Communication Networks (2
nd
Edition) Chapter 2 Solutions
support IP. The salient part of the above figure is that all of the higher layer protocols access the
network interfaces through IP. This is what provides the ability to operate over multiple networks.
2. (c) What economies of scale result from (a) and (b)?
Solution:
Once a network interface for IP is defined for a given network technology, then hosts connected using
the given network technology can connect to the Internet. This allows the reach of the Internet to
grow rapidly, leveraging multiple coexisting networks technologies. Thus investment in new network
technologies extends the reach of the Internet.
3. What difference does it make to the network layer if the underlying data link layer provides a connection-
oriented service versus a connectionless service?
Solution:
If the data link layer provides a connection-oriented service to the network layer, then the network
layer must precede all transfer of information with a connection setup procedure. If the connection-
oriented service includes assurances that frames of information are transferred correctly and in
sequence by the data link layer, the network layer can then assume that the packets it sends to its
neighbor traverse an error-free pipe.
On the other hand, if the data link layer is connectionless, then each frame is sent independently
through the data link, probably in unconfirmed manner (without acknowledgments or
retransmissions). In this case the network layer cannot make assumptions about the sequencing or
correctness of the packets it exchanges with its neighbors.
The Ethernet local area network provides an example of connectionless transfer of data link frames.
The transfer of frames using "Type 2" service in Logical Link Control (discussed in Chapter 6)
provides a connection-oriented data link control example.
4. Suppose transmission channels become virtually error-free. Is the data link layer still needed?
Solution:
The data link layer is still needed for framing the data and for flow control over the transmission
channel. In a multiple access medium such as a LAN, the data link layer is required to coordinate
access to the shared medium among the multiple users.
5. Why is the transport layer not present inside the network?
Solution:
Some of the functions provided by the transport layer can be provided inside the networks, but other
functions cannot. For example, the transport layer provides functions at the end-system to
compensate for the limitations and impairments of the network layer, in order to meet requirements
(e.g. QoS) of the upper layer. For example in TCP/IP, IP provides only best effort service. To provide
the reliable service required by some applications - that is, to compensate for the shortcomings of
best effort service - TCP establishes connections and implements error control on an end-to-end
basis. One can imagine that a service provider could incorporate this error control function at the
edge of its network. On the other hand, one of the main purposes of the transport layer is to allow
multiple processes in the end systems to share a network service. This cannot be achieved inside
the network.
Leon-Garcia/Widjaja 2