In section 5.5, we examined the dominant wired LAN protocol – Ethernet. In the previous section we examined how LAN segments can be connected together via hubs, bridges and routers to form larger LANs. In this section we examine a LAN standard (belonging to the same IEEE 802 family as Ethernet) that is being increasingly deployed for untether (wireless) LAN communication. The IEEE 802.11 standard [Brenner 1997, Crow 1997, IEEE 1999] defines the physical layer and media access control (MAC) layer for a wireless local area network. The standard defines three different physical layers for the 802.11 wireless LAN, each operating in a different frequency range and at rates of 1 Mbps and 2 Mbps. In this section we focus on the architecture of 802.11 LANs and their media access protocols. We'll see that although it belongs to the same standard family as Ethernet, it has a significantly different architecture and media access protocol.
Figure 5.7-1: IEEE 802.11 LAN architecture
Figure 5.7-1 illustrates the principal components of the 802.11 wireless LAN architecture. The fundamental building block of the 802.11 architecture is the cell, known as the basic service set (BSS) in 802.11 parlance. A BSS typically contains one or more wireless stations and a central base station, known as an access point (AP) in 802.11 terminology. The stations, which may be either fixed or mobile, and the central base station communicate amongst themselves using the IEEE 802.11 wireless MAC protocol. Multiple APs may be connected together (e.g., using a wired Ethernet or another wireless channel) to form a so-called distribution system (DS). The DS appears to upper level protocols (e.g., IP) as a single 802 network, in much the same way that a bridged, wired 802.3 Ethernet network appears as a single 802 network to the upper layer protocols.
Figure 5.7-2: An IEEE 802.11 ad hoc network
Figure 5.7-2 shows that IEEE 802.11 stations can also group
themselves together to form an ad hoc network – a network with no
central control and with no connections to the "outside world."
Here, the network is formed "on the fly," simply because there
happen to be mobile devices that have found themselves in
proximity to each other, that have a need to communication, and
that find no pre-existing network infrastructure (e.g., a
pre-existing 802.11 BSS with an AP) in the location. An ad hoc
network might be formed, for example, when people with laptops
meet together (e.g., in a conference room, a train, or a car) and
want to exchange data in the absence of a centralized AP. There
has been a tremendous recent increase in interest in ad hoc
networking, as communicating portable devices continue to
proliferate. Within the IETF, activity in ad hoc networking is
centered around the mobile ad
hoc networks (manet) working group.
Just as in a wired 802.3 Ethernet network, stations in an IEEE 802.11 wireless LAN must coordinate their access and use of the shared communication media (in this case the radio frequency). Once again, this is the job of the media access control (MAC) protocol. The IEEE 802.11 MAC protocol is a carrier sense multiple access protocol with collision avoidance (CSMA/CA). Recall from our study of Ethernet in section 5.5 that a CSMA protocol first senses the channel to determine if the channel is "busy" with the transmission of a frame from some other station. In the 802.11 specification, the physical layer monitors the energy level on the radio frequency to determine whether or not another station is transmitting and provides this carrier sensing information to the MAC protocol. If the channel is sensed idle for an amount of time equal to or greater than the Distributed Inter Frame Space (DIFS), a station is then allowed to transmit. As with any random access protocol., this frame will be successfully received at the destination station if no other station's transmission has interfered with the frame's transmission.
When a receiving station has correctly and completely received a frame for which it was the addressed recipient, it waits a short period of time (known as the Short Inter Frame Spacing - SIFS) and then sends an explicit acknowledgment frame back to the sender. This data link layer acknowledgment lets the sender know that the receiver has indeed correctly received the sender's data frame. We will see shortly that this explicit acknowledgment is needed because, unlike the case of wired Ethernet, a wireless sender can not itself determine whether or not its frame transmission was successfully received at the destination. The transmission of a frame by a sending station and its subsequent acknowledgment by the destination station is shown in Figure 5.7-3.
