802.11 Physical Layer Technologies

The ratification of the 1999 802.11a and 802.11b standards transformed wireless LAN (WLAN) technology from a niche solution for the likes of barcode scanners to a generalized solution for portable, low-priced, interoperable network access. Today, many vendors offer 802.11a and 802.11b clients and access points that provide performance comparable to wired Ethernet. The lack of a wired network connection gives users the freedom to be mobile as they use their devices. Although standardization has been key, the use of unlicensed frequencies, where a costly and time-consuming licensing process is not required, has also contributed to a rapid and pervasive spread of the technology.

802.11 as a standards body actually defined a number of different physical layer (PHY) technologies to be used with the 802.11 MAC. This chapter examines each of these 802.11 PHYs, including the following:

• The 802.11 2.4 GHz frequency hopping PHY

• The 802.11 2.4 GHz direct sequencing PHY

• The 802.11b 2.4 GHz direct sequencing PHY

• The 802.11a 5 GHz Orthogonal Frequency Division Multiplexing (OFDM) PHY

• The 802.11g 2.4 GHz extended rate physical (ERP) layer

802.3 Ethernet has evolved over the years to include 802.3u Fast Ethernet and 802.3z/802.3ab Gigabit Ethernet. In much the same way, 802.11 wireless Ethernet is evolving with 802.11b high-rate direct sequence spread spectrum (HR-DSSS) and 802.11a OFDM standards and the recent addition of the 802.11g ERP. In fact, the physical layer for each 802.11 type is the main differentiator between them.


Wireless Physical Layer Concepts

The 802.11 PHYs essentially provide wireless transmission mechanisms for the MAC, in addition to supporting secondary functions such as assessing the state of the wireless medium and reporting it to the MAC. By providing these transmission mechanisms independently of the MAC, 802.11 has developed advances in both the MAC and the PHY, as long as the interface is maintained. This independence between the MAC and PHY is what has enabled the addition of the higher data rate 802.11b, 802.11a, and 802.11g PHYs. In fact, the MAC layer for each of the 802.11 PHYs is the same.

Each of the 802.11 physical layers has two sublayers:

• Physical Layer Convergence Procedure (PLCP)

• Physical Medium Dependant (PMD)

Figure 3-1 shows how the sublayers are oriented with respect to each other and the upper layers.

The PLCP is essentially a handshaking layer that enables MAC protocol data units (MPDUs) to be transferred between MAC stations over the PMD, which is the method of transmitting and receiving data through the wireless medium. In a sense, you can think of the PMD as a wireless transmission service function that is interfaced via the PLCP. The PLCP and PMD sublayers vary based on 802.11 types.

All PLCPs, regardless of 802.11 PHY type, have data primitives that provide the interface for the transfer of data octets between the MAC and the PMD. In addition, they provide primitives that enable the MAC to tell the PHY when to commence transmission and the PHY to tell the MAC when it has completed its transmission. On the receive side, PLCP primitives from the PHY to the MAC indicate when it has started to receive a transmission from another station and when that transmission is complete. To support the clear channel assessment (CCA) function, all PLCPs provide a mechanism for the MAC to reset the PHY CCA engine and for the PHY to report the current status of the wireless medium.

In general, the 802.11 PLCPs operate according to the state diagram in Figure 3-2. Their basic operating state is the carrier sense/clear channel assessment (CS/CCA) procedure. This procedure detects the start of a signal from a different station and determines whether the channel is clear for transmitting. Upon receiving a Tx Start request, it transitions to the Transmit state by switching the PMD from receive to transmit and sends the PLCP protocol
data unit (PPDU). Then, it issues a Tx End and returns to the CA/CCA state. The PLCP invokes the Receive state when the CS/CCA procedure detects the PLCP preamble and valid PLCP header. If the PLCP detects an error, it indicates the error to the MAC and proceeds to the CS/CCA procedure.

Nonstandard Devices

Although the previous section described how 802.11-standards–based devices access the wireless medium, this section discusses devices that fall outside of the 802.11 standard. These devices use the 802.11 technology in a way that violates or extends an area of the standard and that might prove useful in your network. The specific devices under consideration are:

• Repeater APs

• Universal clients (workgroup bridges)

• Wireless bridges

Although each of these devices provides useful networking tools, you should remember that they are not currently defined in the 802.11 standard, and there are no interoperability guarantees because different vendors may define different mechanisms for implementing these tools. For the reliability of your network, should you choose to use these, you should ensure that they are only interfacing to devices from the same vendor or devices for which the vendor ensures interoperability.


