What is the purpose of jam signal in CSMA/CD?
First, beware that modern switched Ethernet LANs are not CSMA/CD anymore. CSMA/CD was a technique that applied to 10mbit/sec and 100mbit/sec Ethernets that used hubs, not switches. And honestly there were never many 100BASE-TX hubs around; everyone went to switches around that time. The Gigabit Ethernet (1000BASE-T) spec requires switches; there's no such thing as a GigE hub. On modern switched Ethernets, you don't have a shared medium anymore. When you're plugged into a switch, the "collision domain" is only between you and your switch port. And if you're in full duplex mode, which is almost always true with switches, then you don't have any possibility of collision at all. If you can't have a collision, you'll never detect a collision, so you'll never have reason to transmit a jam signal. So back in the days of hubs (and shared cables like thinnet/cheapernet/10BASE-2 coax and thicknet 10BASE-5), here's how it worked: Imagine you h