FDD divides spectrum by frequency, allowing simultaneous transmission. TDD divides spectrum by time, using same frequency for different directions. Both mechanisms are used in LTE and other wireless technologies
Uplink and downlink are two communication links between mobile phone and network. Mobile phone sends signal to network base station (eNodeB, gNodeB). Base station transmits signal to mobile phone. Both devices continuously send and receive signals
USSD enables GSM phones to communicate with network operators' computers. Messages can be up to 182 alphanumeric characters long. Creates real-time connection unlike SMS, enabling faster data exchange. Operates without messaging apps and charges are free
iPhone XS through iPhone 14 Pro Max support eSIM technology. Not all iPhone models support eSIM functionality. iPhone must be unlocked to add eSIM cards
Mobile phones enable calls over radio frequencies while moving within service areas. Modern phones support text messaging, multimedia, and internet access. Feature phones offer basic features, while smartphones provide advanced computing power
4G LTE is the fourth generation cellular data technology, replacing 3G. LTE offers theoretical speeds up to 300Mbps, while HSPA+ reaches 100Mbps. 4G LTE is still standard in many markets despite being replaced by 5G