How do I evaluate the quality of my wireless connection?

Transcription

1 Hw d I evaluate the quality f my wireless cnnectin? Enterprise Cmputing & Service Management A number f factrs can affect the quality f wireless cnnectins at UCB. These include signal strength, pssible prblems that can degrade even a strng signal, and factrs that can affect the data transmissin rate (thrughput). 1 Determining Signal Strength The wireless standard g perates in the GHz (gigahertz) radi frequency (RF) band; RF is measured in decibels (db). Wireless cards ften cme with client sftware that displays signal strength in db r dbm (a variant f db that prvides an exact crrelatin t the pwer f the radi signal in watts). In the samples belw we are lking at the Intel Wireless and the Windws Cnnectin managers t check infrmatin abut the wireless cnnectin. These are but tw examples f what wireless managers may lk like and the infrmatin they prvide. Yu may use a different manager with yur card, but all f these wireless cnnectin managers shuld prvide similar infrmatin. Intel Wireless Sftware (pen Intel PROset Wireless sftware click Advanced Advanced Statistics ) 1. Signal Strength as a Graph 2. Speed & Signal Quality (54.0 Mbps & Very Gd is maximum) 3. Signal Strength in dbm

2 Windws Wireless Sftware (duble-click Wireless Netwrk Cnnectin in Cntrl Panel Netwrk and Sharing Center ) 2 Calculating & Interpreting Signal Strength Nte: The minimum pwer sensitivity n mst g clients is -96dBm (very lw). If yur sftware displays Signal/Nise r in dbm, yu can cnvert this t db by subtracting the minimum pwer sensitivity, - 96dBm, frm the number displayed as r Signal/Nise. Fr example, if the Signal/Nise is -47dBm, yu wuld cnvert this t db as fllws: -47dBm (-96dBm) = 49dB If yur wireless card sftware is indicating that the signal-t-nise rati () is greater than 80dB, yu are getting the maximum available bandwidth, r 54 Mbps (megabits per secnd) fr g. An higher than 80dB wn t increase the amunt f bandwidth beynd this maximum (in the example abve, with an f 49dB, the bandwidth is still 54 Mbps, the maximum available rate). When the drps belw 80dB, hwever, the maximum data rate drps: (db) Maximum Raw Data Rate (Mbps) (db) Maximum Raw Data Rate (Mbps) Maximum Raw (db) Data Rate (Mbps) Even thugh the maximum data rate ges dwn, the cnnectin will still be maintained as lng as yu have an f 4dB r greater Cmputing Services Haas Schl f Business Page 2

3 3 Pssible Prblems (Diagnsis) Other factrs can affect the quality f yur wireless cnnectin. The list belw is nt cmplete, but it may ffer sme explanatin fr pr perfrmance that ccurs even when the signal strength is gd: MULTIPATH: In general, an RF signal grws wider as it is transmitted further. As it spreads, the RF signal will meet bjects in its path that will interfere with the signal in varius ways (e.g. by reflecting it). When the signal is reflected by an bject (e.g. a metal bject) while mving tward a receiver, multiple wave frnts are created, ne fr each reflectin pint. This can result in a large number f waves, depending n hw many reflecting surfaces the riginal signal encunters. Many f these reflected waves are still mving tward the receiver, creating a cnditin knwn as multipath. As the number f reflective surfaces increases, the signal deterirates. SOLUTION: Mve clser t the wireless access pint. Wireless Access Pint Yur Laptp NEAR/FAR: Near/far is a prblem that can happen when multiple wireless users have devices that are very near an access pint, much clser than a user wh s n the radi signal bundary. The device farthest away simply cannt be heard ver the traffic frm the devices clser t the access pint. SOLUTION: Mve the mre distant device clser t the access pint. Wireless Access Pint Yur Laptp NEAR FAR HIDDEN NODE: When yu turn n an g capable laptp r handheld device, it immediately scans the airwaves fr access pints. It quickly evaluates the signal strength f the available access pints, and the number f users per access pint. Based n this, the device will chse the access pint with the strngest RF signal and the fewest users. In hidden nde situatins, at least ne client (nde) is unable t hear ne r mre f the ther clients cnnected t the same access pint. Usually this is because f sme physical bstructin Cmputing Services Haas Schl f Business Page 3

