The initial focus of this chapter is on the most significant issue to solve in the MAC sublayer of LANs and MANs: how to allow a given number of stations to share a common channel to communicate. Among the different solutions that can be devised, the answer successfully adopted by current LANs is introduced and critically commented on.

Such treatment is followed by a description of the general IEEE framework where LAN and MAN standards sit. There finds its place the standard for the implementation of Ethernet LANs, also known as the IEEE 802.3 standard, whose physical layer and MAC sublayer specifications are faced next. An adequate coverage is provided for the admissible topologies, MAC frame format and main algorithms; the evolution that this type of LAN has undergone in the last years is also illustrated, from the original solutions to fast and current gigabit Ethernet networks.

What follows is a description of the reference standards that modern cabling systems have to conform to. The intent is twofold: to make the reader understand (i) how crucial the cabling system is in guaranteeing a successfully operative network; (ii) how important its physical properties are when striving to support higher and higher data rates.

The chapter is closed by a few dense practices, which aim at explaining several real-world issues: the relation between layer-3 and layer-2 addressing schemes; the configuration of the hardware device that allows the station to connect to the LAN and to the Internet; and the layout of the cabling system for a campus network.

This chapter takes the reader into the modern world of computer networks: it first illustrates the main standardization bodies for telecommunications and data networking, then it exposes the general principles that underlie communications in a packet-switched network and describes the features of the main protocols ruling the proper functioning of the Internet.

An adequate, thorough treatment is devoted to the Internet protocol (IP), as its functionalities are the basis on which the entire Internet communication model relies: the original classful and the current classless addressing schemes are presented, followed by the subnetting and supernetting procedures IP allows. Direct and indirect datagram delivery is also introduced. Every topic is complemented by some explanatory examples, whose aim is immediately to translate into useful, working cases, the theoretical concepts.

The chapter is closed by two sections that present the salient attributes of the transmission control protocol (TCP) and of the user datagram protocol (UDP).

Who's who in the telecommunication world

It is appropriate to place in this opening section a brief, yet meaningful overview of the most representative bodies within the telecommunication and networking arena, and of their role. As the topics covered in the next chapters will quantitatively testify, the networking field heavily relies upon standards: they are needed to guarantee for systems and services not only quality, safety, reliability and efficiency, but also interoperability, a magic word when the ultimate objective is to “connect.”

In the previous chapters the most popular solutions for wired and wireless local area networks have been presented. Signal propagation impairments, on one side, and the multiple access technique adopted by the MAC, on the other, contribute to limit both the maximum distance that a LAN is capable of covering and its achievable performance.

However, the need to deploy LANs spanning larger areas, like an entire enterprise or university campus, has pushed toward the implementation of specific devices that allow these limits to be circumvented: repeaters, bridges and switches [65]. Such devices are also used to implement high-performance and flexible cabling schemes, according to the most up-to-date standard requirements.

This chapter describes these commonly used LAN devices, and then discusses a widely accepted solution for traffic isolation using virtual LANs. The chapter ends with a few practical examples of LAN device management and virtual LAN configuration.

Repeaters and bridges

It has already been mentioned that signal quality degrades with distance, so that a maximum span exists, beyond which a receiver cannot correctly recover the associated information.

This constraint can be bypassed using a repeater, an active device that links two or more LAN segments, with the goal of extending the physical connection between two LAN stations. A repeater amplifies a signal received on any input port and repeats it to all output ports. A repeater decodes incoming bit streams, re-encodes and resynchro-nizes them before retransmission. A repeater can then be classified as a layer-1 device. Regarding the synchronism regeneration, a repeater achieves it either by buffering part of the frame and then adding a delay, or by cutting some header bits for the time required to get resynchronized.

In the previous chapters, a few network devices have been presented. Repeaters are layer-1 devices that perform functions related to physical signal processing. Bridges are layer-2 devices that are able to detect frames and interpret their fields, such as MAC addresses. A switch is a multiport bridge, still a layer-2 device, but with high frame-processing performance.

Even if LAN interconnections in wide areas may be done by using bridges, and until the early 1990s this was quite common, the best way for managing wide area connections is through layer-3 devices, usually called routers. Routers perform all functions from layer-1 up to layer-3: they can process packets and are the main devices for network layer management. Indeed, routers represent the most important intermediate systems used to interconnect heterogeneous networks on a geographical scale, building a consistent and cooperative complex system commonly known as an internet.

This chapter illustrates the main concepts behind IP-based routers, discussing the most important functions performed and describing the most common architectures available. Some details of the IP packet processing and forwarding operations are provided, followed by the description of more advanced network layer functionalities, such as packet filtering and address translation. The chapter is concluded by a few practical configuration examples.

What is a router?

A possible way to start presenting routers is to try to compare them with bridges [65]. A bridge physically connects different LANs but, actually, they can be logically considered the same network, with the same network rules and the same network protocol. A router, on the other hand, completely separates LANs and networks, both physically and logically.

The intent of this chapter is to complement the previous discussion on wired LANs, providing a thorough picture of the technologies and standards available today to build wireless network infrastructures. These aim to cover a wide range of areas successfully, spanning from personal to local and metropolitan coverage solutions.

The impressive pace of technological advances of the last decade has acquainted us with technologies that replace cable over very short distances, as testified by palm computers, cellular headsets and plenty of technological “toys” equipped with Bluetooth ports. Wireless LANs have an unprecedented popularity too, rapidly pervading private houses and enterprises; in metropolitan areas, municipalities are also getting into the game, supporting projects of wireless connectivity for communities that would otherwise be confined to low-speed Internet access. This all started with some impressive – at the time – 1 and 2 Mbit/s raw transmission rates that the first Wireless LAN standard guaranteed: current products achieve 54 Mbit/s of raw throughput, rates of 250 Mbit/s are on the way, and the specifications for wireless MAN head to even higher data rates, extended coverage and mobility support.

