Block 7 Digital exchanges 

 - Walton Hall  Open university  1990 
 - 108p.  29cm 
<br/><br/><label>ISBN: </label>0749260491 
<br/><br/><label>Subjects--Topical Terms: </label><br/>Communications engineering
<br/><br/><label>Dewey Class. No.: </label>621.382  / OPE