SQL Newbie

It seems to me that I should be able to do this, but after 5 hrs of trying,
I
can't figure this one out.

I could do this in two queries, but seems like I should be able to do this
in
one. What I am trying to do:Find the highest speed at which each interface of a router has run over
time.

I have three tables, two of which (interface, speed_history) are being used
in
this query (primary / foreign key fields noted as PK / FK):
 router    -> 1:M -> interface         -> 1:M -> speed_history ------------------- ---------------------------
--------------------------
- router_no (int2) PK interface_id (int4) PK      interface_id (int4) PK name (varchar)      router_no (int2) FK
updated_time (timestamp)
 
PK                     link_description (varchar)  speed        (int4)

Data in speed history looks like this:   interface_id  updated_time          speed   1             2005-08-11 08:10:23
450112   1             2005-08-11 10:53:34   501120 <---   1             2005-08-11 10:58:11   450112   2
2005-08-1108:10:23   450112 <---   2             2005-08-11 11:00:44   350234   3             2005-08-11 08:10:23
450112<---
 
The rows of speed_history I want back are marked above with ' <--- '.

Query results should look like:    interface.interface_id    interface.link_description    speed_history.updated_time
speed_history.speed