Answers to the Chapter 8 Q&A Section 421
9
List the possible types of circuits available in an X.25 network deployment.
The circuit types include PVC, SVC incoming only, SVC outgoing only, and SVC
two-way.
10
What effect does changing the default ips and/or ops have on a X.25 transmission?
Altering the input packet size or the output packet size changes the maximum size of the
X.25 packet at the ingress/egress point of the network. The sizes must match in most
situations.
11
What effect does changing the default win and/or wout have on a X.25 transmission?
Altering the window incoming size or the window outgoing size settings changes the
number of packets that can be sent in a single window. The sizes must match on both ends
of the connection.
12
What is a modulo and what are the possible values?
The modulo is a counter that specifies the packet numbering modulus. The default is 8,
and the only other possible value is 128.
13
What was the driving force behind the creation of X.25?
The X.25 specifications detail connections between DTE and DCE for remote terminal
access and computer communications.
14
What entity governs the X.25 standards evolution?
The ITU-T governs X.25. ITU is governed by the United Nations.
15
What is the difference between X.25 DCE and DTE?
The X.25 DCE typically acts as a boundary to the PDN within a switch. The X.25 DTE
typically refers to the router or a PAD.
16
List the X.25 layers.
The layers are Layer 1, Physical; Layer 2, LAPB; and Layer 3, X.25.
17
X.121 addresses are considered to be at which layer of the X.25 model?
X.121 addresses are X.25 layer addresses.
18
List the three LAPB frame formats.
Information, Supervisor, and Unnumbered are the three types of LAPB frames.
19
Does the X.25 DCE have to be the physical DCE?
No. The physical DTE/DCE relationship is independent of the X.25 DTE/DCE
relationship.