Computer Networking 面试题与答案
问题 21. What are the types of Transmission media?
Signals are usually transmitted over some transmission media that are broadly classified in to two categories.
问题 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.
问题 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.
问题 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.
问题 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).
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