Repeater APs

You might find yourself in situations where it is not easy or convenient to connect an AP to the wired infrastructure or where an obstruction makes it difficult for an AP on your wired network to directly associate clients in an area of your deployment. In such a scenario, you can employ a repeater AP. Figure 2-18 shows this scenario, where Elaine is not directly visible to AP2 but she can see AP3, which is not connected to the wired network but can see AP2.

Much like a wired repeater, what a wireless repeater does is merely retransmit all the packets that it receives on its wireless interface. This retransmission happens on the same channel upon which the packet was received. The repeater AP has the effect of extending the BSS and also the collision domain. Although it can be an effective tool, you must take care when employing it; the overlapping of the broadcast domains can effectively cut your throughput in half because the originating AP also hears the retransmit.


Wireless Bridges

If you extend the concept of a workgroup bridge even further to the point where you are connecting two or more wired networks, you arrive at the concept of wireless bridges. Similar to wired bridges, wireless bridges connect networks. You might bridge wirelessly because you need to connect networks that are inherently mobile. Alternatively, the networks to be connected might not be co-located, in which case wireless bridging provides a method for connecting these networks. The main distinction between bridges and workgroup bridges is that the latter are only wirelessly enabling a small network in an office environment, whereas the former can connect larger networks often separated by distances much greater than what is found in the WLAN environment. In fact, many vendors offer products that provide ranges which far exceed the definitions and limitations of 802.11. Figure 2-20 shows a wireless bridging example.

As shown in the figure, one of the bridges assumes the role of the AP in a WLAN network, and the other bridges act as clients. Although the basic 802.11 MAC and PHY sublayer technologies are utilized in wireless bridging, individual vendors have their own proprietary methods for the encapsulation of wired network traffic and for extending the range from a MAC and PHY sublayer perspective. For this reason, once again you should ensure that your wireless bridges are certified to interoperate.

802.11 Medium Access Mechanisms

802.11-based WLANs use a similar mechanism known as carrier sense multiple access with collision avoidance (CSMA/CA). CSMA/CA is a listen before talk (LBT) mechanism. The transmitting station senses the medium for a carrier signal and waits until the carrier channel is available before transmitting.

Wired Ethernet is able to sense a collision on the medium. Two stations transmitting at the same time increase the signal level on the wire, indicating to the transmitting stations that a collision has occurred. 802.11 wireless stations do not have this capability. The 802.11 access mechanism must make every effort to avoid collisions altogether.

CSMA/CA

CSMA/CA is more ordered than CSMA/CD. To use the same telephone conference call analogy, you make some changes to the scenario:

• Before a participant speaks, she must indicate how long she plans to speak. This indication gives any potential speakers an idea of how long to wait before they have an opportunity to speak.
• Participants cannot speak until the announced duration of a previous speaker has elapsed.
• Participants are unaware whether their voices are heard while they are speaking, unless they receive confirmation of their speeches when they are done.
• If two participants happen to start speaking at the same time, they are unaware they are speaking over each other. The speakers determine they are speaking over each other because they do not receive confirmation that their voices were heard.
• The participants wait a random amount of time and attempt to speak again, should they not receive confirmation of their speeches.

The 802.11 implementation of CSMA/CA is manifested in the distributed coordination function (DCF). To describe how CSMA/CD works, it is important to describe some key 802.11 CSMA/CA components first:

• Carrier sense
• DCF
• Acknowledgment frames
• Request to Send/Clear to Send (RTS/CTS) medium reservation

In addition, two other mechanisms pertain to 802.11 medium access but are not directly tied to CSMA/CA:

• Frame fragmentation
• Point coordination function (PCF)

Carrier Sense

A station that wants to transmit on the wireless medium must sense whether the medium is in use. If the medium is in use, the station must defer frame transmission until the medium is not in use. The station determines the state of the medium using two methods:

• Check the Layer 1 physical layer (PHY) to see whether a carrier is present.
• Use the virtual carrier-sense function, the network allocation vector (NAV).

