In this chapter we look at various basic issues that arise in communication over fading channels. We start by analyzing uncoded transmission in a narrowband fading channel. We study both coherent and non-coherent detection. In both cases the error probability is much higher than in a non-faded AWGN channel. The reason is that there is a significant probability that the channel is in a deep fade. This motivates us to investigate various diversity techniques that improve the performance. The diversity techniques operate over time, frequency or space, but the basic idea is the same. By sending signals that carry the same information through different paths, multiple independently faded replicas of data symbols are obtained at the receiver end and more reliable detection can be achieved. The simplest diversity schemes use repetition coding. More sophisticated schemes exploit channel diversity and, at the same time, efficiently use the degrees of freedom in the channel. Compared to repetition coding, they provide coding gains in addition to diversity gains. In space diversity, we look at both transmit and receive diversity schemes. In frequency diversity, we look at three approaches:

• single-carrier with inter-symbol interference equalization,

• direct-sequence spread-spectrum,

• orthogonal frequency division multiplexing.

Finally, we study the impact of channel uncertainty on the performance of diversity combining schemes. We will see that, in some cases, having too many diversity paths can have an adverse effect due to channel uncertainty.

In Chapters 8 and 9, we have studied the role of multiple transmit and receive antennas in the context of point-to-point channels. In this chapter, we shift the focus to multiuser channels and study the role of multiple antennas in both the uplink (many-to-one) and the downlink (one-to-many). In addition to allowing spatial multiplexing and providing diversity to each user, multiple antennas allow the base-station to simultaneously transmit or receive data from multiple users. Again, this is a consequence of the increase in degrees of freedom from having multiple antennas.

We have considered several MIMO transceiver architectures for the point-to-point channel in Chapter 8. In some of these, such as linear receivers with or without successive cancellation, the complexity is mainly at the receiver. Independent data streams are sent at the different transmit antennas, and no cooperation across transmit antennas is needed. Equating the transmit antennas with users, these receiver structures can be directly used in the uplink where the users have a single transmit antenna each but the base-station has multiple receive antennas; this is a common configuration in cellular wireless systems.

It is less apparent how to come up with good strategies for the downlink, where the receive antennas are at the different users; thus the receiver structure has to be separate, one for each user. However, as will see, there is an interesting duality between the uplink and the downlink, and by exploiting this duality, one can map each receive architecture for the uplink to a corresponding transmit architecture for the downlink.

A good understanding of the wireless channel, its key physical parameters and the modeling issues, lays the foundation for the rest of the book. This is the goal of this chapter.

A defining characteristic of the mobile wireless channel is the variations of the channel strength over time and over frequency. The variations can be roughly divided into two types (Figure 2.1):

• Large-scale fading, due to path loss of signal as a function of distance and shadowing by large objects such as buildings and hills. This occurs as the mobile moves through a distance of the order of the cell size, and is typically frequency independent.

• Small-scale fading, due to the constructive and destructive interference of the multiple signal paths between the transmitter and receiver. This occurs at the spatial scale of the order of the carrier wavelength, and is frequency dependent.

We will talk about both types of fading in this chapter, but with more emphasis on the latter. Large-scale fading is more relevant to issues such as cell-site planning. Small-scale multipath fading is more relevant to the design of reliable and efficient communication systems – the focus of this book.

We start with the physical modeling of the wireless channel in terms of electromagnetic waves. We then derive an input/output linear time-varying model for the channel, and define some important physical parameters. Finally, we introduce a few statistical models of the channel variation over time and over frequency.

Objectives

• Comparison of router and bridge.

• IP forwarding.

• Use of ICMP messages in routing.

• The Routing Information Protocol (RIP).

• The Open Shortest Path First (OSPF) protocol.

• Static routing by manually building the routing tables in the routers and hosts.

• Use of Traceroute to find an end-to-end route.

Static and dynamic routing

Routing is the act of transferring packets from a source to a destination using network layer protocol information. It involves two activities, determining optimal routing paths and transporting packets through an internetwork. The key to these two activities is the routing table maintained in each host and router. The routing table records optimal routes and is consulted when a forwarding decision is to be made for an arriving packet. The routing table can be manually set, updated by an ICMP message received, or by routing daemons implementing dynamic routing protocols.

Next-hop routing

When a host needs to send a packet to a destination, it uses its own netmask and the IP address of the destination host to find out the network and subnet ID (or the extended prefix) of the destination.

