IEEE 802.11

Download the IEEE 802.11 standard from IEEE.

Institute of Electrical and Electronics Engineers, Inc. (IEEE) has it's origin in the USA It is a nonprofit organization with "Operations Center" in New Jersey but are counting 375000 members in 150 countries.

In the early days of wireless networks, there were not any standards. The products from one vendor would not work with the products from another vendor. There were differences in the way they used the radio frequencies, how they talked to the medium and a lot of other things.

A work group in the IEEE was put together and on the 26th of June 1997 the 802.11 standard for wireless networks got it's approval.

This standard defines the Wireless LAN Medium Access Control (MAC) and the Physical Layer (PHY). It defines three different physical layers: Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS) and Infrared (IR). The FHSS and DSSS uses radio frequencies on the 2.4GHz Industrial, Scientific and Medical (ISM) band while IR uses infrared light. Still there are no products on the market that uses IEEE 802.11 IR. The FHSS and DSSS products are NOT compatible with each other.

The standard defines:
- The sharing mechanism: Carrier Sense Multiple Access / Collision Avoidance (CSMA/CA)
- Association and disassociation to the access point
- Authentication
- Energy control (Power Save modes)
- Point coordination (not roaming)
- Encryption
- Data rates of 1 and 2Mbps

CSMA/CA

CSMA/CD sends data on the cable and simultaneously listens for collisions. If a collision is detected, it will wait for a random time before it retransmits. This method can't be used on a wireless network as the radio is half-duplex and therefor can not listen while it sends. Instead CSMA/CA listens to the medium before it sends a Request To Send (RTS) to the Access Point. The AP allocates time for the transmission and sends a Clear To Send back to the client. Both the RTS and the CTS contains duration and ID for the transmission. All clients listen for the RTS and CTS packets and stores them in the Network Allocation Vector (NAV). The clients who does not "see" the RTS are sure to see the CTS from the AP and then backs of for the duration of the transmission.

The clients listens for silent medium

The client sends out an RTS

The access point allocates time and sends out an CTS

 

Frequency Hopping Spread Spectrum - FHSS

FHSS radio uses the frequencies from 2.4GHz to 2.4835GHz, a bandwidth of 83.5MHz. It uses 2 and 4 level Frequency Shift Keying (FSK) and divides the total bandwidth into 79 channels of 1MHz each. It then hops between all of these channels in one of 78 orthogonal (non colliding) patterns.

This makes it possible to have approximately 15 access points in an area before interference occurs. The downside is that this technique has a rather low throughput and shorter range than Direct Sequence Spread Spectrum. The standard says 2Mbps maximum but there are products that are capable of 3Mbps (proprietary).

FHSS Frequency Channel Plan

Channel ID	Frequency	Channel ID	Frequency	Channel ID	Frequency	Channel ID	Frequency
2	2402	22	2422	42	2442	62	2462
3	2403	23	2423	43	2443	63	2463
4	2404	24	2424	44	2444	64	2464
5	2405	25	2425	45	2445	65	2465
6	2406	26	2426	46	2446	66	2466
7	2407	27	2427	47	2447	67	2467
8	2408	28	2428	48	2448	68	2468
9	2409	29	2429	49	2449	69	2469
10	2410	30	2430	50	2450	70	2470
11	2411	31	2431	51	2451	71	2471
12	2412	32	2432	52	2452	72	2472
13	2413	33	2433	53	2453	73	2473
14	2414	34	2434	54	2454	74	2474
15	2415	35	2435	55	2455	75	2475
16	2416	36	2436	56	2456	76	2476
17	2417	37	2437	57	2457	77	2477
18	2418	38	2438	58	2458	78	2478
19	2419	39	2439	59	2459	79	2479
20	2420	40	2440	60	2460	80	2480
21	2421	41	2441	61	2461

Hop Set and Sequence

Set 1	0, 3, 6 ,9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57, 60, 63, 66, 69, 72, 75
Set 2	1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, 34, 37, 40, 43, 46, 49, 52, 55, 58, 61, 64, 67, 70, 73, 76
Set 3	2, 5, 8, 11, 14, 17, 20, 23, 26, 29, 32, 35, 38, 41, 44, 47, 50, 53, 56, 59, 62, 65, 68, 72, 74, 77

 

Direct Sequence Spread Spectrum (DSSS)

Direct Sequence Spread Spectrum uses the radio frequencies ranging from 2.4 to 2.4835MHz (83.5MHz bandwidth). It uses a Differential Binary Phase Shift Keying (DBPSK) and Differential Quadruple Phase Shift Keying (DQPSK) modulation and divides the total bandwidth into 13 channels (11 channels in the USA and "World Mode" and 14 channels in Japan). When configured, the radio stays on the selected channel. The term "spread spectrum" is used because the radio uses several frequencies at once.

DSSS Frequency Channel Plan

Channel ID	Frequency (MHz)
1	2412
2	2417
3	2422
4	2427
5	2432
6	2437
7	2442
8	2447
9	2452
10	2457
11	2462
12	2467
13	2472
14	2484

It is recommended to have 30MHz between the center frequencies of the deployed access points to avoid interference. This gives room for 3 access points in an area before interference occurs. Rules of thumb is to use the channels 1,7 and 13 (1, 6, 11 in the US and "World Mode"). Since there are many "World Mode" cards on the market, one should consider not using the channels 12 and 13 to ensure user compatibility. DSSS has a somewhat longer range than FHSS and opens for a potential higher throughput than the 2Mbps that the standard defines.

This is how one should space the channels of a DSSS system. 30Mhz between center frequencies avoids interference with the other access point.

 

FHSS vs DSSS

Both techniques are designed to handle interference in the medium. FHSS does this by hopping between small channels across the entire bandwidth. If a Narrow Band Interference occurs somewhere in the bandwidth, the FHSS just hops to the next position. DSSS uses a broad channel and hopes that nothing covers the entire channel. A narrow band interference occurring across the used channel will stop DSSS communications. FHSS is not as complicated as DSSS and uses less energy, but DSSS has the longer range.

Read more about FHSS, DSSS vs. radio interference here.