Question 1.
What is the principal difference between circuit switches and packet switches?

a) Circuit switches are used in telephony; packet switches are used for data
b) Circuit switches are manually configured; packet switches are entirely dynamic
c) Circuit switches act on entire streams or flows; packet switches act on components within a flow
d) All choices are true

Answer

Question 2.
In a WAN, traffic is switched according to:

a) The combination of source and destination addresses in the Layer 3 header
b) Only the destination in the Layer 3 header
c) A globally-unique Layer 2 identifier
d) A locally-unique Layer 2 identifier

Answer

Question 3.
Which of the following descriptions of media capabilities goes with the listed technology?
(Choose 2)

a) ATM: high-speed media, generally reliable, but Layer 3 is assumed to request retransmission of missing cells
b) Frame Relay: generally reliable connections; Layer 3 is assumed to request retransmission of missing frames
c) X.25: unreliable connections led to its never being used
d) ATM: high-speed media, capable of segregating and treating traffic streams differently

Answer

Answers

Question 1.
What is the principal difference between circuit switches and packet switches?

a) Circuit switches are used in telephony; packet switches are used for data
b) Circuit switches are manually configured; packet switches are entirely dynamic
c) Circuit switches act on entire streams or flows; packet switches act on components within a flow
d) All choices are true

Answer
c) Circuit switches act on entire streams or flows; packet switches act on components within a flow

Explanation
WAN switches move data from one circuit to another; internally to the WAN switch, it is the entire circuit that is switched, not the data. LAN switches move individual packets and switch them from one port to another.
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Question 2.
In a WAN, traffic is switched according to:

a) The combination of source and destination addresses in the Layer 3 header
b) Only the destination in the Layer 3 header
c) A globally-unique Layer 2 identifier
d) A locally-unique Layer 2 identifier

Answer
d) A locally-unique Layer 2 identifier

Explanation
WAN switches generally do not decapsulate a packet as far as the Layer 3 header, since logical addressing is of little or no help in moving traffic at high speed along major arteries. Each flow must be referenced to the Layer 2 address of the next hop for there to be any hope of operating at enough speed for the magnitude of traffic flowing on WAN circuits. The identifier in the Layer 2 header for a given virtual circuit is locally unique and has no significance beyond this connection.
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Question 3.
Which of the following descriptions of media capabilities goes with the listed technology?
(Choose 2)

a) ATM: high-speed media, generally reliable, but Layer 3 is assumed to request retransmission of missing cells
b) Frame Relay: generally reliable connections; Layer 3 is assumed to request retransmission of missing frames
c) X.25: unreliable connections led to its never being used
d) ATM: high-speed media, capable of segregating and treating traffic streams differently

Answer
b) Frame Relay: generally reliable connections; Layer 3 is assumed to request retransmission of missing frames
d) ATM: high-speed media, capable of segregating and treating traffic streams differently

Explanation
ATM is designed for speed and delivering Quality of Service -- different treatment for different (segregated) traffic streams. It is used primarily for voice and video as well as data; voice and video do not have retransmission under Layer 3. ATM was originally intended as a carrier-only technology.

Frame Relay was designed as a low-speed feeder technology to ATM to take advantage of the more reliable media that became available; it does not include the extensive error correction of its predecessor, X.25. Frame Relay assumes any undelivered frames will be handled by a higher-level protocol.

X.25 was widely deployed; it has extensive error detection and correction procedures to compensate for unreliable connections.
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