Wireless networks use radio waves to transmit data and connect devices to the Internet as well as to other networks and applications, which minimizes the need for wired connections. Although a wireless network allows users to access network resources ‘over-the-air’, it is important keep in mind that wireless traffic also traverses the physical wired infrastructure. Therefore, it is imperative to remember that Wireless Local Area Networks are meant to augment, rather than replace, the wired LAN campus infrastructure. This augmentation allows for a flexible data communication system within the enterprise network.
IEEE 802.11 Components
The 802.11 architecture is comprised of several logical and physical components. The 802.11 components that are described in this section are:
- Client or Station (STA)
- Access Point (AP)
- Independent Basic Service Set (IBSS)
- Basic Service Set (BSS)
- Extended Service Set (ESS)
- Distribution System (DS)
The client or station (STA) refers to any appliance that interfaces with the wireless medium and operates as an end user device. The STA contains an adapter card, PC Card, or an embedded device to provide wireless connectivity. Some common examples of STAs include laptop computers, desktop computers, and PDAs with wireless network interface cards.
The wireless Access Point functions as a bridge between the wireless STAs and the existing network backbone for network access. APs serve as the central points in an all-wireless network, or as the connection point between wired and wireless networks. When APs are used in a wireless network, any STA attempting to use the wireless network must first establish membership, or an association, with the Access Point.
An Independent Basic Service Set is a wireless network, consisting of at least two STAs, used where no access to a Distribution System is available. The Distribution System (DS) will be described later in this section. An IBSS is sometimes referred to as an independent configuration or as an ad hoc wireless network. From a logical perspective, an IBSS is very similar to a peer-to-peer network in which no one node performs any server functions.
The 802.11 WLAN infrastructure architecture is based on a cellular architecture which divides the system into cells, referred to as a Basic Service Set (BSS). The BSS is controlled by a Base Station, or more commonly, an Access Point (AP). The cell is restricted to the Access Points coverage area. Clients, or stations, within the cell can then associate themselves with the AP, allowing them to use the wireless LAN.
Access Points may be interconnected using the switched network, creating what is referred to as an Extended Service Set (ESS). The ESS is comprised of overlapping BSS sets (cells) that are usually connected together by some wired medium (Distribution System). In most cases, the ESS allows stations to roam. Roaming is the process of moving from one cell (BSS) to another without losing the wireless connection.
And finally, the Distribution System (DS) allows for the interconnection of the APs of multiple cells (BSSs). This allows for mobility, because STAs can move from one BSS to another BSS. Although the Distribution System could be any type of network, it is almost always a wired Ethernet LAN. However, it should be noted that it is also possible for Access Points to be interconnected without using wires. The three types of DS are integrated, wired, and wireless.
IEEE 802.11 Frames
WLANs are defined in the IEEE 802.11 standards. The IEEE 802 standards define two separate layers for the Data Link layer (Layer 2) of the OSI Reference Model. These two layers are the Logical Link Control (LLC) and Media Access Control (MAC) sublayers. The IEEE 802.11 standards cover the operation of the MAC sublayer and the Physical layer of the OSI Reference Model. The 802.11 frame consists of a 32-byte MAC header, a variable length body between 0 and 2312 bytes, and 4-byte FCS. The 802.11 standard uses three types of frames:
- Control Frames
- Management Frames
- Data Frames
802.11 uses control frames to control device access to the wireless medium. These control frames include the Ready (Request) To Send (RTS), Clear To Send (CTS), and Acknowledgement (ACK) frames. The RTS/CTS function is optional and is employed to prevent frame collisions. After receiving a data frame, the receiving STA will utilize an error checking processes to detect the presence of errors. The receiving STA will send an ACK frame to the sending STA if no errors are found. Receipt of the acknowledgment tells the original sender STAT of the frame that no collisions occurred. However, if the sending STA doesn’t receive an ACK after a period of time it assumes a collision occurred and will retransmit the frame.
802.11 management frames enable stations to establish and maintain communications. There are several management frame subtypes, which include Beacon Frames (Beacons), Association Request Frames, and Authentication Response Frames, amongst several other frames. While not included in this chapter, additional material and detail on the other frame types may be found in the current SWITCH guide which is available online at www.howtonetwork.org.
