I was initially inspired to write this post because a developer on my team (Frank), who incidentally wrote the diagnostic support I'll describe later, responded on this thread: Activating both A+G Wireless Channels.
To begin, I'll itemize the possible ways to connect an extender (MCX) to a Media Center PC (MCPC), in order of best user experience:
The following two topologies likely will not work (you're lucky if you can support even a single SDTV stream)
For the record, 1-3 are the only officially supported topologies for streaming SDTV, and only 1-2 for HDTV. All other topologies are not officially supported, and using these non-supported setups revoke your right to complain about the quality of your experience :) However, it is still possible to connect devices using methods 4-7, but with caveats. I'll talk about topologies 8 and 9 later (dual-band AP section), as these are simply bad news.
Configurations making use of 802.11g wireless: 3, 5, 6, and 7
802.11g uses the 2.4GHz spectrum, and is therefore much more susceptible to common interference sources than 802.11a (currently anyway), which operates in the 5GHz spectrum. Such interference is introduced by your neighbor's WiFi network (on the same or adjacent channel), cordless telephone, microwave oven, etc., and causes performance degradation or even complete loss of connectivity. Further, a single legacy 802.11b station associated (and not even active on the network) with your 802.11g AP will reduce the wireless throughput by as much as half! These potential sources of interference will certainly affect your overall media streaming experience. Ultimately, this is why 802.11a is preferred for Audio/Video (AV) streaming scenarios.
Wireless MCPC, wired MCX: 4 and 5
If it isn't possible to place your MCPC next to a wired connection, then the next best choice is to connect it via WiFi and the extender via Ethernet. As noted above, the best experience when using WiFi will be had with 11a rather than 11g. In addition to interference issues with 11g, any form of WiFi is affected by obstructions such as walls and/or floors. A topology with a number of obstructions between the wireless MCPC and WLAN AP will reduce the throughput available to all MCX devices within the home, and therefore the ability to stream video, such as live or recorded TV. In contrast, if the MCPC is wired and each MCX is wireless, only the MCX separated from the WLAN AP by obstructions (or experiencing interference) will suffer.
Dual-Band AP: 6 and 7
It is extremely advantageous to use a dual-band (concurrently offers 11a and 11g) WLAN AP for digital home installations. All AV devices can be configured to use the 11a radio, and all data-centric devices (PCs that do internet browsing, etc.) to use the 11g radio; physically separating the traffic. This eliminates the possibility of data traffic (e.g. large download or file copy) causing contention with your AV traffic (streaming TV).
Another advantage to a dual-band AP is when you simply cannot use wired Ethernet to connect your MCPC or MCX (both must be connected via wireless). Before I describe the unique advantage a dual-band AP provides for this particular topology, I must first describe the severe problem a single-band AP introduces (i.e. topology 7 and 8). When both the media server (MCPC) and rendering device (MCX) are connected via the same wireless radio (both on 11a for example), we call this a "double-hop;" the first hop from the MCPC to the AP, and the second from the AP to the MCX. Because WiFi is a shared medium (like an Ethernet hub), any transmission onto the air consumes the bandwidth from all attached stations; a double-hop stream consumes twice its normal bandwidth. This additional consumption of bandwidth (in addition to 802.11 protocol overhead and performance degradation caused by interference/obstructions) usually exceeds the available bandwidth on the wireless network; resulting in a poor user experience (visible glitches, etc.) As mentioned above, if the WiFi network performance is not degraded by interference, obstructions, etc., and you have a very patient/forgiving family, SDTV may be possible.
Now that the stern warning is issued, if you absolutely must connect the MCPC and MCX via wireless, use a dual-band AP with topology described by item 6 or 7 above. While all communications between the MCX and MCPC will still have to make two wireless hops (one hop from the MCPC to the AP, and the one from the AP to the MCX) these hops will use separate frequency ranges so they don't have to share the available bandwidth in either frequency range.
Windows Media Center Edition 2005 (Rollup-2) to your rescue
In rollup-2, when you connect an Xbox 360 as an extender, you are given a myriad of diagnostics (courtesy of the QoS team) within the Network Performance Tuner (NPT) that inform you of many scenarios described above:
While these situations cannot be automatically corrected, the goal of these diagnostics is to inform you, our customers, how to take corrective action to fix the network setup.
Hopefully this will help folks connecting their digital home!