It’s been a long time since I’ve had a cordless phone in the house or seen one in business, but I’m expecting that will change. I know that in the US, the 900 MHz spectrum, set aside by the Federal Communications Commission for unlicensed use, was considered a too-successful implementation. Too many people in too dense an environment deployed cordless phones and interfered with each other’s baby monitors, calls and sometimes garage door openers. You know a technology has gone mainstream when there’s cartoons about the technology.
Wireless VoIP refers to the use of Wi-Fi networks, as the wireless technology, to connect Wi-Fi phones to the IP network behind the wireless access point and ultimately connect the wireless VoIP phone to the other elements of the voice communications service, such as the soft-switch (if a public, residential or hosted VoIP service), the PSTN gateway or application server delivering voice mail storage or other service.
The challenge with wireless VoIP is quality.
Wi-Fi was originally engineered as a shared data service, where all users shared in access to the limited resource – spectrum. It was built with a mechanism to allow competing users to back-off for a random time, before transmitting like they did in the original Ethernet days of shared 10 M service with collisions and retransmissions galore. This works well when there are a few users on an Access Point, and when there are many users on an Access Point. The AP simply slows down the average bandwidth for each user when there are many users contending for service. Well, what is a reasonable answer for the store-and-forward attributes of email transmission or web browsing, is in fact, a very bad idea for real-time services such as VoIP where the user notices immediately that packets have been delayed or discarded, affecting their overall experience in negative ways.
More recently, the IEEE committee responsible for the 802.11 family of standards (known as Wi-Fi), introduced amendments and new ideas to allow for Quality of Service (802.11e) – the mechanism specifies that the competing user with real-time applications like VoIP get the bandwidth more reliably and more often than other users and other applications, which of course defeats one of the fundamental assumptions in the shared spectrum application.
The other fundamental consideration is the privacy of communications, particularly voice communications. Because the voice packetization mechanism is relatively simple and since all packets traverse a Wi-Fi environment in a predictable way, some argue that it is resource-simple to capture and decode a conversation where one of the endpoints relies on transmission over a W-Fi network. Attempts at various encryption schemes – WPA, WPA2 – have generally failed as being weak and therefore resource-simple. To be effective, strong encryption needs to be used to assure that eavesdropping is an expensive proposition (resource-intensive). The 802.11 team to the rescue! The 802.11i standard incorporated strong encryption into the standard Wi-Fi transmission.
About 5 years ago, I implemented wireless VoIP in my home. We had four IP extensions on a 3Com NBX in the house. Instead of rolling out Ethernet throughout the building, I purchased little access bridges that would interact with my Wi-Fi router, and in fact would Wi-Fi-ize Ethernet devices, like the four Ethernet phones in the house. I was ultimately unhappy with the quality of the voice communication. The compression of audio between the phones and the gateway were compounded by the large number of places that I would call that also had IP PBXes and gateways installed introducing more transcoding hops and more compression for frequent audio problems. At the time I was also suffering from high frequency hearing loss, which I didn’t know at the time.
Wireless VoIP is a very flexible service, can work in a Wi-Fi shared application environment, but only if the latest standards are deployed and only if both the devices and the APs support Quality of Service and security features. Otherwise, services based on voice-only spectrum, such as DECT ought to be considered as well.
