Enterprise-Class Telephony on Wireless LANs. Tom Alexander CTO VeriWave, Inc. The Leader in Wireless LAN Testing

Transcription

1 Enterprise-Class Telephony on Wireless LANs Tom Alexander CTO VeriWave, Inc. The Leader in Wireless LAN Testing

2 SERIAL SERIAL ETHERNET ETHERNET The Converging Network Voice over IP (VoIP) enables convergence of traditional PBXs and enterprise LANs Advantages: reduced infrastructure, reduced operating costs, advanced services Voice over WLAN (VoWLAN) drives further integration Adds mobility Equipment available today First products in 1996 Cisco, Symbol, SpectraLink, Alcatel, Avaya, NEC, Nortel IP-PBXs being installed Protocols: H.323, SIP, Megaco Cellular/WLAN convergence on the horizon 2

3 Cellular/WLAN Convergence Unlicensed intra-building access attractive to carriers Bundled services Cingular, T-Mobile, Verizon, etc. UMA Network Controller Roaming Access via GSM/GPRS Internet & Cellular Standards being developed Handover, multi-radio handsets Example: Unlicensed Mobile Access GSM/GPRS & WLANs Now part of 3GPP Corporate LAN AP Intra-building Access Via WLAN Handoff 3 Source: UMA Technology website

4 Enterprise Voice Requirements Performance Toll Quality voice Seamless handling of mixed voice/ environments affected by Delay Jitter Packet loss Capacity Reliability & Usability Available across the entire corporate campus Always-On Secure communication Centrally managed by IT affected by Access Point coverage Seamless roaming handoff Security protocols Battery life (talk time) 4

5 Wireless vs. Wired Wired network attributes: High bandwidth Secure Isolated Predictable and fixed topology Wireless network attributes: Limited bandwidth, shared among many users Insecure against both eavesdropping and Denial of Service (DoS) Prone to interference from other networks and from non-wlan devices Highly dynamic topology with special mobility issues 5

6 Network Impairments Packet Loss Jitter Congestion AP AP Call Server & VoIP Gateway PBX Call Server & VoIP Gateway LAN infrastructure introduces impairments into voice traffic Delay, jitter and packet loss Congestion resulting in collisions and dropouts Roaming delays Issues due to contention-based MAC protocol 6

7 Measuring The Reality recent test The goal: characterize network performance Real network: VoIP phones, Access Points & switch Enterprise-class voice and applications What was Measured call quality (R-value, jitter, loss) call quality degradation with loading failover time, call drop counts, R-values varying number of handsets Quality of Service (QoS) schemes artificial delay introduction User Experience Toll quality voice mixed voice/ networks mobile usage conference room environment operational cost inter-campus environment 7 Results:

8 The ITU-T E-Model of Speech Quality Send side SLR OLR dbr point RLR Receive side Ds-factor Weighted echo path loss WEPL Dr-factor Room noise Ps Circuit noise Nc referred to dbr Round-trip delay Tr Coding/Decoding Equipment impairment factor Ie Packet-loss robustness factor Bpl Packet-loss probability Ppl Mean one-way delay T Room noise Pr Sidetone masking rating STMR Listener sidetone rating LSTR (LSTR = STMR + Dr) Absolute delay Ta Quantizing distortion qdu Expectation factor A ITU-T G.17 Objective speech quality metric Measures system characteristics, then calculates voice quality as R Value (-1) Talker echo loudness rating TELR G.17_F1 Directly relates to user satisfaction metrics such as Mean Opinion Score R Value of 8 is approximately equivalent to toll-quality voice (MOS = 4.) 8

9 # # # # # # The Measurement Setup AP VoIP Server & Gateway AP DUT 18 WLAN devices 14 VoIP handsets 2 Enterprise APs 2 Traffic Generator / Analyzer units Wired network VoIP call server/gateway WLAN switch/router Other LAN devices WLAN Switch BSS #1 BSS #2 Traffic Generator & Analyzer Traffic Generator & Analyzer 7 VoIP/WLAN Handsets 7 VoIP/WLAN Handsets 9

10 The Performance Challenge VoIP places significant load on enterprise WLAN equipment Without QoS enabled, very few calls could be supported Sometimes no more than 4-5 calls per AP With QoS, more calls supported, but frequent call drops and poor voice quality scores measured in some cases Injection of small amounts of (1 Mb/s) caused precipitous R value reduction, dropped calls 1 High roaming times seen for failover from primary to backup AP On one particular enterprise WLAN switch: Total time for 128 golden Layer 4 clients to roam: 37 milliseconds Average time for 14 VoIP handsets to roam: seconds Worst-case handset roaming time >1 seconds! Call outages and drops likely

11 R Value Scores Measured 8 TESTED THROUGH 1 ACCESS POINT Near toll quality: R-value above R-value Unacceptable voice quality: R-value below 5 Aruba Chantry Cisco Colubris 2 1 No QOS: 6 calls, no With QOS: 6 calls, no No QOS: 6 calls with With QOS: 6 calls with No QOS: 7 calls, no With QOS: 7 calls, no No QOS: 7 calls with With QOS: 7 calls with Source: David Newman, Network Test 11 Published in Network World Fusion, Jan 25

12 Roaming Times Measured 6 LOWER NUMBERS ARE BETTER 5 Roaming time (seconds) calls failed 1 call, no 6 calls, no 6 calls with 7 calls, no 7 calls with Aruba Chantry Cisco Colubris Unable to test with 6 or 7 calls Not tested with 6 calls Unable to test with 7 calls; 3 calls dropped in 6-call test Source: David Newman, Network Test 12 Published in Network World Fusion, Jan 25

13 Selecting For Success WLAN Access Points & switches are key Must support QoS Preferential treatment of voice over Minimize jitter and packet loss for VoIP traffic Efficient use of wireless medium Fast roaming support is also essential Minimize call drops when users move Avoid dropped calls or dead times when backups take over on equipment failure Network infrastructure must be matched to handsets and VoIP gateways Handsets and APs work together to provide service Roaming is a co-operative function of both handsets and APs 13

14 The Future WLAN QoS protocols 82.11e QoS over WiFi Protocol also assists in better power management for handsets and clients longer battery life Faster roaming support 82.11r Fast Roaming protocols 82.11k Radio Resource Management assists handsets in locating and selecting APs during roaming Better security 82.11i / WPA2 WiFi security protocol 82.11i provides for pre-authentication to speed up roaming 14