IEEE 802.11b

"High Rate" Wireless Local Area Networks

by Kanoksri Sarinnapakorn - March 15, 2001 -

IEEE 802.11b "High Rate" standard wireless local area network (WLAN) operates in the 2.4GHz (2.4 to 2.483 GHz) unlicensed Radio Frequency (RF) band and can transmit up to 11Mbps (Megabits per second). It was released in September 1999 after the Institute of Electronic and Electrical Engineers (IEEE) had released IEEE 802.11 in June 1997.

IEEE 802.11b standard defines only two bottom levels of OSI (Open Systems Interconnection) reference model, the Physical Layer (PHY) and the Data Link Layer (Medium Access Control, MAC sublayer).

Operation Modes

IEEE 802.11b defines two pieces of equipment, a wireless station, which is usually a PC or a Laptop with a wireless network interface card (NIC), and an Access Point (AP),which acts as a bridge between the wireless stations and Distribution System (DS) or wired networks. There are two operation modes in IEEE 802.11b, Infrastructure Mode and Ac Hoc Mode.

Infrastructure Mode

Basis Service Set (BSS)

Extended Service Set (ESS)

Ad Hoc Mode

Independent Basic Service Set (IBSS) or Peer to Peer

IEEE 802.11b Physical Layer

The physical layer (PHY) covers the physical interface between devices and is concerned with transmitting raw bits over the communication channel. IEEE 802.11b physical layer is an extension to IEEE 802.11 physical layer which supports 1 and 2 Mbps. IEEE 802.11b can support higher data rates of 5.5 and 11 Mbps by using Complementary Code Keying (CCK) with Quadrature Phase Shift Keying (QPSK) modulation and Direct-Sequence Spread-Spectrum (DSSS) technology.

In addition, IEEE 802.11b defines dynamic rate shifting, allowing data rates to be automatically adjusted for noisy conditions. This means IEEE 802.11b devices will transmit at lower speeds, 5.5 Mbps, 2 Mbps, and 1 Mps under noisy conditions. When the devices move back within the range of a higher-speed transmission, the connection will automatically speed up again.

Traditionally, IEEE 802.11 uses either a FHSS (frequency-hopping spread spectrum) or DSSS technology. Both are good solutions for transmission data rate of 1 to 2 Mbps.[2] However, in the United States, IEEE 802.11b cannot use FHSS for higher speeds without violating Federal Communications Commission (FCC) regulations.[1]

IEEE 802.11b Data Rate Specifications
Data Rate	Code Length	Modulation	Symbol Rate	Bits/Symbol
1 Mbps	11 (Barker Sequence)	BPSK	1 MSps	1
2 Mbps	11 (Barker Sequence)	QPSK	1 MSps	2
5.5 Mbps	8 (CCK)	QPSK	1.375 MSps	4
11 Mbps	8 (CCK)	QPSK	1.375 MSps	8

Note:	Mbps	Megabits per second
	MSps	Million Symbols per second
	BPSK	Binary Phase Shift Keying uses one phase shift for each bit.
	QPSK	Quadrature Phase Shift Keying uses four rotations (0, 90, 180 and 270 degrees)
	CCK	Complementary Code Keying uses 4 bits per carrier for 5.5 Mbps, and 8 bits per carrier for 11 Mbps

IEEE 802.11b physical layer is split into two parts, the Physical Layer Convergence Protocol (PLCP) and the Physical Medium Dependent (PMD) sublayer. The PMD takes care of the wireless encoding. The PLCP presents a common interface for MAC sublayer to write to and provides carrier sense and CCA (Clear Channel Assessment).[6]

PLCP has two structures, a long and a short preamble. All compliant 802.11b systems have to support the long preamble. The short preamble option is provided in the standard to improve the efficiency of a network's throughput when transmitting special data such as voice, VoIP (Voice-over IP) and streaming video.

The PLCP Frame Format

The PLCP Preamble

Synchronization (Sync) field consists of 128 bits for long preamble and 56 bits for short preamble.
16-bit Start Frame Delimiter (SFD) field is used to mark the start of every frame.

The PLCP Header
- 8-bit Signal or Data rate (DR) field indicates how fast the data will be transmitted.
- 8-bit Service field is reserved for future use.
- 16-bit Length field indicates the length of the ensuing MAC PDU (Medium Access Control sublayer's Protocol Data Unit).
- 16-bit Cyclic Redundancy Code (CRC) field is used for error detecting.

IEEE 802.11b Medium Access Control Sublayer

The MAC sublayer of the IEEE 802.11b serves as the interface between the physical layer and the host device. It supports both Infrastructure and Ad Hoc operation modes. Two robustness features in IEEE 802.11b MAC sublayer are Cyclic Redundancy Check (CRC) and Packet Fragmentation. Each packet has a CRC calculated and attached to ensure that the data are not corrupted in transit. Packet Fragmentation will send large packets in small pieces when sent over the air. This has two advantages. The first advantage is to reduce the need for retransmission because the probability of a packet getting corrupted increases with the packet size. The second advantage is that in case of packet corruption, the node needs to retransmit only one small fragment, therefore it is faster.

Inter Frame Space (IFS)

Four types of Inter Frame Spaces [4]

Short IFS (SIFS) is the period between the completion of packet transmission and the start of the ACK frame. (The minimum IFS)
Point Coordination IFS (PIFS) is SIFS plus a Slot Time.
Distributed IFS (DIFS) is PIFS plus a Slot Time.
Extended IFS (EIFS) is a longer IFS used by a station that has received a packet that it could not understand. This is needed to prevent collisions.

