I (voip247) think that the Sipura SPA-2100 is by far the best hardware SIP adapter as of January 2006 because it totally eliminates the NAT and QoS problems that plague other SIP adapters that don't have built-in routers (like the Sipura 1000/2000/3000 and Linksys PAP2).
Note (from DracoFelis on 5/6/2006):
Now that the SPA-3102 is out, it might be a better choice for a lot of people than the SPA-2100. Like the 2100 the 3102 has a built in router + builtin QoS. But the 3102 also has the 3000's ability to cross-link with a real "phone line" (allowing your to use your normal phone line and VoIP from the same "phone"). And the 3102 also has the 3000's ability to use multiple outbound providers from the same "phone".
At this point, the 2100 has only these two advantages over the newer 3102:
1) The 2100 is a few dollars cheaper.
2) The 2100 allows two phones at once (since it has two phone ports).
But OTOH the SPA-3102 has all these benefits over the 2100:
1) Newer design (which hopefully means it will be supported longer).
2) Ability to fully access/use/control a real phone line (or anything that mimics a phone line, including another VoIP adapter), in addition to VoIP, from the adapter. This also means you can do things like route some calls (911, local calls, toll free calls, etc) via your existing telco line (if you still have one) while still using VoIP for cheap calls elsewhere (all from the "phone" attached to the adapter).
3) Builtin support for multiple (outbound) VoIP accounts, whereas the 2100 only allows one VoIP provider/account per phone jack.
NOTE: In other ways, the SPA-3102 and the SPA-2100 (below) benefits are pretty equal.
- The SPA-2100 eliminates the NAT problem by being both a router and a SIP adapter, which allows it to use your public IP address in SIP messages. The most common NAT problem is one-way audio (the other person can hear you, but you can't hear him). If your SPA-2100 is plugged directly into your DSL/cable modem and you have one public IP address (which is typical), then your SPA-2100 will use the public IP address (rather than a private IP address that would be assigned if a typical home router was in between the DSL/cable modem and the SIP adapter), which eliminates NAT problems.
- The SPA-2100's QoS works very well. TomsNetworking says "The bottom line is that it looks like the recommended TBF-based QoS did the trick of restoring near-perfect call quality with simultaneous continuous uplink data traffic." In contrast, the QoS of the Linksys/Sveasoft/DD-WRT firmware I've tried for my Linksys WRT-54G was unreliable. Even if/when the Linksys firmware improves, using the SPA-2100's QoS eliminates the need for the WRT-54G entirely (and the need to know about WRT-54G hardware versions, firmware versions, firmware reputations, and firmware settings). If a router is used (for wireless or because you have multiple computers), then you don't need to be concerned with the router's QoS ability/inability.
- The SPA-2100 also has a couple more relatively minor benefits: T.38 (QoS for faxes), and G.729 works simultaneously on both lines (which means that if you have a typical 128 kbps uplink, you can make two phone calls and browse the Internet all at the same time).
- Ethernet cables: DSL modem or cable modem
> SPA-2100> router or switch (optional) --> computer. Note that the NAT and QoS problems will only be eliminated if you plug the SPA-2100 DIRECTLY into the DSL/cable modem, without a router in between. Also note that if you only have one computer, you do NOT need a router or switch at all, because the SPA-2100 has one Ethernet WAN jack (for your DSL/cable modem) and one Ethernet LAN jack (for your computer).
- Battery backup: If you want to avoid rebooting-sequence hassles and have your VoIP phone work during power outages, you'll need the DSL/cable modem and Sipura plugged into a battery backup. If your phone is cordless, that also needs to be plugged into the battery backup. Note: The less equipment you plug into the battery backup, the longer the battery power will last during a power outage. So don't plug your Halogen light into the battery backup! ;)
- Settings: Use this form.
- QoS reduces available bandwidth even when there is no active phone call. I don't think there is any good reason to reduce available bandwidth except during an active phone call. (However, ALL the devices I know of that implement QoS do it this way, so it's not just a Sipura problem.)
- There is no max MTU setting. If either your download or upload bandwidth is less than 384 kbps, a typical 1500-byte max MTU setting reduces voice quality compared to a 576-byte max MTU setting. For example, at a typical upload bandwidth of 128 kbps, a 1500-byte packet takes 94 ms to transmit compared to 36 ms for a 576-byte packet, meaning that jitter could be reduced by 58 ms simply by reducing max MTU. http://voxilla.com/index.php?name=PNphpBB2&file=viewtopic&t=6861 (Note: If you are using DSL, you may get part of the benefit of a max MTU setting automatically, since PPPoE's typical 1492-byte max MTU causes automatic fragmentation of outgoing typical 1500-byte packets: http://dslreports.com/forum/remark,13733184 )
- No dynamic DNS. Most cheap prepaid SIP-to-PSTN proxies do not allow incoming SIP calls. A workaround could be to use sip:UserID@myName.dyndns.org (UserID must match whatever's in the Sipura 'User ID' field and the 'SIP Port' must be 5060). However, without dynamic DNS support in the SPA-2100, this workaround doesn't work. Also, ISP monitoring services like Bello don't work if you have a dynamic IP address without dynamic DNS support in your router (the SPA-2100, in this case). The SPA-2100 does have 'Bridge' mode as an alternative to NAT (in 'Lan Setup'), but that does not do any good, since ISPs supply only one dynamic IP address for their cheap accounts. (Note: Polling the Sipura's configuration web page does NOT work as a way of updating the IP address, because loading the Sipura configuration web page causes voice dropouts, even with Sipura's QoS turned on.)
- No ENUM. (Using SIP Broker is a workaround, but ENUM would have higher availability if implemented in firmware.)