A telecoms engineer is explaining to the Graham Dwyer murder trial how mobile phone calls bounce off different cells according to location.
Conor O'Callaghan provided Gardaí investigating Elaine O'Hara's death with call data records between 2011 and 2013 for a number of mobile phones.
Architect Graham Dwyer denies murdering the childcare worker at Killakee Mountain in Dublin in August 2012.
