Hi Makoto,
PTP 450i cannot mix with PTP 550,
If bandwidth is limited . It is advisable to use PTP 550 with two 20 MHz channel instead one 40 MHz channel . This will improve overall performance. Also PTP 450i has application compared to PTP 550, some examples are
1. PTP 450 and PMP 450 share same interface
2. PTP 450 and PMP 450 can use same sync solution
3. PTP 450i has better filtering capability compared to PTP 550, hence it performs better in tougher environment
4. PTP 450i can support narrow channel sizes which is not supported on PTP 550, this is key requirement in many areas.
These are some key differences between PTP 550 and PTP 450i , there are many other minor details not covered here. But in short, these products are similar to some extent but vary in application and deployment style.
Thanks
Sagar