PART II: TERMINOLOGY AND JARGON

Transcription

1 PART II: TERMINOLOGY AND JARGON Wi-Fi is everywhere. Wi-Fi has grown from a technological curiosity to an ubiquitous and non-negotiable service over the last ten years. So, why is it so nebulously handled by IT admins? Why does conference Wi-Fi so often stink to high heaven? Tickets to troubleshoot it land in the help desk s lap, but most of the time, the fixes and administration are handled elsewhere, leaving the average Apple admin out in the cold when it comes to understanding how Wi-Fi works, how to troubleshoot it smartly, and how to handle planning of medium- and large-scale wireless networks that work well. QUESTION: How many IT Directors? Consultants? Help Desk? Other?

2 JARGON Basic Radio Terminology WiFi Jargon How Some of the Terms Relate One or Two Lies

3 BASIC RADIO TERMINOLOGY

4 JARGON Frequency Transmission Power Attenuation Interference Signal Strength, Noise, and SNR (where the gold is) We re going to talk about a series of terms in their radio specific contexts. These are definitions that are helpful primarily in how well they ll fit into the metrics that you can measure and that affect the work that you ll do in the field.

5 JARGON FREQUENCY When it comes down to it, frequency is how often something happens over a period of time. Measured in units of Hz Frequency is the base unit of any radio transmission. Your radio receiver and transmitter are keyed to the same sets of frequencies. When it comes to Wi-Fi, you re either operating on the 2.4GHz frequency band, or the 5GHz frequency bands.

6 FREQUENCY Here is the frequency band chart for the radio spectrum in the United States. Radio, as was discovered by early wireless telegraph operators, is a shared space medium. Part of the reason radio is regulated the way that it is was that in the early days of wireless telegraphy, a news service broadcast their news update and then left the key down on the spark gap transmitter, effectively blocking the other wireless services from getting the story back to headquarters via wireless.

7 FREQUENCY These two arrows show what small areas of the spectrum that Wi-Fi occupies.

8 JARGON TRANSMISSION POWER Transmission requires applying electrical power Increasing power increases transmission range. In Wi-Fi world, power output measured in dbm or mw, varying by vendor choice Transmission Power has

9 TEXT TRANSMISSION POWER All of your WiFi devices transmit All of your WiFi devices AP and STA have transmission power ratings Often the power ratings differ

10 OCCURS WHEN SIGNAL PASSES THROUGH CONSTRUCTION MATERIALS. ATTENUATION

11 INTENSITY OF SIGNAL RADIATING FROM A POINT SOURCE IS INVERSELY PROPORTIONAL TO THE SQUARE OF THE DISTANCE FROM THE SOURCE. ATTENUATION AND THE INVERSE SQUARE LAW Courtesy NASA/JPL/CalTech Standing this close to the sun, if you could do it, would be a fatally brief experience.

12 IN OTHER WORDS, THE FURTHER YOU GET AWAY FROM A SIGNAL SOURCE, THE LESS INTENSE IT IS. ATTENUATION AND THE INVERSE SQUARE LAW Courtesy NASA/JPL/CalTech While if you re standing on Mars, the intensity of the overall sunlight is vastly lower. You can t breathe here, but you don t burn in a nuclear fusion-powered hellfire.

13 λ 2.4GHZ = 12.5CM 5.745GHZ = 5.47CM WAVELENGTH MATTERS The wavelength of the transmission affects how it is degraded by a given material. Longer wavelengths can go greater distances and through more difficult media. 2.4GHz s longer wavelength is more effective at penetrating structured walls. Why is that? It has everything to do with how these signals intersect with construction materials. The tighter the signal (the shorter the wavelength), the higher likelihood of reflection at the material boundary.

