Select the correct statement$($s$)$ regarding SONET and SDH$.$

a$.$ STS describes the SONET electrical signal, while STM describes the SDH electrical signal

b$.$ OC describes the optical signal on both SONET and SDH

c$.$ An STS$-3$ equals an STM$-1,$ which is $155\mathrm{.}52$ Mbps

d$.$ All of the above are correct