Select the correct statements regarding Asynchronous Transfer Mode $($ATM$)$:

a$.$ ATM is a variable$-$length cell standard that supports voice, video and data

b$.$ ATM is connection$-$oriented

c$.$ ATM is a connectionless network layer protocol similar to IP$,$ in which fixed cell sizes enable predicable latency and quality$-$of$-$service $($QoS$)$

d$.$ a and b are correct statements