14 JARGON ATTENUATION Materials generally impose some level of attenuation, and you ll find listings in a lot of applications and documentation. Drywall imposes some, brick and concrete impose a lot Some other unexpected things impose it as well. Foundation Wall -15dB Brick/Concrete/Blocks -15dB Elevator or metal obstacle -10dB Metal Rack -6dB Drywall/Sheetrock -3dB Non-tinted Glass -3dB Wood -3dB Cubicles -2dB

15 JARGON INTERFERENCE Something that modifies or disrupts a signal as it travels Unwanted signal added to wanted signal Have someone talking while I talk. We can all have conversations at once, and it will be substantially harder to discern the signal (your conversation) from the conversations around you (noise). Moreso, though, interference is often in a different language. if Chris starts clapping while I m speaking, it s even harder to hear. When it comes to Wi-Fi, much of the interference that will make it hard to build great networks will be other signals, but the hardest to cope with can be interference of a non variety.

16 THIS ISN T ALL, BUT WE RE GOING TO COME BACK TO IT. INTERFERENCE

17 JARGON SIGNAL (STRENGTH) Measure of an electric field at a defined distance of from a transmitter.

18 JARGON SIGNAL STRENGTH In WiFi, we typically refer to RSSI (Received Signal Strength Indicator) as our measure.

19 JARGON SIGNAL STRENGTH In WiFi, these numbers appear goofy and nonsensical; they re non-standardized. The higher the integer, the better the overall signal -85 to -45 are not unusual -85 is bad.

20 JARGON SO, UH, WHAT S A DBM? Sir Mix-A-Lot, Possibly

21 JARGON DBM Decibel Milliwatts Measure of Power But not a linear measure Rules of 3 and 10 apply Just a reminder that we re working in a logarithmic world. I won t spend time on the mathematics here, because I was promised there would be no math, but there are rules that apply here. A decrease of 3 dbm is equivalent to a loss of 50% of your received power in milliwatts. A decrease of 10 dbm is equivalent to a loss of 1000% (a factor of ten) of your received power. The increase does the opposite. Increase of 3dBm is equivalent of doubling your received signal strength, and increase of 10dBm is ten times the signal. Again, we re working with exponential scales here because of the inverse square law. But those numbers are harder to fathom, and that s why we do a lot of our work in dbm, which assumes a perfect 1mW of power at 0, and goes downward from there. That s why we re working with negative numbers.

22 JARGON RULES OF THREE AND TEN +3 dbm, multiply by 2-3 dbm, divide by dbm, multiply by dbm, divide by 10

23 JARGON NOISE Superposition of white noise onto a signal, from a variety of sources A source of interference.

24 MEASURED & REPORTED BY THE MAC OS X COREWLAN FRAMEWORK NOISE

25 THE INFORMATION IS THEREFORE AVAILABLE IN A VARIETY OF TOOLS. NOISE

26 INDIVIDUALLY, SIGNAL AND NOISE ALONE AREN T WORTH A BUCKET OF WARM SPIT. BUT

27 WHERE THE MAGIC HAPPENS. SIGNAL TO NOISE RATIO (SNR)

28 JARGON SIGNAL TO NOISE RATIO (SNR) Subtract the Noise measure from the Signal (RSSI) level The larger the number, the better the SNR Apple s WiFi Menu item does not explicitly provide this

29 (-72) - (-95) = 23 RSSI - NOISE = SNR

30 WIFI-SPECIFIC TERMINOLOGY

31 JARGON BASIC TERMS WiFi Equipment (The Pieces) Frequency Band PHY Mode Channel Co-Channel and Adjacent Channel Interference

32 JARGON ADVANCED JARGON: DOWN THE RABBIT HOLE MIMO Spatial Streams Guard Interval MCS Index Transmit Rate

33 BASIC WIFI TERMINOLOGY

34 JARGON THE PIECES AP (Access Point): A central transmitter. STA (Station): A client WiFi device. The official documentation uses this, so we ll try to use it as well. (You see AP all the time, but STA not as much)

