EMB ERJ 145 ER/LR
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 13
Communication Breakdown; Confusion; Situational Awareness
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic; Flight Crew Became Reoriented
Aircraft; Human Factors
I was expecting Aircraft X to check on frequency. However Aircraft Y was the aircraft which actually checked on. I hadn't noticed different call sign nor was I expecting a similar sounding aircraft to check in; hence I descended Aircraft Y to 21000 feet. A moment later Aircraft X checked on frequency which caused suspicion and called up Aircraft Y after hearing across the room another controller calling Aircraft Y. I quick looked the sector and saw the event in progress. I told the controller I will stop Aircraft Y at 29000 feet; promptly called close proximity traffic and after coordination I made a communication change to next controller sector.This was a similar sounding call sign event. By aircraft checking on frequency and not stating their assigned altitude didn't help me to recognize that incorrect call sign from expectation of another aircraft; still a recognition issue none the less.
Aircraft Y was on frequency along with Aircraft X.  Aircraft X was given a frequency change and there was some garbled read back. Aircraft Y asked if the communication switch was for them and I said yes. Aircraft Y then checked onto the next sector frequency with no altitude in the check in. That sector's controller descended Aircraft Y thinking he was talking to Aircraft X. Meanwhile Aircraft Z was a departure that was climbed to 30000 feet to go under Aircraft Y. I noticed Aircraft Y start to descend and tried to call him to ask why he was descending. I could not and then turned Aircraft Z and climbed him to 33000 feet. At this time the other sector controller realized that he had Aircraft Y and stopped their descent at 29000 feet as we had traffic at 28000 feet. The callsigns are problematic as a lot of controllers say they confuse the two and that the pilots will question who it was for as they hear the similar callsigns. Also in this issue the numbers were very close. If Aircraft Y had checked onto the frequency and stated their altitude it may have caused the controller to question him as that sector only owns to 23000 feet.
Center Controllers reported the wrong aircraft took a frequency change and descent clearance due to similar sounding callsigns.