Ethernet hub is a network device use to connect different devices including nodes, servers, printers etc. Hub works on physical layer (Layer-1) when it receives message on any port broadcast it to all the devices connected with hub. CSMA/CD made for the hub to minimize the problem of collusions.
Collusions occur when more than one node transfers data on the same time. Hub is simply multi-port repeater, regenerate the signals and forward to all the connected devices this mechanism is know as broadcast.
In hub bandwidth is shared among the ports; mean increase the number of connected devices reduces the speed of the network. Hub consists of single collusion domain means one shared area in the hub is utilize by all connected devices for network traffic transfer. Now a day’s hub is obsolete replaced by the data link device (2-layer) switch, with the ability of multiple collusion domains and better hard based ASIC technology insists the corporate users to opt for switch rather than hub. Hub is feasible to use in small networks but in complex network using hub is a bad idea, its better to go with switch. Hub physical looks like a star topology but logically based on bus topology. The analogy of hub is a one way road on which traffic is moving in both directions with no traffic management, in this case there is a greater chance of collusions. CSMA/CD is like a traffic police standing on road to manage the traffic and reduce the chance of collusions