1. http://www.ofcom.org.uk/static/archive/ra/topics/mpsafety/school-audit/mobilework.htm.
2. http://timesofindia.indiatimes.com/city/kochi/Illegal-interception-of-police-radio-rampant/articleshow/11879303.cms.
3. http://www.networkcomputing.com/unified-communications/android-software-connects-calls-without-mobile-carriers/d/d-id/1095782?.
4. http://www.policemag.com/channel/technology/news/2012/10/05/raytheon-s-communications-upgrade-assists-interoperability.aspx.
5. http://www.hcsdemo.com/c/en/us/products/collateral/physical-security/ip-interoperability-collaboration-system/datasheet_c78-582927.html.