the CCNA guide states that the CSMA/CD protocol is used to stop devices on the same medium e.g. same wire like on a bus network transmitting simultaneously to avoid collisions. Now please tell me, if 5 hosts are connected to a hub (one on each wire) does CSMA help here? I always thought it didnt because they're on separate wires on each port on the hub, and the hub forwards all frames onto all interfaces by default, therefore making hubs half duplex, but does CSMA even come into play if the hosts are on separate wires? Another thing, if you have a full duplex ethernet cable and connect to a hub, why can't run it full duplex? Can a switch only run full duplex if a full duplex ethernet cable is connected? Does the host/device on the other end have to run full duplex too?
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3 Answers
the CCNA guide states that the CSMA/CD protocol is used to stop devices on the same medium e.g. same wire like on a bus network transmitting simultaneously to avoid collisions.
That really isn't correct. the CD part is Collision Detection. CSMA/CA (CA for collision avoidance) is used by other protocols, like Wi-Fi, is to try to avoid collisions.
A hub is really just a wire, and each port is electrically connected to all the other ports, so all the hosts are on the same wire, even though it looks like each has its own wire. If two devices connected to a hub send a frame at the same time, both frames will be repeated out the other interfaces at the same time. That is a collision; the electrical signal will interfere with each other, and the devices on the other ports will not understand either frame.
You seem to be confusing hubs with switches. Each port on a switch represents a separate collision domain. Switch ports are not all connected together the same way hubs are. Switches determine when, and to which port(s), to switch frames.
The patch cables are the same for half or full duplex operation. Devices connected to a switch do not need to be the same duplex as each other since each switch port represents a separate collision domain.
I always thought it didnt because they're on separate wires on each port on the hub, and the hub forwards all frames onto all interfaces by default,
Hubs don't work with complete frames, they work at a lower level than that. When data starts arriving on one port it immediately starts being sent out on the other ports, there are no buffers (though there will be slight pipeline delays). If a hub detects transmission on multiple ports at the same time it will generate a "jam signal".
Thus from the point of view of CSMA/CD hubs are transparent. Devices still check that the line is idle before transmitting and if a collision happens it will be seen by all devices on the collision domain.
Another thing, if you have a full duplex ethernet cable and connect to a hub, why can't run it full duplex?
Full duplex and hubs are incompatible concepts. Full duplex implies that multiple nodes can successfully transmit at the same time but a multiport repeater (aka hub) can only handle one node transmitting at a time.
Can a switch only run full duplex if a full duplex ethernet cable is connected?
For a given Ethernet physical layer standard there isn't really such a thing as a "full duplex cable" or a "half duplex" cable. Coaxial Ethernet is fundamentally half-duplex. The common variants of Twisted pair and fiber Ethernet are full-duplex at the electrical level but can operate in a half-duplex mode where sinultanious transmission and reception is treated as a collision.
Does the host/device on the other end have to run full duplex too?
For a segment to operate correctly in full-duplex mode the devices at both ends of the segment must support full-duplex and be in full-duplex mode (Either through autonegotiation or manual configuration).
If for some reason one end of the segment is in full-duplex mode and the other is in half duplex mode then you get a duplex mismatch and terrible network performance. In particular this is likely to happen if a port in autonegotiation mode is connected to a port with autonegotiateion disabled and mode forced to full-duplex.
CSMA/CD is the LAN access method used in Ethernet.When a device wants to access to the network it checks whether the network is free or not.If the network is not free,the device waits a random amount of time before retrying.
CSMA/CD is used with devices operating in half-duplex mode only. CSMA/CD helps devices connecting to half-duplex switch ports operate correctly
