoln Lab in Lexington,
if the device is connecting to the Internet from the same location; it
happens because the local network through which the device is connecting
is likely to have multiple addresses available to it.
While IP addressing can handle portability, it has some trouble with
mobility. Being connected is a heavy-duty operation-it sends signals
down the line that the IP address, which designates the physical location
of the device, is such-and-such. An IP address, say for routing a piece of
email or downloading a movie, needs to be stable for at least the duration of the data being transferred to it. That stability is quite complicated to
achieve when the user is mobile, and the problem has not been fully solved
yet. (What appears to be IP connectivity on a moving vehicle-for example,
the web browsing now available on some planes-is actually a local IP
network connecting to the Internet via a cell link. The IP address does not
change as the device moves.)
If Internet communications have not fully sorted out how to handle
highly mobile communications, another community had that problemand solution-as its raison d'etre. Cell phones allow users to communicate
while moving. This is not always easy; for example, if the speaker is on a
high-speed train, special technology is needed so that the call is not
dropped.52 There are two parts to enabling the mobility of cell phone users.
Cell phones (called mobile phones in Europe) rely on a network of radio
towers to transmit the communication. But a cell phone is not fixed in
space and so the system also needs a way to determine that the user has
paid for services. Thus each user is registered in a Home Location Register
(HLR), a very large database that stores subscriber information (including
to what services users are entitled) about all users in that home location.
Users are assigned to a particular HLR based on their phone number.53 That
solves part of the problem, but, of course, cell phone subscribers are
mobile. So as a subscriber moves about the network, information about
the user's privileges on the network must also travel. Otherwise there is
simply too much delay in completing calls while the network checks back
with the HLR. In addition to the HLR there is a local, smaller database
called the Visitor Location Register (VLR) that contains a portion of the
subscriber's information.
Calls are handed from tower to tower as the phone subscriber moves
during a conversation. These towers, or base stations, are divided into
groups called Location Areas. When a cell phone is turned on, the phone
identifies itself through its phone number to the nearest tower (note that
unlike a telephone on the PSTN, a cell telephone number is definitely not
a program used to define the phone's location). By checking in with the
appropriate HLR (easy to determine from the phone number), the base
station grants the phone service.
When a roaming phone is initially turned on, and maybe every thirty
minutes after that, a signaling message is sent from the phone to the HLR.54
This enables the HLR to route calls to the subscriber. The first time a
roaming subscriber tries to make a call, the HLR is queried: to what services
is the subscriber entitled? After that, the VLR is established in the new
Location Area; all queries go to it, rather than the HLR. (This is why the initial call made while roaming may take longer to connect than subsequent ones.) A new VLR is created whenever a subscriber crosses into a
new Location Area.
2.6 Voice on the Data Network
In the Internet, voice communications systems did not happen until
several decades after written applications (such as email). There was one
exception: in 1973 an engineering team led by Danny Cohen of the Information Sciences Institute (ISI) of the University of Southern California used
the ARPANET to make calls between ISI and MIT Lincoln Lab in Lexington,