11.6 IEEE 802.11 Medium Access Control Layer

The IEEE 802.11 family MAC layer is common to all types of PHY layers mentioned previously and this section discusses features of the MAC layer. Many of the functions have been modeled after the previously introduced IEEE 802.3 LAN with modifications and additional functions needed for wireless capability. The goal of the IEEE 802.11 family of MAC standards is to provide equivalent or better functions as compared with the IEEE 802.3 LAN MAC layer.

As mentioned before, a common LLC layer is used across the other LAN standards and IEEE 802.11 family WLANs that creates an easy bridge between WLANs and LANs. The IEEE 802.3 LANs provide a mechanism for multiple users to share a medium, and this continues, albeit with some changes, in the IEEE 802.11 family. The IEEE 802.3 LANs are very reliable, and this needs to be realized in the IEEE 802.11 family, keeping in mind the issues confronting wireless mediums.

Other features of MAC are more particular to the wireless nature of IEEE 802.11. Mechanisms such as cyclic redundancy checksum (CRC) and packet fragmentation have been added to cope with the wireless media. Security-and mobility- related features have also been added due to the wireless aspect of IEEE 802.11.

11.6.1 Contention Resolution ”CSMA/CA

Medium access control, as the name implies, has to provide a fair and consistent method for users to access the shared medium. IEEE 802.3 LANs introduced carrier sense multiple access/collision detection (CSMA/CD) for sharing the physical medium. The IEEE 802.3 Ethernet LAN devices "sense" or listen to the wired medium for any other transmissions from other nodes in the IEEE 802.3 LAN. IEEE 802.3 frames are sent if there is no one sending data.

If there is a collision in the physical medium, the IEEE 802.3 LAN detects this collision and waits a random amount of time before it transmits again. This mechanism works well with wired LANs as the sending device can listen to the transmissions and detect if there are collisions.

PHYSICAL CARRIER SENSE

The IEEE 802.11 physical medium is also shared among all devices. The same CSMA mechanism, where the physical medium or carrier is sensed, can be employed in the IEEE 802.11 family as well. This is because the sending stations can listen for any transmissions before sending frames using the wired medium. However, detection of collisions is not easy in wireless mediums, as typically the transmission overpowers the sending station's ability to detect or hear any collision, and expensive equipment is required for simultaneous reception and transmission.

Therefore, the collision detection used in IEEE 802.3 LANs is not suitable in the IEEE 802.11 family. The change is to use a collision avoidance (CA) mechanism, where the receiving station sends an acknowledgment (ACK) packet to confirm that the packet has arrived uncorrupted.

This is referred to as the basic access mechanism and is formally referred to as the distribution coordination function (DCF) in the MAC layer of the IEEE 802.11 family. This explicit ACK mechanism adds some extra overhead as compared to IEEE 802.3 LANs. This method of ACKs for packet transmission adds additional reliability in an otherwise noise-filled and unreliable wireless medium.

One of the problems associated with the basic access mechanism is that it assumes that the sending station can listen to all stations and sense the carrier. However, due to physical obstructions and the fact that not all stations may be within hearing range of each other, all nodes may not be able to listen to each other. This is referred to as the hidden node problem.

HIDDEN NODE PROBLEM

As shown in Figure 11-5, there are three stations with IEEE 802.11 connectivity. Stations A and B are in range with each other, and stations B and C are within range of each other. Therefore, if station A were to transmit to station B, any transmission by station C at the same time would corrupt packets arriving at station B. Station C is not aware that there are other stations transmitting to station B as well.

Figure 11-5. Hidden node problem and its resolution.

To alleviate this problem, on top of the basic access mechanism mentioned earlier, two more frames are exchanged. This requires the sending station, in our example station A, to first send a request to send (RTS) message to station B. Station B, if it receives this correctly, will respond with a clear to send (CTS) message. Station B's CTS is heard by both stations A and C. Only station A will transmit at this time, as it received a clear to send from station B.

These two messages on top of the basic access mechanism of CSMA/CA create a solution for dealing with wireless medium access and the hidden node problem. Stations hearing a CTS will wait until the ACKs are sent for the packet transmission. Once the ACKs are heard, the other stations can contend for resources. The use of the RTS/CTS mechanism is optional and is typically used for large packets.