Figure 5.7-3: data transmission and acknowledgment in IEEE 802.11
Figure 5.7-3 illustrates the case when the sender senses the channel to be idle. What happens if the sender senses the channel busy? In this case, the station performs a backoff procedure that is similar to that of Ethernet. More specifically, a station that senses the channel busy will defer its access until the channel is later sensed idle. Once the channel is sensed idle for an amount of time equal to DIFS, the station then computes an additional random backoff time and counts down this time as the channel is sensed idle. When the random backoff timer reaches zero, the station transmits its frame. As in the case of Ethernet, the random backoff timer serves to avoid having multiple stations immediately begin transmission (and thus collide) after a DIFS idle period. As in the case of Ethernet, the interval over which the backoff timer is randomizes is doubled each time a transmitted frame experiences a collision.
We noted above that unlike the 802.3 Ethernet protocol, the wireless 802.11 MAC protocol does not implement collision detection. There are a couple of reasons for this:
The ability to detect collisions requires the ability to both send (one's own signal) and receive (to determine if another station's transmissions is interfering with one's own transmission) at the same time. This can be costly.
More importantly, even if one had collision detection and sensed no collision when sending, a collision could still occur at the receiver. This situation results from the particular characteristics of the wireless channel. Suppose that station A is transmitting to station B. Suppose also that station C is transmitting to station B. With the so-called hidden terminal problem, physical obstructions in the environment (e.g. a mountain) may prevent A and C from hearing each others transmissions, even though A's and C's transmissions are indeed interfering at the destination, B. This is shown in Figure 5.7-4(a). A second scenario that results in undetectable collisions at the receiver results from the fading of a signal's strength as propagates through the wireless medium. Figure 5.7-4(b) illustrates the case where A and C are placed such that their signal strengths are not strong enough for them to detect each others' transmissions, and yet their transmissions are strong enough to have interfered with each other at station B.
Figure 5.7-4: hidden terminal problem (a) and fading (b)
Given these difficulties with detecting collisions at a wireless receiver, the designers of IEEE 802.11 developed an access protocol which aimed to avoid collisions (hence the name CSMA/CA) , rather than detect and recover from collisions (CSMA/CD). First, the IEEE 802.11 frame contains a duration field in which the sending station explicit indicates the length of time that its frame will be transmitting on the channel. This value allows other stations to determine the minimum amount of time (the so-called network allocation vector, NAV) for which they should defer their access, as shown in Figure 5.7.3.
The IEEE 802.11 protocol can also use a short Request To Send (RTS) control frame and a short Clear To Send (CTS) frame to reserve access to the channel. When a sender wants to send a frame, it can first send a RTS frame to the receiver, indicating the duration of the data packet and the ACK packet. A receiver that receives an RTS frame responds with a CTS frame, giving the sender explicit permission to send. All other stations hearing the RTS or CTS then know about the pending data transmission and can avoid interfering with those transmissions. The RTS, CTS, DATA and ACK frames are shown in Figure 5.7-5. An IEEE 802.11 sender can operate either using the RTS/CTS control frames, as shown in Figure 5.7-5, or can simply send its data without first using the RTS control frame, as shown in Figure 5.7-3.
Figure 5.7-5: Collision Avoidance using the RTS and CTS frames
The use of the RTS and CTS frames helps avoid collisions in three important ways:
Because the receiver's transmitted CTS frame will be heard by all stations within the receiver's vicinity, the CTS frame helps avoid both the hidden station problem and the fading problem.
Because the RTS and CTS frames are short, a collision involving a RTS or CTS frame will only last for the duration of the whole RTS or CTS frame. Note that when the RTS and CTS frames are correctly transmitted, there should be no collisions involving the subsequent DATA and ACK frames.
In our discussion above, we have only highlighted some of the
key aspects of the 802.11 protocol. Additional protocol
capabilities such as time synchronization, power management,
joining and leaving a network (i.e., roaming stations) are
covered in the full IEEE 802.11 standard. See [Brenner 1997, Crow
1997, IEEE 1999] for details.
Copyright James F. Kurose and Keith W. Ross 1996–1999. All rights reserved.