Answers to the Chapter 3 Q&A Section 711
4
What is the main purpose(s) of Layer 5?
Layer 5 (session layer) controls the conversation between two endpoints. Although
the term used is session, the term conversation more accurately describes what is
accomplished. The session layer ensures that not only communication but also useful
sets of communication between endpoints is accomplished.
5
What is the main purpose(s) of Layer 4?
Layer 4 (transport layer) provides end-to-end error recovery, if requested.
6
What is the main purpose(s) of Layer 3?
Layer 3 (network layer) defines logical addressing and routing as a means to deliver data
across an entire network. IP and IPX are two examples of Layer 3 equivalent protocols.
7
What is the main purpose(s) of Layer 2?
The data link layer defines addressing specific to a particular medium as part of the means
to provide delivery of data across that medium. It also includes the protocols used to
determine what device(s) accesses the media at any point in time.
8
What is the main purpose(s) of Layer 1?
Layer 1 (physical layer) is responsible for encoding energy signals onto the medium and
interpreting a received energy signal. Layer 1 also defines the connector and cabling
details.
9
Describe the process of data encapsulation as data is processed from creation until it exits
a physical interface to a network. Use the OSI model as an example.
Data encapsulation represents the process of a layer adding a header (and possibly a
trailer) to the data as it is processed by progressively lower layers in the protocol
specification. In the context of OSI, each layer could add a header so that--other than the
true application data--there would be six other headers (Layers 2 to 7) and a trailer for
Layer 2, with this L2-PDU being encoded by the physical layer onto the network media.
10
Describe the features required for a protocol to be considered connectionless.
Unordered low-overhead delivery of data from one host to another is the service provided
in most connectionless protocol services.
11
Name at least three connectionless protocols.
LLC type 1, UDP, IPX, IP, and PPP are some examples of connectionless protocols.
Remember, Frame Relay, X.25, and ATM are connection-oriented, regardless of whether
they define error recovery.
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