Physical Carrier Sense

IEEE 802.11b uses Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). CSMA/CA is referred to as the Distributed Coordination Function (DCF). This requires each station to listen for other users. If the channel is idle, the station may transmit. However if it is busy, each station must wait until transmission stops at which time the receiver sends ACK. Then each station must wait for a time equal to DIFS, plus a random number of slot times for next transmission in order to avoid collisions over the medium.

The CSMA/CA also includes an optional Point Coordination Function (PCF), which is used to set up an access point as a point coordinator. In this function, the point coordinator assigns priority to each client in a given transmission frame. The PCF option is used for multimedia traffic.[2]

Virtual Carrier Sense

An assumption of Physical Carrier Sense is that every station can "hear" all other stations. This is not always true. In "The Hidden-Node Problem," wireless stations A, B and C all can see Access Point P. A and B can see each other, and B and C can see each other, but A cannot see C.

To handle this problem, IEEE 802.11b specifies an optional Request to Send/Clear to Send (RTS/CTS), 4-Way Handshake, protocol. This protocol reduces the probability of a collision on the receiver area. When a sending station wants to transmit data, it first sends an RTS and waits for the Access Point to reply with a CTS. Since all stations in the network can hear the Access Point, the CTS causes them to delay any intended transmissions, allowing the sending station to transmit and receive a packet acknowledgment(ACK).

The IEEE 802.11b MAC Frame Format [4]

Frame Control (FC): protocol version and frame type (management, data and control).
Duration/ID (ID)

Station ID is used for Power-Save poll message frame type.
The duration value is used for the Network Allocation Vector (NAV) calculation.

Address fields (1-4) contain up to 4 addresses (source, destination, transmittion and receiver addresses) depending on the frame control field (the ToDS and FromDS bits).
Sequence Control consists of fragment number and sequence number. It is used to represent the order of different fragments belonging to the same frame and to recognize packet duplications.
Data is information that is transmitted or received.
CRC contains a 32-bit Cyclic Redundancy Check (CRC).

The usage of different Address fields (1-4) according to ToDS and FromDs bits setting
ToDS	FromDS	Address 1	Address 2	Address 3	Address 4
0	0	DA	SA	APA	N/A
0	1	DA	APA	SA	N/A
1	0	APA	SA	DA	N/A
1	1	RA	TA	DA	SA

Note:	DA	Destination Address		SA	Source Address
	RA	Recipient Address		TA	Transmitter Address		APA	Access Point Address

Frame Control Format [4]

Protocol Version indicates the version of IEEE 802.11 standard.
Type & Subtype: Type - Management, Control and Data , Subtype - RTS, CTS, ACK etc
To DS is set to 1 when the frame is sent to Distribution System (DS)
From DS is set to 1 when the frame is received from the Distribution System (DS)
More Fragment is set to 1 when there are more fragments belonging to the same frame following the current fragment
Retry indicates that this fragment is a retransmission of a previously transmitted fragment. (For receiver to recognize duplicate transmissions of frames)
Power Management indicates the power management mode that the station will be in after the transmission of the frame.
More Data indicates that there are more frames buffered to this station.
WEP indicates that the frame body is encrypted according to the WEP (wired equivalent privacy) algorithm.
Order indicates that the frame is being sent using the Strictly-Ordered service class.

Type and Subtype of the Frame Control Field
Type (b3 b2)	Type Description	Subtype (b7 b6 b5 b4)	Subtype Description
00	Management	0000	Association Request
00	Management	0001	Association Response
00	Management	0010	Reassociation Request
00	Management	0011	Reassociation Response
00	Management	0100	Probe Request
00	Management	0101	Probe Response
00	Management	0110-0111	Reserved
00	Management	1000	Beacon
00	Management	1001	ATIM
00	Management	1010	Disassociation
00	Management	1100	Deauthentication
00	Management	1101-1111	Reserved
01	Control	0000-0001	Reserved
01	Control	1010	PS-Poll
01	Control	1011	RTS
01	Control	1100	CTS
01	Control	1101	ACK
01	Control	1110	CF End
01	Control	1111	CF End + CF-ACK
10	Data	0000	Data
10	Data	0001	Data + CF-ACK
10	Data	0010	Data + CF-Poll
10	Data	0011	Data + CF-ACK + CF-Poll
10	Data	0100	Null Function (no data)
10	Data	0101	CF-ACK (no data)
10	Data	0110	CF-Poll (no data)
10	Data	0111	CF-ACK + CF-Poll(no data)
10	Data	1000-1111	Reserved
10	Data	0000-1111	Reserved

References

[1] 3com, "IEEE 802.11b Wireless LANs Wireless Freedom at Ethernet Speeds," [Online document],25 April 2000. Available URL: http://www.3com.com/technology/tech_net/white_papers/503072.html

[2] Al Petrick, "Standard and Protocols IEEE 802.11 b Wireless Ethernets," [Online document], June 2000. Available URL: http://www.csdmag.com/main/2000/06/0006stand.htm

[3] Angela Champness, "IEEE 802.11 DSSS: The Path To High Speed Wireless Data Networking," [Online document]. Available URL: http://www.wi-fi.net/downloads/weca80211boverview.pdf

[4] Breezecom, "IEEE 802.11 Technical Tutorial," [Online document]. Available URL: http://www.breezecom.com/Materials/PDFFiles/802.11Tut.pdf

[5] Dr. Arian Durresi, "Wireless Data Networking IEEE 802.11 & Overview of IEEE 802.11b," [Online document]. Available URL: http://www.cis.ohio-state.edu/~durresi/presentations/802.11.pdf

[6] Joel Conover, "Anatomy of IEEE 802.11b Wireless," [Online document], 7 August 2000, Available URL: http://www.networkcomputing.com/1115/1115ws2.html

[7] "Introduction to IEEE 802.11," [Online document]. Available URL: http://www.intelligraphics.com/articles/80211_article.html

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