A wireless network consists of the following:
If client 1 is connected to the access point via 802.11g and client 2 is connected to the same access point via 802.11n, will client 2 see a degradation in throughput and bandwidth?
If client 1 is connected to the access point via 802.11b, client 2 is connected to the access point via 802.11g and client 3 is connected to the access point via 802.11n, will client 3 see a degradation in throughput and bandwidth?
Just to add a clip to the great info from Eric (+5 points for this buddy! Loved the party analogy ;-)
Backward Compatibility with Existing Platforms
Like today's 802.11g platforms, 802.11n networks will be backward compatible with clients built under previous 802.11a/b/g wireless standards. 802.11n wireless access points will interoperate easily in mixed environments. They can be configured to concurrently support both conventional 802.11a/b/g transmissions over 20-MHz channels and higher-bandwidth 802.11n transmission over 40-MHz channels. However, the introduction of 802.11a/b/g clients into an 802.11n network will impact overall throughput rates for the network, and decrease aggregate throughput of the access points. This impact differs depending on which type of wireless client is introduced.
An 802.11n network can incorporate 802.11a/g clients with minimal performance loss. However, as with today's 802.11g networks, operating in a mixed environment that includes 802.11b clients can substantially affect throughput. (The throughput of today's 802.11g networks may drop from 25 Mbps to as low as 7 Mbps when 802.11b clients enter the environment, and 802.11n networks will suffer a comparable performance decrease.) However, organizations deploying 802.11n have the option of configuring the network to exclude 802.11b clients on some channels or on all channels to ensure that throughput remains high.
Organizations that can benefit from 802.11n but still rely on 802.11b/g devices (and plan to continue using those devices for the foreseeable future) can plan to deploy 802.11n in mixed mode in both the 2.4-GHz frequency and 5-GHz frequency. Organizations will experience the greatest performance improvements in the 5-GHz frequency given the relatively fewer number of existing 802.11a clients, the greater availability of spectrum, and the fact that the 5-GHz frequency has less interference than the 2.4GHz frequency.
From this excellent Overview doc;
Hope this helps!