Ques 21. What are the types of Transmission media?
Signals are usually transmitted over some transmission media that are broadly
classified in to two categories.
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Ques 22. Guided Media:
These are those that provide a conduit from one device to another that include
twisted-pair, coaxial cable and fiber-optic cable. A signal traveling along any of these
media is directed and is contained by the physical limits of the medium. Twisted-pair
and coaxial cable use metallic that accept and transport signals in the form of
electrical current. Optical fiber is a glass or plastic cable that accepts and transports
signals in the form of light.
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Ques 23. Unguided Media:
This is the wireless media that transport electromagnetic waves without using a physical conductor. Signals are broadcast either through air. This is done through radio communication, satellite communication and cellular telephony.
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Ques 24. What is Project 802?
It is a project started by IEEE to set standards to enable intercommunication
between equipment from a variety of manufacturers. It is a way for specifying
functions of the physical layer, the data link layer and to some extent the network
layer to allow for interconnectivity of major LAN protocols.
It consists of the following:
• 802.1 is an internetworking standard for compatibility of different LANs and MANs across protocols. • 802.2 Logical link control (LLC) is the upper sublayer of the data link layer
which is non-architecture-specific, that is remains the same for all IEEE-
defined LANs.
• Media access control (MAC) is the lower sublayer of the data link layer that
contains some distinct modules each carrying proprietary information specific
to the LAN product being used. The modules are Ethernet LAN (802.3), Token
ring LAN (802.4), Token bus LAN (802.5).
• 802.6 is distributed queue dual bus (DQDB) designed to be used in MANs.
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Ques 25. What is Protocol Data Unit?
The data unit in the LLC level is called the protocol data unit (PDU). The PDU
contains of four fields a destination service access point (DSAP), a source service
access point (SSAP), a control field and an information field. DSAP, SSAP are
addresses used by the LLC to identify the protocol stacks on the receiving and
sending machines that are generating and using the data. The control field specifies
whether the PDU frame is a information frame (I - frame) or a supervisory frame (S -
frame) or a unnumbered frame (U - frame).