A View of the LANscape
An essential element of any organization's data-processing operation is a local area network (LAN). A LAN is needed to interconnect equipment on the user's premises and to provide a means to efficiently connect to outside services and other corporate sites. Over the years, a wide variety of technologies and configurations have been turned into products, making life difficult for managers and network administrators. Fortunately, in recent years, a small group of LAN approaches have come to dominate the market, making life much easier for those who have to select and maintain LANs. In this article, we look at the key product types on the LAN scene today:
IEEE 802.11 Wireless LAN
The most widely used LANs today are based on Ethernet, a scheme that has been around for about 25 years. An experimental, 3 Mbps version was developed in the mid-1970s by Xerox. Then, in 1980, Xerox, Intel, and Digital Equipment Corporation jointly issued a specification for a 10 Mbps Ethernet system and freely licensed the technology. Thus, the commercial version of Ethernet was born, and it quickly came to dominate the marketplace. By the mid-1980s, a standard for Ethernet was issued by the IEEE 802.3 working group. As explained in another article in this series, the IEEE 802 committee was set up to develop a range of standards for LANs; the 802.3 standard differs in minor ways from the Xerox specification, but is also referred to as Ethernet. The 802.3 working group quickly became—and remains—the most important part of the IEEE 802 effort.
The original commercial Ethernet operates over a bus topology LAN using coaxial cable. In a bus topology LAN, all stations attach, through appropriate hardware interfacing known as a tap, directly to a linear transmission medium, or bus. Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus. A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. At each end of the bus is a terminator, which absorbs any signal, removing it from the bus.
Moving to Higher Speeds
Although Ethernet quickly became the dominant force in the marketplace, because of its simplicity and robustness, the shortcomings of a 10 Mbps capacity became apparent. Personal computers and microcomputer workstations began to achieve widespread acceptance in business computing in the early 1980s and have now achieved virtually the status of the telephone: an essential tool for office workers. Until relatively recently, office LANs provided basic connectivity services—connecting personal computers and terminals to mainframes and midrange systems that ran corporate applications, and providing workgroup connectivity at the departmental or divisional level. In both cases, traffic patterns were relatively light, with an emphasis on file transfer and electronic mail. The LANs that were available for this type of workload, primarily Ethernet and token ring, are well suited to this environment.
In recent years, two significant trends have altered the role of the personal computer and therefore the requirements on the LAN:
The speed and computing power of personal computers has continued to enjoy explosive growth. Today's more powerful platforms support graphics-intensive applications and ever more elaborate graphical user interfaces for the operating system.
MIS organizations have recognized the LAN as a viable and indeed essential computing platform, resulting in the focus on network computing. This trend began with client/server computing, which has become a dominant architecture in the business environment and the more recent intranetwork trend. Both of these approaches involve the frequent transfer of potentially large volumes of data in a transaction-oriented environment.
The effect of these trends has been to increase the volume of data to be handled over LANs and, because applications are more interactive, to reduce the acceptable delay on data transfers. The earlier generation of 10 Mbps Ethernets and 16 Mbps token rings are simply not up to the job of supporting these requirements.
The following are examples of requirements that call for higher-speed LANs:
Centralized server farms. In many applications, there is a need for user or client systems to be able to draw huge amounts of data from multiple centralized servers, called server farms. An example is a color publishing operation, in which servers typically contain tens of gigabytes of image data that must be downloaded to imaging workstations. As the performance of the servers themselves has increased, the bottleneck has shifted to the network. Switched Ethernet alone would not solve this problem because of the limit of 10 Mbps on a single link to the client.
Power workgroups. These groups typically consist of a small number of cooperating users who need to draw massive data files across the network. Examples are a software development group that runs tests on a new software version, or a computer-aided design (CAD) company that regularly runs simulations of new designs. In such cases, large amounts of data are distributed to several workstations, processed, and updated at very high speed for multiple iterations.
High-speed local backbones. As processing demand grows, LANs proliferate at a site, and high-speed interconnection is necessary.
Ethernet Meets the Challenge
In response to the need for greater speed, in the mid-1990s the IEEE 802.3 committee introduced a 100 Mbps version known as Fast Ethernet. Unlike the traditional bus arrangement of classical Ethernet, Fast Ethernet uses a hub or switch to achieve greater performance.
To clarify the distinction between buses, hubs, and switches, Figure 1a shows a typical bus layout of a traditional 10 Mbps Ethernet. In this configuration, all the stations must share the total capacity of the bus, which is 10 Mbps.
In a hub, there is a central point to which wires are strung from individual stations on the LAN. In this arrangement, a transmission from any one station is received by the hub and retransmitted on all of the outgoing lines. Again, the total capacity of the LAN is the same as that of the access lines from each station, namely 10 Mbps. The hub has several advantages over the simple bus arrangement. It exploits standard building wiring practices in the layout of cable. In addition, the hub can be configured to recognize a malfunctioning station that's jamming the network, and to cut that station out of the network. Figure 1b illustrates the operation of a shared-medium hub. Here again, station B is transmitting. This transmission goes from B across the transmit line from B to the hub, and from the hub along the receive lines of each of the other attached stations.
We can achieve greater performance with a LAN switch. In this case, the central element acts as a switch, similar to a packet switch. An incoming frame from a particular station is switched to the appropriate output line to be delivered to the intended destination. At the same time, other unused lines can be used for switching other traffic. Figure 1c shows an example in which B is transmitting a frame to A and at the same time C is transmitting a frame to D. In this example, the current throughput on the LAN is 20 Mbps, although each individual device is limited to 10 Mbps.
The LAN switch has several attractive features:
No change is required to the software or hardware of the attached devices to convert a bus LAN or a hub LAN to a switched LAN. In the case of an Ethernet LAN, each attached device continues to use the Ethernet medium access-control protocol to access the LAN. From the point of view of the attached devices, nothing has changed in the access logic.
Each attached device has a dedicated capacity equal to that of the entire original LAN, assuming that the layer 2 switch has sufficient capacity to keep up with all attached devices. For example, in Figure 1c, if the layer 2 switch can sustain a throughput of 20 Mbps, each attached device appears to have a dedicated capacity for either input or output of 10 Mbps.
The layer 2 switch scales easily. Additional devices can be attached to the layer 2 switch by increasing the capacity of the layer 2 switch correspondingly.
Using the switch technology, the IEEE 802.3 committee has now gone on to introduce 1 Gbps Ethernet LANs and is working on 10 Gbps LANs. All of these options are discussed in another article in this series.