By Jim Geier
December 11, 2002
Wireless LANs are not totally wire free. Learn important aspects of the distribution system, which provides interconnectivity among access points.
The majority of wireless LANs requires more than one access point to supply adequate radio frequency (RF) signal coverage throughout a facility. To enable roaming between multiple access points and connections to wired network resources, the 802.11 standard specifies a distribution system, which provides wired interconnections between access points. The 802.11 standard says that the distribution system may be of any technology, such as Ethernet, token ring, or any other network type. The majority of actual installations, however, utilize Ethernet.
Ethernet, defined by the IEEE 802.3 standard, has been evolving over two decades. The IEEE 802 group first began development of it in February 1980 (hence “802”). The 802 group actually established several competing wired LAN standards, which are Ethernet (802.3), token bus (802.3), and token ring (802.5). The Ethernet standard has become the basis of most LAN deployments for many years.
The initial versions of 802.3 specify 10 Megabits per second (Mbps) data rates over coaxial cable, twisted pair wire and optical fiber. Newer versions of 802.3 extend data rates to 100Mbps and 1 Gigabit per second (Gbps). The 802.3 MAC Layer uses carrier sense multiple access (CSMA), which is also the heart of the 802.11 protocol.
An 802.3-based distribution system (also referred to as the “wired backbone”) consists of switches or hubs that tie together users (PCs and access points) equipped with 802.3 network interface cards (NICs). The switch or hub is somewhat analogous to an 802.11 access point. The main difference, obviously, is that the hub or switch provides the connections over a physical medium and an access point uses radio waves.
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A hub offers a single collision domain among multiple wired users. When one user’s Ethernet NIC sends data, all other stations connected to the LAN will hold off sending data until the medium is idle. A traditional access point most closely resembles a hub.
A switch is more sophisticated than a hub and connects one user to another without blocking access of other users. The switch improves throughput because of the smaller resulting collision domains. Users don’t have to wait until others are finished before sending data.
In a wireless LAN, the switch or hub connects access points together. This creates a wired backbone enabling wireless LAN roaming protocols to work. Most access points accommodate connection to a switch or hub via an RJ-45 connector and twisted pair (Category 5) wiring. The Cat5 cable can be up to 100 meters (roughly 300 feet) long. As a result, you need to plan the installation of hubs or switches to avoid exceeding this distance. If distances exceed the 100 meters, you can interconnect switches via optical fiber and place switches close enough to access points in various parts of the facility.
Distribution System Deployment Tips
Consider the following when deploying a distribution system for a wireless LAN:
- Use a hub for smaller deployments. If the wireless LAN only consists of a couple of access points, then you can probably get by with a hub. There’s no need to pay extra money for a switch for smaller deployments in homes and small offices. In fact, DSL and cable modem interfaces generally come equipped with a built-in, four-port Ethernet hub.
- Utilize switches for enterprise-wide deployments. A larger wireless LAN with many access points will benefit from the use of Ethernet switches. For very large networks, consider implementing a master switch that interconnects a series of smaller switches. Connect the switches together using optical fiber to improve security and increase the range of the distribution system.
- Select the appropriate data rate. In most cases, 10Mbps Ethernet will suffice to support interconnections among 802.11b access points. Ethernet provides adequate throughput to handle the corresponding packet transfer rates. Use 100Mbps Ethernet switches, however, as the distribution system for 802.11a-based wireless LANs.
- Maximize the use of Power over Ethernet (PoE). In order to save considerable time and money when installing electrical wiring for supplying power to access points, always consider the use of PoE equipment as part of the distribution system. This also makes it possible to more effectively control power distribution in a way that increases the security of the network. For example, management software can automatically shut off the power to the access points during off-hours or when a breach of security occurs.
- Create a separate IP domain for the wireless LAN. Some network devices continuously send broadcast packets that propagate freely throughout Ethernet networks. Access points will also forward these broadcast packets to all users on the wireless LAN. In many enterprise scenarios, the broadcast packets will flood the wireless LAN and severely limit the performance for wireless users. So separate the wireless LAN from the rest of the corporate network through a router or separate virtual LAN (VLAN).
Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs and offers computer-based training (CBT) courses on wireless LANs.