Appendix G: Exercises - Codes: The Guide to Secrecy from Ancient to Modern Times - page 589

9.8. Digital cell phones not only digitize speech, but also compress it. Why is
this necessary?
9.9. There is a standard for wireless networks that competes with IEEE 802.11
(see pages 340-353), called Bluetooth , G.2 which is a network that does
not require a server or other central access source. In other words, the
devices on a Bluetooth network can find each other independently and
can communicate directly with each other, i.e., a peer-to-peer network (see
page 220). Since Bluetooth allows not only telephones, but also computers,
personal desktop assistants (PDA)s, and even TVs and audio equipment
to communicate with one another, it may be used in concert with 802.11.
Bluetooth devices have microchips embedded in them containing software,
called a link controller , allowing one Bluetooth device to recognize another,
by continually sending out signals in search of other such devices in its
range. The software also contains profiles encoded into each device so
that one can determine if it is appropriate to form a connection (cell
phones and TVs need not communicate, for instance). Once connection
is established between two or more Bluetooth devices, this is a network
called a piconet . Bluetooth devices that are placed too close to one another
may have their radio frequencies interfere with one another. Speculate as
to how Bluetooth fixes this problem (via some monitoring technique).
9.10. If a cell phone has a microbrowser installed, it can interface with the
WWW to display a Web page using a protocol called Wireless Access
Protocol (WAP), which employs its own language, the Wireless Markup
Language (WML). To accomplish this, the cell phone must send a request
that is routed through a landline (wire-based network), which is sent to a
Web server where the page is located. Then the page is sent to to what is
called a WAP gateway. Assuming the WWW page is in HTML, speculate
on how the WAP gateway would deal with the page in order for it to be
displayed on the cell phone once it is relayed back.
9.11. A step above cell phones are satellite phones, which employ GSM tech-
nology, so the phone must have a SIM installed (see page 347). A signal is
sent from the phone to a satellite, which receives the call. Assuming there
are numerous satellites in the system, how would this system of satellites
work in a fashion similar to a cell phone network?
9.12. Exploration satellites, such as the Voyager series, send signals back to
earth that contain pictures and other data. The transmitter on some of
these satellites can be a mere 23 watts. Given an antenna of fourteen feet
on board, and a signal in the 8-GHz band, sent to NASA satellites that
are 100 feet in diameter, explain how the signal can reach earth.
( Hint: See Exercise 9.5. )
G.2 Thenameisa(rough)translationfromtheDanishword Blatand , thesurnameoftheking
of Denmark from 940 to 985 AD. King Harald Blatand united Denmark and Norway. Thus,
it was deemed appropriate to name a uniting communications feature after this man.