Book "communication systems" by Dr. Essam Abboud and Dr. Ghassan flouh (Damascus University) The seven layers (OSI Layers) | ● ۩ ۝ ۩ ● Mustafa Hamdan communication and Information Technology Blog ● ۩ ۝ ۩ ● copy saved November 11, 2016 on the website of Wayback misheen. Page 486 book "communication systems" (Damascus University)