Computer Networks Top Down Approach Solution

Chapter 1- Computer Networks

1) What is the difference between a host and an end system?
	There is no difference. Throughout this text, the words "host" and "end system" are used interchangeably.
	Examples of end-systems include PCs, workstations, Web servers, mail servers, PDAs, Internet-connected game consoles, etc.

2) The word protocol is often used to describe diplomatic relations. How does Wikipedia describe diplomatic protocol?
	From Wikipedia: Diplomatic protocol is commonly described as a set of international courtesy rules. These well-established
	and time-honored rules have made it easier for nations and people to live and work together. Part of protocol has always
	been the acknowledgement of the hierarchical standing of all present. Protocol rules are based on the principles of civility

3)Why are standards important for protocols?
	Standards are important for protocols so that people can create networking systems and products that interoperate

4)List six(6) access technologies. Classify each one as home access, enterprise access or wide-area wireless access

	i) Dial-up Modem over Telephone Line - Home Access
	ii) DSL over telephone line: Home Acess or Small Office
	iii) Cable to HFC: Home Access
	iv) 100 Mbps switched Ethernet: Enterprise Access
	v) Wifi(802.11): Home Access and Enterprise
	vi) 3G and 4G technology: Wide-Area Wireless Access

5)Is HFC(Hybrid Fibre-Coaxial) transmission rate dedicated or shared among users?
	HFC bandwidth is shared among the users.
  Are collisions possible in a downstream HFC channel? Why or Why not?
	On the downstream channel, all packets emanate from a single source, namely, the head end. Thus, there are no collisions
	in the downstream channel.

6) What is transmission rate of Ethernet LANs?
	Ethernet LANs have a transmission rate of 10 Mbps, 100Mbps, 1Gbps, 10Gbps

7) What are some of the physical media that Ethernet can run over?
	Today, Ethernet most commonly runs over over twisted-pair copper wire. It also can run over fibers optic links.
8) Dial-up 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 rate is shared or dedicated.
	Dial up modems: up to 56 Kbps, bandwidth is dedicated; ADSL; up to 24 Mbps downstream and 2.5 Mbps upstream, bandwidth is dedicated; HFC, rates up to 42.8 Mbps and upstream rates of up to 30.7 Mbps, bandwidth is shared. FTTH: 2-10Mbps upload; 10-20 Mbps download; bandwidth is not shared.
9) Describe the most popular wireless Internet access technologies today. Compare and Contrast them.
	There are two popular wireless Internet access technologies today:
	a) Wifi(802.11) In a wireless LAN, wireless users transmit/receive packets to/from a base station (i.e, wireless access point) within a radius of few tens of meters. The base station is typically connected to the wired Internet and thus serves to connect wireless users to the wired network.
	b) 3G and 4G wide-area wireless access networks. In these systems, packets are transmitted over the same wireless infrastructure used for cellular telphones, with the base station thus being managed by a telecommunications provider. This provides wireless access to users within a radius of tens of kilometers of the base station.

10) Suppose there is exactly one packet switch being a sending host and a receiving host. The transmission rates between the sending hos and the switch and between the switch and the receiving host are R1 and R2, respectively. Assuming that the switch uses store-and-forward packet switching, what is the total end-to-end delay to send a packet of length L?(Ignore queing, propagation delay, and processing delay.)
	At time t0 the sending host begins to transmit. At time t1 = L/R1, the sending host completes transmission and the entire packet is received at the router(no propagation delay). Because the router has the entire packet at time t1, it can begin to transmit the packet to the receiving host at time t1. At time t2 = t1 + L/R2, the router completes transmission and the entire packet is received at the receiving host (again, no propagation delay). Thus, the end-to-end delay is L/R1 + L/R2

11) What advantage does a circuit-switched network have over a packet-switched network? What advantage 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. FDM requires sophistacted analog hardware to shift signal into appropriate frequency bands. Time division multiplexing (TDM) has an advantage over Frequency division multiplexing(FDM) as it gives bandwidth saving and there is low interference between multiplexed signals.
12) Supppose users share a 2 Mbps link. Also suppose each user transmits continuously at 1 Mbps when transmitting, but each user transmits only 20 percent of the time.
	a) When circuit switching is used, how many users can be supported?
		2 users can be supported because each user requires half of the link bandwidth.
	b) For the remainder of this problem, suppose packet switching is used. Why will there be essentially no quering delay before the link if two or fewer users transmit at the same time? Why will there be a queuing delay if three users transmit at the the same time?
		Since each users requires 1Mbps when transmitting, if two or fewers users transmit simultaneously, a maximum of 2Mbps will be required. Since the available bandwidth of the shared link is 2Mbps, there will be no queuing delay before the link. Whereas, if three users transmit simultaneously, the bandwidth required will be 3Mbps which is more than the available
		bandwidth of the shared link. In this case, there will be queuing delay before the link.
	c) Find the probability that a given user is transmitting.
		Probability that a gien user is transmitting = 0.2
	d) Suppose now there are three users. Find the probability that at any given time, all three users are transmitting simultaneously.
	   Find the fraction of time during which the queue grows.