35 JARGON FREQUENCY BAND Shorthand that describes a collection channels in the same general area of the spectrum.

36 THE 2.4 GHZ BAND CONSISTS OF ONE COLLECTION OF CHANNELS, WHILE THE 5 GHZ BAND CONSISTS OF ANOTHER COLLECTION. FREQUENCY BAND

37 JARGON FREQUENCY BAND: 2.4 GHZ 11 (1-11) Channels in the United States 2412 MHz MHz The original frequency band, used for b, g, and early n

38 JARGON FREQUENCY BAND: 5 GHZ Many more channels, depending on how you look at them 5180 MHz-5340 MHz, 5500 MHz-5700 MHz, 5745 MHz MHz n and ac

39 2 160MHz 6 80MHz MHz MHz

40 JARGON PHY MODE A shorthand way of stating the standard in use within the family b g a n ac Can tell you a lot about the theoretical capabilities of a network. Note that standards are mostly confined to a single frequency band, but that this isn t completely true.

41 JARGON CHANNEL Shorthand description of a defined frequency band in the spectrum. Channel 149 = 5745 MHz (5.745 GHz)

42 CAUSED BY EXTRANEOUS POWER IN A NEARBY CHANNEL TO THAT WHICH IS BEING USED FOR COMMUNICATION. ADJACENT CHANNEL INTERFERENCE

43 ADJACENT CHANNEL INTERFERENCE CENTURYLINK 2308 IS BROADCASTING ON CH. 9, WHICH OVERLAPS WITH EVERYTHING ON 6, 11, AND THE OTHER NON-9 CHANNELS BETWEEN. In the 2.4GHz spectrum, there are 11 channels defined as part of the spectrum. These channels are spaced at 5MHz apart, but the signal spread of the OFDM subcarriers in g/n is 20MHz (802.11b uses 22MHz channels with DSSS). So, the only clear non-overlapping channels in 2.4 are 1, 6 and 11. Century Link as depicted here is hanging out on 9, right in between 6 and 11, and so will have to cope with some signal interference with the edge subcarriers of the signals of channel 6 and 11. We ll talk in a minute about why that might be desirable.

44 JARGON CO-CHANNEL INTERFERENCE WiFi standard requires that devices using the same channel be polite to one another. When one device transmits, all other devices on that channel must wait. The transmitting device has the Conch Technically this is not interference.

45 THIS BEHAVIOR IS DETERMINED ON A CHANNEL-BY-CHANNEL BASIS. CONSEQUENTLY, TWO DIFFERENT NETWORKS ON THE SAME CHANNEL THAT ARE DETECTABLE BY THE OTHER WILL EFFECTIVELY INTERFERE WITH ONE ANOTHER. CO-CHANNEL INTERFERENCE

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47 GREENPUPPY (CH. 6) AND SMOKY (CH. 11) BROADCAST ON DIFFERENT CHANNELS WITH NO OVERLAP, AND THEREFORE DO NOT INTERFERE WITH ONE ANOTHER. CO-CHANNEL INTERFERENCE

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49 GREENPUPPY (CH. 6) AND MOO-G (CH. 6) BROADCAST ON THE SAME CHANNEL, SO THEY AND THE CLIENTS CONNECTED TO THEM MUST BE POLITE TO ONE ANOTHER; EFFECTIVELY, THEN INTERFERE WITH ONE ANOTHER. CO-CHANNEL INTERFERENCE

50 THE MORE APS AND STAS YOU HAVE ON THE SAME CHANNEL, THE MORE CCI WILL HURT YOUR OVERALL PERFORMANCE. CO-CHANNEL INTERFERENCE

51 REMEMBER THAT CO-CHANNEL INTERFERENCE OCCURS BY DESIGN. YOU LL SOMETIMES SEE SOURCES REFER TO IT AS COOPERATION AS A RESULT. CO-CHANNEL INTERFERENCE COOPERATION

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53 JARGON TRANSMIT RATE (TX) The agreed upon rate at which a STA can communicate with an AP Expressed in megabits per second (Mbps)

54 ADVANCED JARGON DOWN THE RABBIT HOLE

55 JARGON ADVANCED JARGON MIMO Spatial Streams Guard Interval MCS Index

56 JARGON MIMO: MULTIPLE-INPUT, MULTIPLE-OUTPUT Increase your transmit and receive capacity by using multiple transmit and receive antennas to send more data. Uses multiple signal paths, which can mitigate interference and attenuation. It s pronounced with a hard G, like gif.