11.6.2 Addressing

The same MAC address format that is used in the IEEE 802.3 LANs is used in the IEEE 802.11 family as well. This is the same 48-bit address as shown in Figure 11-6.

Figure 11-6. MAC address.

The use of this address is similar to the IEEE 802.3 LANs, where the source and the destination are identified by a MAC address, referred to in the IEEE 802.11 family standards as source address (SA) and destination address (DA). However, unlike the IEEE 802.3 LANs, additional types of addresses (using the same MAC address format) have been defined in the IEEE 802.11 family. These are the transmitter address (TA), receiver address (RA) and the basic service set identifier (BSS ID). These additional addresses facilitate mobility as well as multicast-related operations.

11.6.3 Reliability and Robustness

The benefit of wired LANs is the inherent reliability that accompanies them. One of the key requirements of the IEEE 802.11 family MAC layer is to be robust against interference in the 2.4-GHz band . The cyclic redundancy checksum (CRC)-32 that was used in IEEE 802.3 LANs is continued here as well. The MAC frames include a frame check sequence (FCS) field that contains the CRC for the packets, and the receiver checks for any errors. Another new mechanism added for robustness is a fragmentation function.

Packet fragmentation allows large packets to be broken into smaller packets. This is useful when transmitting in congested environments or when there is heavy interference. The MAC layer reassembles the fragments, and this prevents the upper layers from having to retransmit. Also, using smaller fragments minimizes the amount of retransmissions, especially as large packets tend to get corrupted.

11.6.4 Security

One of the key additions to the IEEE 802.11 family is new security mechanisms. The wired LAN is inherently secure due to the physical medium used (coaxial cables or wires used). The cable needs to be plugged in to obtain connectivity. Users connect to the wired LAN within the secure confines of a building, and privacy is assured as the information travels within the home or enterprise, where the wired LAN is deployed. Typically the wired LAN must be physically compromised to tap the data.

The concern with wireless is that anyone within proximity of the wireless devices can transmit and listen to the wireless transmission. To prevent this, the IEEE 802.11 standards created wired equivalent privacy (WEP), which includes mechanisms for authentication of the 802.11 users and as well privacy of the packets. WEP is an optional feature of the MAC layer. However, if devices are Wi-Fi certified, then WEP is a requirement for Wi-Fi certification.

Authentication is the mechanism that verifies that you are who you say you are. This is important in the IEEE 802.11 family and employs a shared key mechanism. Using a shared WEP encryption key, which is input into both the sending and receiving stations, both stations independently encrypt the same piece of information. The encrypted result provided by the sending station is compared with the receiving station's result. If the results are exactly the same, then the sending station is authenticated.

Privacy is very important to wireless LANs due to casual eavesdropping. Again, WEP provides an encryption mechanism where the content of the data packets to be sent passes through an encryption algorithm. The receiver needs to have the same shared key to decrypt the encrypted packets. WEP uses a symmetric cipher algorithm, where the same shared key is used for encryption and decryption.

Although WEP is the security mechanism in the IEEE 802.11 family, there are some important considerations when securing wireless LAN. IEEE does not mandate a mechanism to administer or manage these shared keys. Therefore, if the keys are compromised, significant security concerns may result. Also, the authentication mechanism is only in one direction, where the receiver authenticates the sending station. The preference is for bidirectional authentication and for keys to change periodically. Many of these concerns, including the strength of the WEP algorithm, are being considered actively by the IEEE. A new working group was created in 2001, known as IEEE 802.11i, to address the deficiencies of the MAC layer with respect to security.

11.6.5 Power Management

With the advent of the IEEE 802.11 family, clearly one of the requirements of the MAC layer was to support the users going wireless or "untethered" for a significant amount of time. The IEEE 802.11 MAC layer allows the devices to turn off their receiver and transmitter to conserve power.

The MAC layer provides the ability for stations to inform other stations, whether they are continuously listening known as continuous aware mode, or listening intermittently to save battery, known as power save mode (PSM).

The PSM is requested by the station and lets recipients know how frequently they will wake up to check if there are any messages for them. Any incoming packets destined for the device in PSM will have to be queued. The recipient may be informed of waiting packets using broadcast mechanisms.