The ring of steel is the popular name for the security and surveillance cordon consisting of a few hundred CCTV cameras surrounding the City of London , installed in the 1990s to deter the Provisional IRA and other threats .
The ring of steel is the popular name for the security and surveillance cordon consisting of several hundred CCTV cameras surrounding the City of London , the financial district at the heart of Greater London . 
 The measures were installed in the 1990s to deter the Provisional IRA and other threats .