Figure 11-1 illustrates the exchange of management frames between an Access Point (AP) and a client (STA) using passive scanning to synchronize itself with the Access Point:
Fig. 11-1. STA Association with an AP when using Passive Scanning
Figure 11-2 illustrates the exchange of management frames between an Access Point (AP) and a client (STA) using active scanning to synchronize itself with the Access Point:
Fig. 11-2. STA Association with an AP when using Active Scanning
And finally, data frames are sent by any STA and contain higher layer protocol information or data. As is the case with management frames, the 802.11 standard supports several data frame types. Additional information on the 802.11 standard data frames may be found in the current SWITCH guide which is available online.
IEEE 802.11 Standards
At the physical (PHY) layer, IEEE 802.11 defines a series of encoding and transmission schemes for wireless communications the most common of which are the Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), and Orthogonal Frequency Division Multiplexing (OFDM) transmission schemes. Although Infra Red (IR) also exists at this layer, very little development of this standard has occurred due to line-of-sight limitations. The 802.11 standards described in this section are:
- IEEE 802.11 (original)
- IEEE 802.11b
- IEEE 802.11a
- IEEE 802.11g
- IEEE 802.11n
The original IEEE 802.11 standard defined WLANs that provided up to 2 Mbps throughput. The original standard specified the FHSS and DHSS transmission schemes and the S-Band Industrial, Scientific, and Medical (ISM) frequency band, which operates in the frequency range of 2.4 to 2.5 GHz. The original 802.11 standard is also sometimes referred to as IEEE 802.11 legacy and is now considered obsolete because the throughput is too slow for most applications.
The 802.11b standard is an extension to 802.11 that operates in the same unregulated 2.4 GHz band as the original 802.11 standard. Devices operating on 802.11b use DSSS modulation for higher speeds. The data rates on a channel can vary according to client capabilities and conditions. However, the only possible data rates are 1, 2, 5.5, and 11 Mbps. The 5.5 Mbps and 11 Mbps are two new speeds added to the original specification. The 2.4-GHz band consists of 14 channels, each 22 MHz wide. In North America, the Federal Communications Commission (FCC) allows channels 1 through 11. Most of Europe can use channels 1 through 13. In Japan only channel 14 is used. APs or clients uses a spectral mask or a template to filter out a single channel based around a center frequency.
The IEEE 802.11a standard is an extension to 802.11 that applies to wireless LANs and provides up to 54 Mbps. This standard uses OFDM and does away with spread spectrum. As a result, it is not compatible with 802.11b or 802.11g and therefore this standard is seldom used any more.
802.11a equipment operates at 5GHz. This higher frequency range means that 802.11 signals are absorbed more readily by walls and other solid objects in their path due to their smaller wavelength and, as a result, cannot penetrate as far as those of 802.11b. The FCC has allocated 300MHz of RF spectrum for unlicensed operation in the 5GHz block referred to as the Unlicensed National Information Infrastructure (U-NII) band.
The 802.11g standard also works in the same 2.4GHz range as 802.11b. IEEE 802.11g operates at a bit rate as high as 54 Mbps, but uses the S-Band ISM and OFDM. However, unlike 802.11a, 802.11g is backward compatible with 802.11b can operate at the 802.11b bit rates and use DSSS.
Like 802.11a, 802.11g uses 54 Mbps in ideal conditions and the slower speeds of 48 Mbps, 36 Mbps, 24 Mbps, 18 Mbps, 12 Mbps, and 6 Mbps in less-than-ideal conditions.
IEEE 802.11n improves on 802.11a and 802.11g maximum data rate with a significant increase in the rate from 54 Mbps to approximately 600 Mbps. The 802.11n standard includes several enhancements to the previously described 802.11 standards. The enhancements include MIMO, 40-MHz operation, Frame aggregation at the MAC sublayer, and backward compatibility, which makes it possible for multiple 802.11 a, 802.11b, 802.11g and 802.11n devices to coexist.