Local Area Networks

The next step up is the LAN (Local Area Network). A LAN is a privately owned network that operates within and nearby a single building like a home, of-fice or factory. LANs are widely used to connect personal computers and consu-mer electronics to let them share resources (e.g., printers) and exchange informa-tion. When LANs are used by companies, they are called enterprise networks.

Wireless LANs are very popular these days, especially in homes, older office buildings, cafeterias, and other places where it is too much trouble to install cables. In these systems, every computer has a radio modem and an antenna that it uses to communicate with other computers. In most cases, each computer talks to a device in the ceiling as shown in Fig. 1-8(a). This device, called an AP (Access Point), wireless router, or base station, relays packets between the wireless computers and also between them and the Internet. Being the AP is like being the popular kid as school because everyone wants to talk to you. However, if other computers are close enough, they can communicate directly with one an-other in a peer-to-peer configuration.

There is a standard for wireless LANs called IEEE 802.11, popularly known as WiFi, which has become very widespread. It runs at speeds anywhere from 11

Ethernet switch

Ports To rest of

network To wired network

Access point

Figure 1-8. Wireless and wired LANs. (a) 802.11. (b) Switched Ethernet.

to hundreds of Mbps. (In this book we will adhere to tradition and measure line speeds in megabits/sec, where 1 Mbps is 1,000,000 bits/sec, and gigabits/sec, where 1 Gbps is 1,000,000,000 bits/sec.) We will discuss 802.11 in Chap. 4.

Wired LANs use a range of different transmission technologies. Most of them use copper wires, but some use optical fiber. LANs are restricted in size, which means that the worst-case transmission time is bounded and known in ad-vance. Knowing these bounds helps with the task of designing network protocols.

Typically, wired LANs run at speeds of 100 Mbps to 1 Gbps, have low delay (microseconds or nanoseconds), and make very few errors. Newer LANs can op-erate at up to 10 Gbps. Compared to wireless networks, wired LANs exceed them in all dimensions of performance. It is just easier to send signals over a wire or through a fiber than through the air.

The topology of many wired LANs is built from point-to-point links. IEEE 802.3, popularly called Ethernet, is, by far, the most common type of wired LAN. Fig. 1-8(b) shows a sample topology of switched Ethernet. Each com-puter speaks the Ethernet protocol and connects to a box called a switch with a point-to-point link. Hence the name. A switch has multiple ports, each of which can connect to one computer. The job of the switch is to relay packets between computers that are attached to it, using the address in each packet to determine which computer to send it to.

To build larger LANs, switches can be plugged into each other using their ports. What happens if you plug them together in a loop? Will the network still work? Luckily, the designers thought of this case. It is the job of the protocol to sort out what paths packets should travel to safely reach the intended computer.

We will see how this works in Chap. 4.

It is also possible to divide one large physical LAN into two smaller logical LANs. You might wonder why this would be useful. Sometimes, the layout of the network equipment does not match the organization’s structure. For example, the

SEC. 1.2 NETWORK HARDWARE 21 engineering and finance departments of a company might have computers on the same physical LAN because they are in the same wing of the building but it might be easier to manage the system if engineering and finance logically each had its own network Virtual LAN or VLAN. In this design each port is tagged with a

‘‘color,’’ say green for engineering and red for finance. The switch then forwards packets so that computers attached to the green ports are separated from the com-puters attached to the red ports. Broadcast packets sent on a red port, for example, will not be received on a green port, just as though there were two different LANs. We will cover VLANs at the end of Chap. 4.

There are other wired LAN topologies too. In fact, switched Ethernet is a modern version of the original Ethernet design that broadcast all the packets over a single linear cable. At most one machine could successfully transmit at a time, and a distributed arbitration mechanism was used to resolve conflicts. It used a simple algorithm: computers could transmit whenever the cable was idle. If two or more packets collided, each computer just waited a random time and tried later.

We will call that version classic Ethernet for clarity, and as you suspected, you will learn about it in Chap. 4.

Both wireless and wired broadcast networks can be divided into static and dynamic designs, depending on how the channel is allocated. A typical static location would be to divide time into discrete intervals and use a round-robin al-gorithm, allowing each machine to broadcast only when its time slot comes up.

