Wireless Security.pdf - PDF Archive

24 Chapter 2
Transmission of the message across the Internet is achieved through a number of deviceto-device
hops involving the PHY and data link layers of each routing or relaying device
in the chain. At each step, the data link layer of the receiving device determines the MAC
address of the next immediate destination, and the PHY layer transmits the packet to the
device with that MAC address.
On arrival at the receiving computer, the PHY layer will demodulate and decode the
voltages and frequencies detected from the transmission medium, and pass the received
data stream up to the data link layer. Here the MAC and LLC elements, such as a
message integrity check, will be extracted from the data stream and executed, and the
message plus instructions passed up the protocol stack. At Layer 4, a protocol such as
Transport Control Protocol (TCP), will ensure that all data frames making up the message
have been received and will provide error recovery if any frames have gone missing.
Finally the e-mail application will receive the decoded ASCII characters that make up the
original transmitted message.
Standards for many layers of the OSI model have been produced by various organizations
such as the Institute of Electrical and Electronics Engineers (IEEE). Each standard details
the services that are provided within the relevant layer and the protocols or rules that must
be followed to enable devices or other layers to call on those services. In fact, multiple
standards are often developed for each layer, and they either compete until one emerges
as the industry “ standard ” or else they peacefully coexist in niche areas.
The logical architecture of a wireless network is determined principally by standards that
cover the data link (LLC plus MAC) and PHY layers of the OSI model. The following
sections will give a preliminary introduction to these standards and protocols, while more
detailed descriptions will be found in Parts III to V where local area (LAN), personal area
(PAN) and metropolitan area (MAN) wireless networking technologies are described,
respectively.
The next section starts this introductory sketch one layer higher—at the Network layer—not
because this layer is specific to wireless networking, but because of the fundamental
importance of its addressing and routing functions and of the underlying Internet Protocol (IP).
2.2 Network Layer Technologies
The Internet protocol (IP) is responsible for addressing and routing each data packet
within a session or connection set up under the control of transport layer protocols such
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