Objectives

• The Cisco Internet Operating System software.

• Configuring a Cisco router.

• Transparent bridge configuration and operation.

• The spanning tree algorithm.

Ethernet bridges

Use of bridges

Bridges are link layer devices. As illustrated in Fig. 0.3, when two network segments with different link and physical layer protocols are connected, the bridge performs a two-way translation of the protocols. The data section of a transit frame is extracted and re-encapsulated in the frame format used by the next-hop network segment.

However, there are several reasons to use bridges to connect networks even with identical protocols, rather than using a large network without bridges. First, network segments could be far away from each other but still work within the same logical network. In this case, two remote network segments can be linked by two bridges via a point-to-point wide area link. Second, there is a limit on the maximum length of the shared medium in a local network.

Lab operation mechanism

This guide is based on a lab course offered at Polytechnic University, Brooklyn, New York, USA. The course personnel were a faculty member who delivered lectures on TCP/IP protocols and two lab instructors who were responsible for the setup and maintenance of the lab, and assisting students during experiments. Each experiment, consisting of the exercises in one chapter of this book, typically took the students two to three hours to complete.

Due to the limitation of space and equipment, the whole class was divided into small groups of eight (i.e., the number of workstations in the lab). The groups were scheduled to use the lab in different time slots. In the labs, each student was assigned a workstation and was required to perform the experiments independently. If required, two or more students could share a workstation and perform the experiments together.

As the title of this book suggests, this book is a minimalist approach to teaching TCP/IP using laboratory-based experiments. It is minimalist in that it provides one, possibly idiosyncratic, choice of topics at a depth we felt was sufficient to learn the basics of TCP/IP. The intention was not to write a reference text on the subject. The laboratory was important in giving students the experience of observing the TCP/IP protocols in action. The act of observing and drawing some conclusions from those observations, brings to life the often dry study of network protocols, and motivates students to learn more about them.

Appendix A is necessary reading only for the instructor who is in charge of setting up the lab. We have attempted to keep costs down so that only the most Scrooge-like University administrator would raise an eyebrow over the cost of the lab equipment (as for lab space, that may be another matter!). We assume that the students have a basic background in networking, perhaps from a previous course, or perhaps as part of a course that back loads the experiments in this book after providing a general lecture-based introduction to networks.

The Internet

The Internet is a global information system consisting of millions of computer networks around the world. Users of the Internet can exchange email, access to the resources on a remote computer, browse web pages, stream live video or audio, and publish information for other users. With the evolution of e-commerce, many companies are providing services over the Internet, such as on-line banking, financial transactions, shopping, and online auctions. In parallel with the expansion in services provided, there has been an exponential increase in the size of the Internet. In addition, various types of electronic devices are being connected to the Internet, such as cell phones, personal digital assistants (PDA), and even TVs and refrigerators.

Today's Internet evolved from the ARPANET sponsored by the Advanced Research Projects Agency (ARPA) in the late 1960s with only four nodes. The Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite, first proposed by Cerf and Kahn in [1], was adopted for the ARPANET in 1983. In 1984, NSF funded a TCP/IP based backbone network, called NSFNET, which became the successor of the ARPANET. The Internet became completely commercial in 1995.

Objectives

• TCP connection establishment and termination.

• TCP timers.

• TCP timeout and retransmission.

• TCP interactive data flow, using telnet as an example.

• TCP bulk data flow, using sock as a traffic generator.

• Further comparison of TCP and UDP.

• Tuning the TCP/IP kernel.

• Study TCP flow control, congestion control, and error control using DBS and NIST Net.

TCP service

TCP is the transport layer protocol in the TCP/IP protocol family that provides a connection-oriented, reliable service to applications. TCP achieves this by incorporating the following features.

• Error control: TCP uses cumulative acknowledgements to report lost segments or out of order reception, and a time out and retransmission mechanism to guarantee that application data is received reliably.

• Flow control: TCP uses sliding window flow control to prevent the receiver buffer from overflowing.

• Congestion control: TCP uses slow start, congestion avoidance, and fast retransmit/fast recovery to adapt to congestion in the routers and achieve high throughput.

The TCP header, shown in Fig. 0.16, consists of fields for the implementation of the above functions. Because of its complexity, TCP only supports unicast, while UDP, which is much simpler, supports both unicast and multicast. TCP is widely used in internet applications, e.g., the Web (HTTP), email (SMTP), file transfer (FTP), remote access (telnet), etc.