Static allocation wastes channel capacity when a machine has nothing to say dur-ing its allocated slot, so most systems attempt to allocate the channel dynamically (i.e., on demand).

Dynamic allocation methods for a common channel are either centralized or decentralized. In the centralized channel allocation method, there is a single enti-ty, for example, the base station in cellular networks, which determines who goes next. It might do this by accepting multiple packets and prioritizing them accord-ing to some internal algorithm. In the decentralized channel allocation method, there is no central entity; each machine must decide for itself whether to transmit.

You might think that this approach would lead to chaos, but it does not. Later we will study many algorithms designed to bring order out of the potential chaos.

It is worth spending a little more time discussing LANs in the home. In the future, it is likely that every appliance in the home will be capable of communi-cating with every other appliance, and all of them will be accessible over the In-ternet. This development is likely to be one of those visionary concepts that nobody asked for (like TV remote controls or mobile phones), but once they arrived nobody can imagine how they lived without them.

Many devices are already capable of being networked. These include com-puters, entertainment devices such as TVs and DVDs, phones and other consumer electronics such as cameras, appliances like clock radios, and infrastructure like utility meters and thermostats. This trend will only continue. For instance, the average home probably has a dozen clocks (e.g., in appliances), all of which could

adjust to daylight savings time automatically if the clocks were on the Internet.

Remote monitoring of the home is a likely winner, as many grown children would be willing to spend some money to help their aging parents live safely in their own homes.

While we could think of the home network as just another LAN, it is more likely to have different properties than other networks. First, the networked de-vices have to be very easy to install. Wireless routers are the most returned con-sumer electronic item. People buy one because they want a wireless network at home, find that it does not work ‘‘out of the box,’’ and then return it rather than listen to elevator music while on hold on the technical helpline.

Second, the network and devices have to be foolproof in operation. Air con-ditioners used to have one knob with four settings: OFF, LOW, MEDIUM, and HIGH. Now they have 30-page manuals. Once they are networked, expect the chapter on security alone to be 30 pages. This is a problem because only com-puter users are accustomed to putting up with products that do not work; the car-, television-, and refrigerator-buying public is far less tolerant. They expect pro-ducts to work 100% without the need to hire a geek.

Third, low price is essential for success. People will not pay a $50 premium for an Internet thermostat because few people regard monitoring their home tem-perature from work that important. For $5 extra, though, it might sell.

Fourth, it must be possible to start out with one or two devices and expand the reach of the network gradually. This means no format wars. Telling consumers to buy peripherals with IEEE 1394 (FireWire) interfaces and a few years later retracting that and saying USB 2.0 is the interface-of-the-month and then switch-ing that to 802.11g—oops, no, make that 802.11n—I mean 802.16 (different wire-less networks)—is going to make consumers very skittish. The network interface will have to remain stable for decades, like the television broadcasting standards.

Fifth, security and reliability will be very important. Losing a few files to an email virus is one thing; having a burglar disarm your security system from his mobile computer and then plunder your house is something quite different.

An interesting question is whether home networks will be wired or wireless.

Convenience and cost favors wireless networking because there are no wires to fit, or worse, retrofit. Security favors wired networking because the radio waves that wireless networks use are quite good at going through walls. Not everyone is overjoyed at the thought of having the neighbors piggybacking on their Internet connection and reading their email. In Chap. 8 we will study how encryption can be used to provide security, but it is easier said than done with inexperienced users.

A third option that may be appealing is to reuse the networks that are already in the home. The obvious candidate is the electric wires that are installed throughout the house. Power-line networks let devices that plug into outlets broadcast information throughout the house. You have to plug in the TV anyway, and this way it can get Internet connectivity at the same time. The difficulty is

SEC. 1.2 NETWORK HARDWARE 23 how to carry both power and data signals at the same time. Part of the answer is that they use different frequency bands.

In short, home LANs offer many opportunities and challenges. Most of the latter relate to the need for the networks to be easy to manage, dependable, and secure, especially in the hands of nontechnical users, as well as low cost.