User Agents (UAs)Ben Teitelbaum <firstname.lastname@example.org> (November 2003)
To call SIP.edu users, the calling party needs a SIP phone, or more precisely, a "SIP user agent."
The SIP user agent provides an interface, which the caller uses to enter an alphanumeric SIP.edu URI (e.g. sip:email@example.com). Once the call is placed, the UA initiates SIP signaling to the destination domain's SIP proxy, and establishes voice communication with a receiving SIP UA.
In the initial phase of SIP.edu, the receiving SIP UA will almost always be a gateway that will terminate the SIP portion of the call and gateway to a TDM interface on the campus voice switch (PBX). SIP UAs can receive calls that are placed directly to an IP address/port pair to which they are listening or can receive calls that are placed through a SIP proxy to which they have registered.
A SIP Phone: Hard Phone or Soft Phone
When SIP user agents are implemented in hardware as IP appliances, they are known as hard phones. Hard phones resemble traditional POTS desk phones and are available at prices ranging from $65 to $600 each.
SIP user agents that use software are known as soft phones . They run on a PC and make use of the PC's sound card for audio input and output. SIP soft phones are generally very cheap or free.
Some soft phones are designed for use on personal digital assistants (e.g. iPAQ). These will not be considered further here.
Whether the calling SIP user agent is a hard phone or a soft phone, it must meet several requirements:
Entering alphanumeric SIP URIs
On hard phones: Entering an alphanumeric SIP.edu URI (e.g. sip:firstname.lastname@example.org) on a hardphone's telephone keypad can be very cumbersome. As a work-around, many hardphone vendors provide a web interface that can be used for entering alphanumeric SIP URIs. Additionally, many hardphones have built-in directories and programmable speed-dial buttons that eliminate the need to enter any given SIP.edu URI more than once.
On soft phones: Soft phones have the advantage of supporting alphanumeric input via the PC keyboard. However, because softphones rely on the PC for audio input and output (I/O), there are audio hardware considerations that apply to soft phone configuration that do not for hard phones.
Using general-purpose audio devices, such as conventional PC speakers or the built-in microphone and speakers on a laptop, is not recommended. The quality of these devices is often very low, the placement of the microphones sub-optimal, and, without good acoustic echo cancellation (AEC), echo problems are rife.
The table below summarizes the differences between hard and soft phones.