CIS240
Homework Assignment 1

1. Why are standards important for protocols?
In the absence of standardization, manufacturers and organizations felt free to 'enhance' the protocol, creating incompatible versions on their networks. In some cases, this was deliberately done to discourage users from using equipment from other manufacturers. There are more than 50 variants of the original bi-sync protocol. One can assume that a standard would have prevented at least some of this from happening.

2. Dialup modems, HFC, DSL, and FTTH are all used for residential access. For each of these access technologies, provide a range of transmission rates, and comment on whether the transmission is shared or dedicated.
Dial up modems: up to 56 Kbps, bandwidth is dedicated; ISDN: up to 128 kbps, bandwidth is dedicated; ADSL: downstream channel is .5-8 Mbps, upstream channel is up to 1 Mbps, bandwidth is dedicated; HFC, downstream channel is 10-30 Mbps and upstream channel is usually less than a few Mbps, bandwidth is shared. FTTH: 2-10Mbps upload; 10-20 Mbps download; bandwidth is not shared. 3. Describe the Wifi (802.11, 3G, and 4G wireless Internet access technologies. Compare and contrast them. a) 802.11 is the original Wifi and is in almost all homes, universities, businesses and soon to be in vehicles. In most cities, you can stand on a corner and receive many signals. This wireless can be accessed by laptops, smartphones, notebooks and other devices. Wireless users need to be within meters of the base station. b) 3G is used with the cellular industry and unlike Wifi, a user need only be within a few Kilometers as opposed to a few meters of a base station. This is packet-switched wide-area wireless access technology at speeds in excess of 1Mbps. c) 4G is even higher speed wide area technology can achieve rates in excess of 10Bpbs and is being called LTE (Long Term Evolution), 4. What advantage does a circuit-switched network have over a packet-switched network? What advantages does TDM have over FDM in a circuit switched network?
A circuit-switched network can guarantee a certain amount of end-to-end bandwidth for the duration of a call. Most packet-switched networks today (including the Internet) cannot make any end-to-end guarantees for bandwidth.

5. Describe how a Botnet can be created, and how it can be used for a DDoS attack.
Creation of a botnet requires an attacker to find vulnerability in some application or system. After finding the vulnerability, the attacker needs to scan for hosts that are vulnerable. Any system that is part of the botnet can automatically scan its environment and propagate by exploiting the vulnerability. An important property of such botnets is that the originator of the botnet can remotely control and issue commands to all the nodes in the botnet. Hence, it becomes possible for the attacker to issue a command to all the nodes that target a single node.

6. Consider sending a packet from a source host to a destination host over a fixed route. List the delay components in the end-to-end delay. Which of these delays are constant and which are variable?
The delay components are processing delays, transmission delays, propagation delays, and queuing delays. All of these delays are fixed, except for the queuing delays, which are variable.

7. What is the difference between a virus and a worm?
a) Virus - Requires some form of human interaction to spread. Classic example: E-mail viruses.
b) Worms - No user replication needed. Worm in infected host scans IP addresses and port numbers, looking for vulnerable processes to infect.

8. Consider an application that transmits data at a steady rate (for example, the sender generates an N-bit unit of data every k time units, where k is small and fixed). Also, when such an application starts, it will continue running for a relatively long period of time. Answer the following questions and justify your answer. a. Would a packet-switched network or a circuit-switched network be more appropriate for this application? Why? b. Suppose that a packet-switched network is used and the only traffic in this network comes from applications described above. Furthermore, assume that the sum of the application data rates is less than the capacity of each link and every link. Is some form of congestion control needed? Why?
a) A circuit-switched network would be well suited to the application described, because the application involves long sessions with predictable smooth bandwidth requirements. Since the transmission rate is known and not bursty, bandwidth can be reserved for each application session circuit with no significant waste. In addition, we need not worry greatly about the overhead costs of setting up and tearing down a circuit connection, which are amortized over the lengthy duration of a typical application session.
b) Given such generous link capacities, the network needs no congestion control mechanism. In the worst case, all the applications simultaneously transmit over one or more particular network links. However, since each link offers sufficient bandwidth to handle the sum of all of the applications' data rates, no congestion will occur.

9. Suppose you would like to urgently deliver 40 terabytes of data from Boston to Los Angeles. You have available a 100 Mbps dedicated line for the transfer. Would you prefer to transmit the data via this link or instead use FedEx overnight delivery? Explain.
40 terabytes = 40*1012 *8bits. If using the dedicated link, it will take approximately 37 days. But with FedEx overnight delivery, you can guarantee the data arrives in one day, and it only costs you no more than $100.

10. List five tasks that a layer can perform. Is it possible that one (or more) of these tasks can be performed by two (or more) layers?
Five generic tasks are error control, flow control, segmentation and reassembly, multiplexing, and connection setup. Yes, these tasks can be duplicated at different layers. For example, error control is often provided at more than one layer.