SANs, LANs, MANs, and WANs

SANs, LANs, MANs, and WANs

Characterize Network

One way to characterize networks is according to their size. Two wellknown examples are LANs (local area networks) and WANs (wide area networks); the former typically extend less than 1 km, while the latter can be worldwide. Other networks are classified as MANs (metropolitan area networks), which usually span tens of kilometers. The reason such classifications are interesting is that the size of a network often has implications for the underlying technology that can be used, with a key factor being the amount of time it takes for data to propagate from one end of the network to the other; we discuss this issue more in later articles.

An interesting historical note is that the term “wide area network” was not applied to the first WANs because there was no other sort of network to differentiate them from. When computers were incredibly rare and expensive, there was no point in thinking about how to connect all the computers in the local area—there was only one computer in that area. Only as computers began to proliferate did LANs become necessary, and the term “WAN” was then introduced to describe the larger networks that interconnected geographically distant computers.

Another kind of network that we need to be aware of is SANs (system area networks). SANs are usually confined to a single room and connect the various components of a large computing system. For example, HiPPI (High Performance Parallel Interface) and Fiber Channel are two common SAN technologies used to connect massively parallel processors to scalable storage servers and data vaults. (Because they often connect computers to storage servers, SANs are sometimes defined as storage area networks.) Although this article does not describe such networks in detail, they are worth knowing about because they are often at the leading edge in terms of performance, and because it is increasingly common to connect such networks into LANs and WANs.

Bandwidth and Throughput

Bandwidth and 

Throughput

Bandwidth and throughput are two of the most confusing terms used in networking. While we could try to give you a precise definition of each term, it is important that you know how other people might use them and for you to be aware that they are often used interchangeably. First of all, bandwidth is literally a  measure of the width of a frequency band. For example, a voice-grade telephone line supports a frequency band ranging from 300 to 3300 Hz; it is said to have a bandwidth of 3300 Hz− 300 Hz = 3000 Hz. If you see the word “bandwidth” used in a situation in which it is being measured in hertz, then it probably refers to the range of signals that can be accommodated.

When we talk about the bandwidth of a communication link, we normally refer to the number of bits per second that can be transmitted on the link. We might say that the bandwidth of an Ethernet is 10 Mbps. A useful distinction might be made, however, between the bandwidth that is available on the link and the number of bits per second that we can actually transmit over the link in practice. We tend to use the word “throughput” to refer to the measured perfor- mance of a system. Thus, because of various inefficiencies of implementation, a pair of nodes connected by a link with a bandwidth of 10 Mbps might achieve a throughput of only 2 Mbps. This would mean that an application on one host could send data to the other host at 2 Mbps.

Finally, we often talk about the bandwidth requirements of an application—the number of bits per second that it needs to transmit over the network to perform acceptably. For some applications, this might be “whatever I can get”; for others, it might be some fixed number (preferably no more than the  available link bandwidth); and for others, it might be a number that varies with time. I will provide more on this topic later in this blog.

Keep reading^^….