Regardless of the application, the time delay and jitter of the VoIP system will be a design consideration. As already noted, the two following issues relate to time delay and jitter:
- Signaling for call set up, tear down and other call control communications will be delayed. (Worst case delay is the principal concern.)
- Jitter in the voice traffic/bearer channel will cause delay.
The ITU has set recommendations for the maximum round trip delay in a voice system and the perceived quality of the voice channel. This recommendation is defined in ITU G.113 and is provided in the following table:
G.113 Delay Specification | |
0 to 150 msec | acceptable to most applications |
150 to 400 msec | acceptable for international connections |
> 400 msec | acceptable for public network operation |
Under normal operations, the roundtrip delay should be less than 150 msec. The following diagram illustrates the possible delays in the communication path for an enterprise network with one layer of routing.
Round Trip Delay Source | Delay Symbol | G.711 (5msec) | G.726 (10msec) |
Encode / Decode | Tenc_dec | 5.5 | 5.5 |
Assemble / Disassemble | Tasm | 10 | 20 |
Jitter Buffer (1) | Tbuf | 10 | 20 |
WLAN Access Delay | Taccess | 10 | 20 |
Access Point Routing | Tap_route | 10 | 10 |
Enterprise Routing | Trouting | 5 | 5 |
Backbone Delay (residual) | Tbackbone | 99.5 | 69.5 |
Total | 150 | 150 |
Tabel 12
These figures indicate that the G.726 residual backbone delay will be less than 70 msec, but there is some concern in the industry that in countries with large geographic areas, such as the U.S., Canada and others, the backbone network delay may exceed 70 msec.
For a VoIP cordless phone, the equivalent diagram of the communication path would remove the enterprise router and feature a minimum of 10 msec of additional delay margin (the maximum delay for the POTS PSTN network). The residential delay path is shown in the following diagram:
In a cellular phone system, a great deal of effort is expended on the handoff operation. The handoff is typically completed in 35 msec with 50 msec being worst case. WLAN systems do not have the interconnect processing capabilities or higher-order switching intelligence that is built into cellular networks. In a WLAN, the following capabilities are relevant to the network's ability to hand off active phone calls:
- The WLAN must know when a link has been lost. (This can be a simple rule, such as losing more than N/M packets.)
- AP probe and associate.
- Currently, tests by the University of Maryland show that an AP probe takes place in 250 to 400 msec.
- Significant effort to improve both AP-to-AP handoffs and authentication are being addressed by the IEEE 802.11 committee's 802.11i (security) and 802.11e (QoS) task groups.
- Authentication, security and routing updates.
- Delays of more than three seconds have been caused by centralized authentication servers.
During the handoff between 802.11 APs, there will be short but noticeable loss of voice packets. On the positive side, VPN and call agent servers have timeouts on the order of tens of seconds. While some VoIP packets may be lost in a WLAN, connection should be maintained.
Several proposals for solving the hand-off delay problem have been proposed. These include:
- Nearest neighbor or sub-net authorization proxy (authorization across a sub-net).
- Highest QoS priority to an AP's probing services.
- Shadow registration in enterprise WLANs. That is, a subscriber will be pre-registered and authorized within a sub-net.