noobie join question

Hi List,
I am having trouble trying to figure out
how to get the result listed at the bottom.

I have 3 tables units, types of units which has a description of the units,
and a table that list associations of the units. I can't figure out
how to do the proper joins. Any pointers would be appreciated.

create table types (
        id integer,
        descr varchar(30)
);

COPY types (id, descr) FROM stdin;
1    descr 1
2    descr 2
3    descr 3
4    descr 4
\.

create table units (
        uid integer,
        udevice varchar(30),
        utype integer
);

COPY units (uid, udevice, utype) FROM stdin;
1    aaaaa    1
2    bbbbb    1
3    ccccc    4
4    ddddd    3
\.


create table assoc (
        aid integer,
        src_id integer,
        dest_id integer
);

COPY assoc (aid, src_id, dest_id) FROM stdin;
1    1    2
2    1    3
3    3    4
4    4    2
\.

desired result
aaaaa | descr 1 | bbbbb | descr 1
aaaaa | descr 1 | ccccc | descr 4
ccccc | descr 4 | ddddd | descr 3
ddddd | descr 3 | bbbbb | descr 1


Thanks,
Steve