IEEE 802.11g Standard

IEEE 802.11g is a convergence of IEEE 802.11a and IEEE 802.11b. The latter provides the data rates of IEEE 802.11a at the 2.4 GHz band, thus supporting interoperability with older IEEE 802.11 and IEEE 802.11b devices.

Four Physical Layers

IEEE 802.11g uses DSSS or OFDM or both at the 2.4 GHz ISM band to provide data rates of up to 54 Mbps. Combining DSSS and OFDM is achieved by defining four physical layers for IEEE 802.11g. These are defined as extended rate physicals (ERPs), which co-exist during a frame exchange. Therefore, the sender and receiver have the option to select and use one of the four layers as long as they both support it. The four physical layers are:

1. ERP-DSSS/CCK is the old physical layer used by IEEE 802.11b, which uses DSSS with CCK modulation. The data rates are the ones supported by IEEE 802.11b.
2. ERP-OFDM is a new physical layer. OFDM is used to provide IEEE 802.11a data rates at the 2.4 GHz band.
3. ERP-DSSS/PBCC is introduced in IEEE 802.11b and provides the same data rates as the DSSS/CCK physical layer. It uses DSSS technology with the PBCC coding algorithm. IEEE 802.11g extends the set of data rates to include 22 and 33 Mbps.
4. DSSS-OFDM is a new physical layer that uses a combination of DSSS and OFDM. The packet physical header is transmitted using DSSS while the packet payload is transmitted using OFDM. It supports interoperability.

The first two physical layers are mandatory — every IEEE 802.11g device must support them.

Parameters of IEEE 802.11g Physical Layers

The physical layer packet overhead for an IEEE 802.11 packet consists of two parts:

1. Physical Layer Convergence Protocol (PLCP) preamble is used for synchronization,
2. PLCP header that holds packet information related to the physical layer.

The PLCP preamble is too long and adds considerable overhead in the WLAN system. An option to support a shorter preamble (called short preamble — surprise, surprise!) was introduced to reduce packet overhead and improve network performance. If both the sender and receiver support this option, the communication is performed using the short preamble. IEEE 802.11g makes the short preamble mandatory.

The ERP Network Attribute

The default values for the slot time and minimum contention window for IEEE 802.11b are 20 μs and 31 slots, respectively. IEEE 802.11g adopts these values. However, when stations transmit at ERP-OFDM data rates (6 - 54 Mbps) with a shorter preamble of 20 μs, these two values degrade network performance. The most appropriate values in this case are those defined in the IEEE 802.11a standard (9 μs and 15 slots), where stations transmit exclusively at OFDM data rates.

IEEE 802.11g incorporates dynamic adjustment for both values by defining an ERP network attribute, which is a flag published to the stations via a beacon frame (a control frame that contains network information). The ERP attribute is enabled if all stations associated to a WLAN are capable of supporting ERP-OFDM data rates — the values for the slot time and minimum contention window then depend on the WLAN operation mode (BSS or independent BSS (IBSS)).

• For BSS operation, the slot time parameter is set to 9 μs and the minimum contention window parameter is set to 15 slots.
• For IBSS operation, the slot time is set to 20 μs and the minimum contention window is set to 15 slots.

For both modes, all frame exchanges are performed using ERP-OFDM data rates.

Interoperability Aspects and Protection Mechanism

In an IEEE 802.11g network, a station can choose between 14 data rates and four physical layers in order to transmit packet most efficiently. Inevitably, this raises interoperability problems.

The types of stations that may exist are:

• ERP stations support the ERP-OFDM physical layer. They are equipped with a pure IEEE802.11g wireless interface.
• Non-ERP stations that support short preamble are equipped with a newer IEEE 802.11b wireless interface that supports up to 11 Mbps, but its firmware is upgraded in order to support the use of short preamble.
• Non-ERP stations that do not support short preamble are equipped with an IEEE802.11b wireless interface of an older release of an IEEE 802.11 wireless interface that does not support short preamble.

Let's say a network consists of ERP and non-ERP stations. ERP stations communicate with each other using ERP-OFDM packet. Non-ERP stations are unable to detect OFDM transmission. Therefore, when an ERP station transmits, non-ERP stations won't be able to detect the transmission and assume that the medium is idle. If a non-ERP station starts transmitting, it causes a collision.

There are two solutions:

1. Use the DSSS-OFDM physical layer so that all stations are able to detect the DSSS-transmitted PLCP preamble and header, and refrain from transmitting.
2. Use RTS/CTS frames to protect the OFDM transmission. When non-ERP and ERP stations co-exist, all RTS and CTS frames must be transmitted using the ERP-DSSS physical layer so that all stations are informed about the incoming transmission even if the data packet is transmitted using OFDM.

In addition to the second solution, IEEE 802.11g defines an alternative protection mechanism called CTS-to-self to prevent collisions caused the DSSS/OFDM interoperability problem.

The CTS-to-Self Mechanism

This alternative reduces the overhead in a WLAN system but does not efficiently deal with the hidden station problem.

Figure(a) below illustrates the RTC-CTS mechanism. When station A wants to send a packet to station C, it sends a RTS frame (arrow 1), which is received by stations B and C (arrow 2). Stations B and C send a CTS frame (arrow 3) that are received by all stations (arrow 4), including station D, which is hidden from station A. Although station D didn't receive the RTS, it receives the CTS frame from station C and refrains from transmitting.

RTS/CTS vs. CTS-to-self

Figure(b) shows the CTS-to-self mechanism. When station A wants to send a packet to station C, it sends a CTS frame (arrow 1), which is received by stations B and C (arrow 2). Both stations refrain from transmission. However, because station D is not within range of station A, it does not detect the CTS frame and is unaware of station A's transmission. If station D starts transmitting, it will result in a collision.

CTS-to-self should only be used if stations can detect each other's transmission. Otherwise, RTS/CTS should be used.

Source D. Vassis, G. Kormentazas, A. Rouskas, I. Maglogiannis, "The IEEE 802.11g Standard for High Data Rate WLANs", IEEE Network, Volume 19, Issue 3, May-June 2005