多項選擇題Which two statements about the DiffServ model are true？ （）

A. A class can be identified as a single application， multiple applications with similar service needs， or be based on the source or destination IP addresses.
B. A drawback of the DiffServ model is that it does not provide backward compatibility with devices that can only use the ToS field.
C. DiffServ uses the DiffServ field in the MAC header to mark frames into behavior aggregates （BAs）. 
D. The DiffServ field occupies the same eight bits of the MAC header that were previously used for the ToS field.
E. The first six high-order bits of the DiffServ field are used to identify the Resource Reservation Protocol （RSVP） value.
F. The primary goal of DiffServ is scalability.


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2.多項選擇題Which two statements are true about voice ports on a router？ （）

A. Calls to the PSTN can be made via the use of FXO or T1/E1 trunk ports.
B. Calls to the PSTN can be made via FXS or T1/E1 trunk ports.
C. Analog and IP phones can be connected to the VoIP network via FXO or T1/E1 trunk ports.
D. Calls between analog phones that are attached to the FXS ports in a VoIP network can be completely processed by voice-enabled routers.

3.多項選擇題Which two statements are true about the digital audio in a VoIP network？ （）

A. Standard encoding techniques create an uncompressed digital data rate of 4000 bps.
B. Standard encoding techniques create an uncompressed digital data rate of 8000 bps.
C. Standard encoding techniques create an uncompressed digital data rate of 64，000 bps.
D. Voice quality is not a concern if compression is not used.
E. Two methods of quantization are linear and logarithmic.
F. Two methods of compression are u-law and a-law.

4.多項選擇題Which two statements are true about analog to digital conversion of voice signals for use in digital telephony networks？ （）

A. The three required steps in the analog to digital conversion are sampling， encoding， and compression.
B. The three required steps in the analog to digital conversion are sampling， quantization， and compression.
C. The three required steps in the analog to digital conversion are sampling， quantization， and encoding.
D. The output of the sampling process is a pulse code modulation （PCM） signal.
E. The output of the sampling process is a pulse amplitude modulation （PAM） signal.