Access Time Delay

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:

1. Signaling for call set up, tear down and other call control communications will be delayed. (Worst case delay is the principal concern.)
2. Jitter in the voice traffic/bearer channel will cause delay.

VoIP signaling and voice traffic are not separate communications channels. VoIP packets exist as virtual communications within a single channel. Only queuing priorities can ensure timely delivery of voice packets when other types of packets are competing for services in an IP network. This situation is complicated when a wireless user is moving and there are AP-to-AP handoffs in the network. Further delays are added into the WLAN as the user must associate with an AP, authorization must take place and the handoff must be completed.

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

Table 11

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.

Figure 10

Because delay in the PSTN backbone network is beyond the control of mobile device manufacturers, the following table illustrates a residual delay budget for the backbone network (Tbackbone) using short duration G.711 and G.726 VoIP packet structures. The data for this table is taken from measurements that were performed on VoIP software implementations and from worst case delays in an extensive wireless enterprise network.

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

(1) Tbuf can be set as large as Tbackbone
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:

Figure 11 Any effects communication delay could have on voice quality for a fixed implementation of VoIP over WLAN is readily ameliorated, but larger issues arise when a user is handed off from one AP to another.

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:

1. The WLAN must know when a link has been lost. (This can be a simple rule, such as losing more than N/M packets.)
2. AP probe and associate.
   1. Currently, tests by the University of Maryland show that an AP probe takes place in 250 to 400 msec.
   2. 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.
3. Authentication, security and routing updates.
   1. Delays of more than three seconds have been caused by centralized authentication servers.

Clearly, dramatic improvements are needed in AP probe, authentication and routing update operations. WLANs have delays for handoffs nearly 10 times greater than handoffs in cellular systems.

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:

1. Nearest neighbor or sub-net authorization proxy (authorization across a sub-net).
2. Highest QoS priority to an AP's probing services.
3. Shadow registration in enterprise WLANs. That is, a subscriber will be pre-registered and authorized within a sub-net.