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 2005-08-11 08:10:23 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