DQDB Non-Isochronous Protocol Answer | Generated by AI
Question: In the DQDB dual bus subnet, which media access control protocol is specified to provide non-isochronous services?
A. Carrier Sense Multiple Access
B. Pre-arbitrated Access
C. Distributed Queue Access
D. Time Division Multiple Access
Your answer: D
Correct answer: C
Answer:
Correct answer is C – Distributed Queue Access (分布式排队访问)
Explanation:
DQDB (Distributed Queue Dual Bus) is a metropolitan area network (MAN) standard defined in IEEE 802.6. It uses two unidirectional buses (forward and reverse bus) and is designed to support both isochronous (time-sensitive, constant bit rate) and non-isochronous (asynchronous, variable bit rate) traffic.
In DQDB, there are two main media access control mechanisms:
-
QA (Queued Arbitrated) slots → use Distributed Queue Access protocol
→ mainly used for non-isochronous (connectionless, best-effort, asynchronous) services, such as data traffic, LAN-like communication, file transfer, etc. -
PA (Pre-Arbitrated) slots → use pre-assigned fixed slots
→ mainly used for isochronous services (constant bit rate, delay-sensitive), such as voice and video.
Therefore:
- The protocol that provides non-isochronous services in DQDB is Distributed Queue Access (分布式排队访问), which corresponds to option C.
Why the other options are incorrect:
-
A. Carrier Sense Multiple Access (CSMA)
This is the access method used in Ethernet (IEEE 802.3), not in DQDB. -
B. Pre-arbitrated Access
This exists in DQDB, but it is used for isochronous services, not non-isochronous. -
D. Time Division Multiple Access (TDMA)
While DQDB uses slot-based access (similar in concept to TDMA), the specific mechanism name for non-isochronous traffic in DQDB is Distributed Queue Access, not simply called TDMA.
Summary:
DQDB dual-bus subnet → non-isochronous services → Distributed Queue Access (QA protocol) → correct answer is C.
References:
- IEEE 802.6 standard (DQDB) documentation
- Computer network textbooks covering MAN standards (e.g., Tanenbaum, Kurose & Ross, etc.)