57 JARGON MIMO Different configurations possible Equipment that usually specifies so on the spec sheet (unless the vendor is Apple) Antenna configuration represented by: # Tx Antenna x # Rx Antenna - 3x3

58 THE NUMBER OF TRANSMIT AND RECEIVE ANTENNAS AT EACH END OF A WIRELESS CONNECTION CONTRIBUTE TO THE OVERALL PERFORMANCE AVAILABLE. THE LOWEST NUMBER IMPOSES A LIMIT BY DEFINING THE NUMBER OF AVAILABLE SPATIAL STREAMS. MIMO

59 JARGON SPATIAL STREAMS Each transmit/receive antenna generates a spatial stream (a separate transmission of data) Hardware spec sheets display the number of spatial streams after the Tx Rx, e.g. Tx Rx: Streams 3 3 : : 2

60 OKAY, BUT WHY DO I CARE ABOUT MIMO AND SPATIAL STREAMS? They get you partway to determining how fast your connection will go, but you need to know a few other things to get there.

61 JARGON CHANNEL WIDTH Standard channel width was 20 MHz n allows the creation of wider channels that use 40 MHz of spectrum, contributing to higher data rates ac allows creation of 80 MHz and 160 MHz channels.

62 40 MHZ CHANNEL WIDTHS IN 2.4?! WEEHAWKEN. DAWN. GUNS. DRAWN.

63 BECAUSE THE SPECTRUM IS FINITE AND CONSTRAINED, EACH DOUBLING OF CHANNEL WIDTH ROUGHLY HALVES THE NUMBER OF AVAILABLE CHANNELS. CHANNEL WIDTH

64 JARGON GUARD INTERVAL Guard interval is the period of time that a transmitter will pause between transmissions to prevent them from interfering with one another 800ns (Guard Interval) and 400ns (Short Guard Interval) Shorter pauses can increase your data rate

65 JARGON MODULATION AND CODING SCHEME (MCS) Describes how data is encoded combined with a rating that describes how efficiently the data stream is being used to transmit data. It s a bit like orchestrating a piece of music. The more instruments you can use at once, the richer the sound. Sometimes you can play a string quartet. Sometimes you can play a symphony with a double brass band. The Modulation & Coding Scheme is based on signal strength and agreed

66 JARGON THIS LANDING IS GONNA GET PRETTY INTERESTING. Wash

67 TEXT MCS INDEX Number of Spatial Streams + Modulation & Coding Scheme determines MCS Index And note that this is a lowest common denominator evaluation. If you have an AP capable of 3, but clients are only capable of 2, you get two!

68 TEXT MCS INDEX TO TRANSMIT RATE Now cross-reference MCS Index against Channel Width and Guard Interval to generate Transmit Rate

69 CROSS REFERENCE USING WHAT, EXACTLY? MCS INDEX TO TRANSMIT RATE

70 Borrowed from WLAN Pros

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72 TX RATE IS A (NECESSARY) LIE

73 JARGON TX RATE IS A (NECESSARY) LIE WiFi is a half-duplex technology: it can only transmit or receive; it can not do both simultaneously In the best possible case, the maximum throughput is approximately half of the reported transmit rate Numerous environmental factors will reduce it.

74 Here s an example iperf test showing the transfer of data. You can see the Bandwidth listed at left, around 135Mbps, and the Tx Rate of 270Mbps