Computer Network Unit 1 q what are the topologies in computer n/w ?

Q 9. Explain architecture of DQDB.
Ans: DQDB: Distributed Queue Dual Bus Defined in IEEE 802.6
Data Over Cable Service Interface Distributed Queue Dual Bus (DQDB) is a Data-link layer communication protocol for Metropolitan Area Networks (MANs), specified in the IEEE 802.6 standard, designed for use in MANs. DQDB is designed for data as well as voice and video transmission based on cell switching technology (similar to ATM). DQDB, which permits multiple systems to interconnect using two unidirectional logical buses, is an open standard that is designed for compatibility with carrier transmission standards such as SMDS, which is based on the DQDB standards.
For a MAN to be effective it requires a system that can function across long, ¡°city-wide¡± distances of several miles, have a low susceptibility to error, adapt to the number of nodes attached and have variable bandwidth distribution. Using DQDB, networks can be thirty miles long and function in the range of 34 Mbps to 155 Mbps. The data rate fluctuates due to many hosts sharing a dual bus as well as the location of a single host in relation to the frame generator, but there are schemes to compensate for this problem making DQDB function reliably and fairly for all hosts.
The DQDB is composed of a two bus lines with stations attached to both and a frame generator at the end of each bus. The buses run in parallel in such a fashion as to allow the frames generated to travel across the stations in opposite directions. 
the basic DQDB architecture:

Q 10. Explain FDDI Frame Format.
Ans :The following figure shows the frame format of an FDDI data frame and token:

FDDI Frame Fields

Preamble -- A unique sequence that prepares each station for an upcoming frame.
Start Delimiter -- Indicates the beginning of a frame by employing a signaling pattern that differentiates it from the rest of the frame.
Frame Control -- Indicates the size of the address fields, whether the frame contains asynchronous or synchronous data, and other control information.