Practice Problems - Chapter 7
Computer Networks

1. What is a computer network?

A computer network is a set of independent computer systems connected by telecommunications links for the purpose of sharing information and resources.

2. What is a network node?

The individual computers on the network are referred to as nodes (also hosts or end systems).

3. Explain what a circuit-switched network is and what a packet-switched network is. Give an example of each.

1. A circuit-switched network is one in which a circuit is temporarily established between the caller and the callee. This circuit lasts for the duration of the communication and is then terminated. This has it's roots in the early days of networking when it was common to transmit data via switched, dial-up telephone lines.

2. Packet-switched technology is utilized by most wide area networks (WANS). A message must "hop" from one node to another to get from the source to the destination. The unit of transmission is a packet ( a block of information with a fixed size). A message may be composed of many packets, which each "hop" their own way from source to destination. At the destination all the packets are re-assembled into the original message. The packets are basically routed from switch to switch to switch. Thus the term "packet-switched". Note that packets do not all have to follow the same route.

4. What is meant by the term ‘bandwidth’ ?

Bandwidth is the communication capacity of the network transmission medium. It is measured in terms of the number of Kilobytes (KB), Megabytes (MB) or GigaBytes(GB) of information that can be transmitted per second. A byte is 8 bits. Sometimes bandwidth is quoted in terms of Kilobits (Kb), Megabits(Mb) or Gigabits(Gb) per second. When bandwidth is quoted, make sure it is clear whether you are dealing with bits or bytes.

If you think of the transmission medium joining any two points in a network as a pipe, the bandwidth of that section of the network is related to the pipe's diameter. The bigger the pipe's diameter, the more information can flow through it in a given period of time, and the bigger the bandwidth. The 'pipes' joining major nodes of the internet are much bigger than the 'pipe' hooking your individual pc to your internet service provider. This is because the major nodes must handle much more information.

5. What is a local area network (LAN) ? Give an example.

A local area network connects hardware devices such as computers, printers, and storage devices that are all in close proximity. Examples include the networking of machines in a single room, building or campus, e.g. the computer science department network, or the Kent campus network.

6. What is a wide area network (WAN) ? Give an example

A wide area network connects devices that may be across town, the country or the oceans. These types of networks extend over wide areas of public property. Because of this the users must purchase telecommunications services from an external provider. Most WANs use a store-and-forward, packet-switched technology to deliver messages. Examples of WANs are the cell phone network you use, regional internet service providers the backbone of the internet, or the network of a national or multinational corporation.

7. What is an Internet Service Provider (ISP) ?

An internet service provider is a wide-area network, whose purpose is to provide a pathway from a specific network to other networks, or from an individual to other networks.

8. What is a network communication protocol?

A network communication protocol is a mutually agreed upon set of rules, conventions, and agreements for the efficient and orderly exchange of information.

9. Ethernet is a common technology used for local area networks. The two standard ways to construct an Ethernet LAN are via a shared cable or via a hub. Explain the topologies of these two techniques using diagrams to supplement your description.

See pages 297 through 299 in the textbook.

10. The internet communication protocol is broken up into several independent layers called a protocol hierarchy or a protocol stack. Each layer addresses one aspect of the overall communications task.

a. What is this protocol named?

TCP/IP (Transport Control Protocol / Internet Protocol)

b. List the 5 layers of this protocol stack and briefly describe the responsibilities of each layer.

5.    Application
4.    Transport (TCP)
3.    Network (IP)
2a.  Medium Access Control
1.    Physical layer.

1. Physical layer: these protocols govern the exchange of binary digits across the physical communication channel, e.g. fiber optics, copper wire, wireless radio channel. It's goal or responsibility is to create a "bit pipe" between two computers. This level does not guarantee error-free transmission of these bits of information

2a and 2b are referred to as the data link protocols. Their responsibility is to create an error-free "message pipe" between two computers, directly connected by a physical link This is done in two steps.

2a. The medium access control protocols determine how to allocate the shared communication link, among competing machines which all want to send messages sometime or other. Basically these rules determine who gets access to the shared line when multiple nodes want to send at the same time. The most common of these protocols is Ethernet, which uses a contention-based approach in which all nodes compete equally for access. In other words, a node may send when no one else is sending.  Message collisions are detected and rules provide how to deal with this.

2b. The logical link protocols ensure that once two computers have access to the shared line that the message traveling across the line arrives correctly. An automatic repeat request (ARQ) algorithm is employed. The message to be sent is broken up into individual packets of fixed-size and is combined with a sequence number, error check data and wrapped in starting and ending bit sequences. The message is sent packet by packet using the ARQ algorithm, assuring that the entire message arrives correctly.

3. Network Layer: This layer assures that messages are transmitted correctly between two nodes which are not directly connected, but are connected via a network. The two critical responsibilities of this layer are 1) creating a universal addressing scheme for all network nodes and 2) delivering messages between any two nodes in the network.

All nodes in the network must be running the same network layer protocols. On the internet, the network protocol is called IP or internet protocol. the IP address is 32 bits, often written as 4 numbers each in the range of 0-255 e.g. 141.140.1.5

In this layer, symbolic host names such as macalester.edu are converted to IP addresses using a Domain Name Service (DNS), which is a massive database spread over thousands of machines each of which contain certain name-to-IP address mappings.

Also in this layer, various routing algorithms are used to transmit message packets from source node A through a series of intermediate nodes to destination node B.

4. Transport layer: application programs transfer data to the internet via a specific communications port. Multiple applications on a given computer (node) may be connected to the internet at any given time. It is the responsibility of this layer to assure that there is error free message transmission between  the proper ports (programs) on node A and B. Basically it creates a "program-to-program" message delivery service. Certain programs always use the same port number e.g. HTTP uses port 80. The transport layer protocol used on the internet is usually Transport Control Protocol (TCP).

5. Application layer protocols are the rules for implementing the end-user services provided by the network. These services are the reason networks exist in the first place. HTTP protocol is used by the World Wide Web to access and deliver web pages.

When a user on machine A types a URL e.g. http://www.macalester.edu/about/history.html into the web browser ( or clicks on a hyperlink), the browser first requests that the TCP layer establishes a connection between itself and port 80 (the web server program) of a machine called www.macalester.edu. The browser then sends a get message to the web server to request the page /about/history.html . The web server receives the request, processes it and sends a response message. Once the message is delivered by the TCP layer to your web browser's port, the page is displayed and the TCP connection terminated.