$3.$

$($a$)$ Differentiate between a hub, a switch, and a router in computer

networks. At which level of the OSI model do they each operate?

$($b$)$ What function is provided by the Transmission Control Protocol

$($TCP$)$ ports? Briefly describe a situation in which TCP ports might

be of use and how the purpose of a given port on a remote machine

is decided.

$($c$)$ With the aid of a diagram, explain what is meant by the hidden

terminal problem in a wireless network. Outline one technique for

solving it$.$

$($d$)$ Writing a program in assembly language is presumably more

difficult than writing a program in a high$-$level language. Give two

reasons which justify the above statement and explain why assembly

language is still used to program some applications.

$($e$)$ Use the relation CUSTOMER, given below, to write an SQL query

that lists the customer$$s name and city for all customers that are

located in either Liverpool or Cardiff.

CUST$\_$NUMBER CUST$\_$NAME CITY

$1$ Merci Lyndon Manchester

$2$ Andrew Santiago London

$3$ Arnie Staples Cardiff

$4$ Maria Stones Stoke$-$on$-$Trent

$5$ Ibrahim Safari Liverpool