4 between it and ther users. As a result, there can be prblems in the way the clients share the available bandwidth, causing data cllisins r bit errrs. When a bit errr ccurs, the clients need t re-transmit the data. These cllisins can result in significantly degraded data transmissin rates in the wireless netwrk. DYNAMIC RATE SHIFTING (DRS): Adaptive (r Autmatic) Rate Selectin (ARS) & Dynamic Rate Shifting (DRS) are terms used t describe hw bandwidth is adjusted dynamically by wireless clients. This adjustment in speed ccurs as distance increases between the client and the access pint (r pssibly if interference increases). As the distance grws greater, the signal strength will decrease t a pint where the current data rate cannt be maintained. As the signal strength drps, the client will drp its data rate t the next lwer specified data rate, fr example, frm 11 Mbps t 5.5 Mbps, r frm 2 Mbps t 1 Mbps. INAPPROPRIATE AP SELECTION: Sme wireless clients will attempt t cnnect t a distant high bandwidth (802.11g/n) access pint with a weak signal, rather than cnnecting t a nearby lw bandwidth (802.11b/g) access pint with a strng signal. The result can be drpped cnnectin with reduced thrughput. 4 Thrughput (Pssible Explanatins) Thrughput is a measure f the speed f yur wireless cnnectin. Defined as the amunt f data transmitted in a given time perid, thrughput is based n many factrs. Five imprtant factrs fr yur cnsideratin SHARED BANDWIDTH: All wireless clients cnnected t an access pint must share the bandwidth available. As mre peple cnnect, the cntentin fr the available bandwidth increases, causing interference and increased transmissin errr rates. POSSIBLE SOL N - cnnect t anther access pint which des nt have as many peple using it. INTERFERENCE FROM ANOTHER RADIO FREQUENCY SOURCE: b/g perates in the radi frequency (RF) band between 2.4 and GHz. This is an unlicensed range f frequencies, meaning the Federal Cmmunicatins Cmmissin (FCC) allws anyne t use it. Unfrtunately fr b/g users, micrwave vens and 2.4 GHz crdless phnes perate in the same frequency band. The radi signal emanating frm such devices can severely degrade an b/g signal. POSSIBLE SOL N - mve away frm surces that perate in the 2.4 GHz frequency range. SECURITY: b/g netwrks are susceptible t hacking and data theft by unauthrized users. EVERY wireless user shuld use sme frm f security, mst ften SSL. This adds mre verhead t the wireless data packets, abslutely necessary but using valuable bandwidth. Althugh g allws fr 54 Mbps maximum thrughput, a user will typically nly get abut Mbps f data. DISTANCE: Greater distances between the transmitter (access pint) and receiver (client, yu) will cause the thrughput t decrease because f an increase in the number f errrs can recgnize these errrs and retransmits data. Als, g is cnfigured t make discrete jumps t specified data rates (54, 48, 36, 24, 18, 12, 9, and 6 Mbps). If 54 Mbps cannt be maintained because f errrs & degrading signal strength, then the device will drp t 48 Mbps, then dwn t 36 Mbps, all the way dwn t 6 Mbps, with the eventual lss f the cnnectin. Remember that, because f verhead, yu will nly receive abut half f the available bandwidth. POSSIBLE SOL N - mve clser t a wireless access pint r stp mving away frm ne. B/G INTEROPERABILITY: The presence f an b participant significantly reduces the speed f an g netwrk. When bth b and g clients are cnnected t an g access pint, the perfrmance f the g clients can suffer. RESOLVED - Airbears currently nly allws fr g clients and s des nt experience this prblem. Cmputing Services Haas Schl f Business Page 4

5 5 Recmmendatins Avid cngested wireless netwrks and mve clser t access pints when in and arund Haas. Use wired cnnectins in any f the classrms, arund Haas, r in the main lab (yu are able t check ut an Ethernet cable frm Haas Media Services in S300). Make sure yur wireless card is capable f using a r g - yu will be unable t cnnect t Airbears therwise. If yu cannt get past the Airbears Wireless Vulnerability Scan, please d ne f the fllwing: Shut dwn and cmpletely pwer ff yur cmputer fr 20 t 30 minutes and then try again. Cntinue t use yur cmputer with the wireless card disabled fr 20 t 30 minutes and then try again. While ding ne f the abve, check-ut an Ethernet cable frm Haas Media Services and cntinue t use the internet frm a wired cnnectin. Cmputing Services Haas Schl f Business Page 5