The logical organization of the chapter reflects the colloquial description above: the first section is devoted to wireless LANs and to the corresponding set of specifications, belonging to the IEEE 802.11 family of standards; the second section deals with the issue of fixed – and mobile – radio broadband access that wireless MANs have to provide. For the sake of completeness, the next sections rapidly look at wireless personal area networks (WPANs) and wireless mesh networks. The chapter is closed by some practical – and crucial – considerations on traffic monitoring and security issues in wireless networks.

This is the chapter where the reader first approaches the world of networks. The preliminary step to take consists in providing a succinct introduction to the analysis of the signals that, either in the electrical or in the optical domain, carry data information across networks.

Also, before throwing onto the floor the first Lego bricks and the instructions to let the reader build the composite view of modern computer networks, a review of the “fabric” of these bricks is needed. A light excursus is thus provided, to gain a basic acquaintance with copper wires, fiber optics and the radio channel, each transmission medium being employed in specific networking contexts.

Network classification comes next: it is a useful exercise to gain familiarity with the main ideas and the terminology peculiar to the world of networks. This is followed by the introduction of the crucial notion of delay, as well as by a miscellanea of concepts that cross different areas, spanning from sources of network traffic to service classification, from performance metrics to quality of service.

Following a pattern that will shape the exposition throughout the entire book, a direct experience is proposed and commented at the end of the chapter, to let the reader confront the real world via a first-hand adventure.

Signals: time and frequency analysis

Indeed, our aim is to talk about networks. Before doing so, however, we have to recall the fundamentals of signal analysis, to understand what type of information we are moving around.

This final chapter is dedicated to a brief overview of some of the most important solutions used by network operators, on the one hand, for providing access to their customers, on the other, as wide area network (WAN) technologies are adopted in the network core. The chapter starts with an overview of the xDSL family, one of the most popular access technologies adopted today. Then a historical perspective of WAN technologies is given, going from the X.25 and ISDN solutions to the more recent frame relay service and ATM protocol. The attempt made by IETF to join the flexibility of the connectionless IP world with the advantages of the connection-oriented ATM approach is, finally, the basis for introducing the principles of MPLS, a very popular technology currently used by operators to offer enhanced IP-based connectivity services to their customers, including quality of service guarantees and tunneling capabilities. Given the importance of the last topic, the chapter closes with a section dedicated to a practical example of MPLS router configuration.

The xDSL family

The xDSL acronym refers to a set of solutions and technologies developed for supporting the transmission of high speed data in existing copper access networks. It also supports data transmissions at long distances but at lower bit rates.

The ITU-T recommendation G.995.1 [91] provides an overview of the digital subscriber line family. In the following ADSL, with the recent ADSL2 and ADSL2+, HDSL, SHDSL, VDSL and VDSL2 will be presented.

In Chapter 6, the main functions and architectures of routers have been presented and discussed. Routers are the basic active devices employed in networks and their fundamental task is to forward packets from node to node toward their final destination, taking the most adequate decisions concerning the network path to follow [65]. More specifically, routers perform three main functions: routing table creation and update, table look-up for forwarding decision and physical datagram forwarding from input ports to output ports.

The first of these functions is called routing and consists of running algorithms and implementing protocols suitable to the specific routing problem and network topology size [81].On the one hand, routing algorithms are executed to create and maintain routing tables, once sufficient information about the network topology has been gathered. On the other hand, routing protocols are employed between routers to exchange the network topology information necessary for executing the algorithms.

This chapter illustrates the basic principles of routing, describing the most important algorithms and protocols. The hierarchical approach to the routing problem adopted by the Internet is also discussed, including a brief overview of the most popular IP routing protocols, namely RIP and OSPF. A couple of practical examples of IP routing configurations conclude the chapter.

Routing algorithms

Routing algorithms are characterized by the way the routing table is created and possibly updated. Generally speaking, such an algorithm should be correct, fair, stable, optimal, fast, simple, and adaptive to network topology changes and to traffic conditions. It is evidently difficult to meet all these requirements with just a single, ideal algorithm, so any real routing algorithm usually provides a trade-off on some of them.

The topics this book touches lie within the networking field, an exciting area that in the last 20 years has experienced a stunning growth and gained an increasing popularity. Just as previous ages of modern society have been marked by technological advancements that significantly shaped them, from transistors to personal computers, our life is now molded by emails, our work and leisure time clocked by websites, our children daily accompanied by the Internet. What lies behind this boiling surface? What infrastructures and communication rules allow us to connect to the Internet from home through an ADSL connection? How does information travel on a high-speed backbone from our office to an overseas destination? Through a rigorous yet practical approach, the aim of this volume is to provide all the concepts needed for a thorough knowledge of networking technologies, as well as to breed the development of agile skills in modern network design.

After laying the common language foundations and the basic concepts and terminology within the field, the book is committed to a critical treatment of the technologies, protocols and devices adopted in contemporary networks. A special emphasis is placed on building an effective competence in all subject areas: hence, each topic is complemented by guided and commented practices, where proficiencies are challenged through real problems. The aim is to strengthen the abilities needed for present-day network design. The chapter structure reflects the authors’ pedagogical view: first build good foundations and gain expertise in each topic, then consolidate and confront real networking issues.