The station can check the PHY and detect that the medium is available. But in some instances, the medium might still be reserved by another station via the NAV. The NAV is a timer that is updated by data frames transmitted on the medium. For example, in an infrastructure BSS, suppose Martha is sending a frame to George (see Figure 2-4). Because the wireless medium is a broadcast-based shared medium, Vivian also receives the frame. The 802.11 frames contain a duration field. This duration value is large enough to cover the transmission of the frame and the expected acknowledgment. Vivian updates her NAV with the duration value and does not attempt transmission until the NAV has decremented to 0.


Note that stations only update the NAV when the duration field value received is greater than what is currently stored in their NAV. Using the same example, if Vivian has a NAV of 10 milliseconds, she does not update her NAV if she receives a frame with a duration of 5 milliseconds. She updates her NAV if she receives a frame with a duration of 20 milliseconds.

DCF

The IEEE-mandated access mechanism for 802.11 networks is DCF, a medium access mechanism based on the CSMA/CA access method. To describe DCF operation, we first define some concepts. Figure 2-5 shows a time line for the scenario in Figure 2-4.

In DCF operation, a station wanting to transmit a frame must wait a specific amount of time after the medium becomes available. This time value is known as the DCF interframe space (DIFS). Once the DIFS interval elapses, the medium becomes available for station access
contention.

In Figure 2-5, Vivian and George might want to transmit frames when Martha's transmission is complete. Both stations should have the same NAV values, and both will physically sense when the medium is idle. There is a high probability that both stations will attempt to transmit when the medium becomes idle, causing a collision. To avoid this situation, DCF uses a random backoff timer.

The random backoff algorithm randomly selects a value from 0 to the contention window (CW) value. The default CW values vary by vendor and are value-stored in the station NIC. The range of values for random backoff start at 0 slot times and increment up to the maximum value, which is a moving ceiling starting at CWmin and stopping at a maximum value known as CWmax. For the sake of this example, assume that the CWmin value begins at 7 and CWmax value is 255. Figure 2-6 illustrates the CWmin and CWmax values for binary random backoff.

A station randomly selects a value between 0 and the current value of the CW. The random value is the number of 802.11 slot times the station must wait during the medium idle CW before it may transmit. A slot time is a time value derived from the PHY based on RF characteristics of the BSS .

Getting back to the example, Vivian is ready to transmit. Her NAV timer has decremented to 0, and the PHY also indicates the medium is idle. Vivian selects a random backoff time between 0 and CW (in this case, CW is 7) and waits the selected number of slot times before transmitting. Figure 2-7 illustrates this process, with a random backoff value of four slot times.


The Acknowledgment Frame

A station receiving a frame acknowledges error-free receipt of the frame by sending an acknowledgment frame back to the sending station. Knowing that the receiving station has to
access the medium and transmit the acknowledgment frame, you would assume that it is possible for the acknowledgment frame to be delayed because of medium contention. The transmission of an acknowledgment frame is a special case. Acknowledgment frames are allowed to skip the random backoff process and wait a short interval after the frame has been received to transmit the acknowledgment. The short interval the receiving station waits is known as the short interframe space (SIFS) . The SIFS interval is shorter than a DIFS interval by two slot times. It guarantees the receiving station the best possible chance of transmitting on the medium before another station does.

Referring to Vivian's transmission to George, Vivian deferred her transmission attempt for four slot times. The medium was still available, so she transmitted her frame to George, as depicted in Figure 2-9. The AP receives the frame and waits a SIFS interval before sending an acknowledgment frame.


The Hidden Node Problem and RTS/CTS

Vivian might be unable to access the medium because of another station that is within range of the AP yet out of range of her station. Figure 2-10 illustrates this situation. Vivian and George are in range of each other and in range of the AP. Yet neither of them is in range of Tony. Tony is in range of the AP and attempts to transmit on the medium as well. The situation is known as the hidden node problem because Tony is hidden to Vivian and George.

802.11 Frame Fragmentation

Frame fragmentation is a MAC layer function that is designed to increase the reliability of frame transmission across the wireless medium. The premise behind fragmentation is that a frame is broken up into smaller fragments, and each fragment is transmitted individually, as depicted in Figure 2-13. The assumption is that there is a higher probability of successfully transmitting a smaller frame fragment across the hostile wireless medium. Each frame fragment is individually acknowledged; therefore, if any fragment of the frame encounters any errors or a collision, only the fragment needs to be retransmitted, not the entire frame, increasing the effective throughput of the medium.