What is meant by single broadcast domain and single collision domain?

Answer 1

A collision domain is a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters, in particular, when using early versions of Ethernet. A network collision occurs when more than one device attempts to send a packet on a network segment at the same time. Collisions are resolved using carrier sense multiple access with collision detection in which the competing packets are discarded and re-sent one at a time. This becomes a source of inefficiency in the network.[1]

Only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network in order to avoid data collisions. Because only one device may be transmitting at any one time, total network bandwidth is shared among all devices. Collisions also decrease network efficiency on a collision domain; if two devices transmit simultaneously, a collision occurs, and both devices must retransmit at a later time.

Collision domains are found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. Collision domains are also found in wireless networks such as Wi-Fi.

Modern wired networks use a network switch to eliminate collisions. By connecting each device directly to a port on the switch, either each port on a switch becomes its own collision domain (in the case of half duplex links) or the possibility of collisions is eliminated entirely in the case of full duplex links.

A broadcast domain is basically a VLAN. The broadcast domain defines how far a Layer-2 broadcast will propagate on the network, which is to say the broadcast will hit every device on the VLAN, or every device on the "subnet". Routers block broadcasts by design. If you need to leave your broadcast domain (get off your local subnet) then you jump up to layer 3 and go through a router to talk to machines on some other broadcast domain. The layer 2 broadcast will typically traverse all hubs, bridges and switches in a single VLAN. If you question was in the context of using a "single broadcast domain" just be careful not to do that for a relatively large network with several hundred or thousands of nodes, or the network performance and/or the end-station performance will suffer because of all the broadcast traffic.

Answer 2

Collision domain, first what is a collision:

Maybe the question you have is, what is the difference between a layer 2 and layer 3 broadcast.

Collisions are considered part of the physical layer in the ISO model. The physical layer is the cables. An example would be copper, also referred to as transmission media or medium. A collision occurs when two or more computers attempt to transmit data at the same time onto the same media (cable).

Ethernet uses a media access method known as contention (meaning all network cards have equal access to the media).

The complete definition is CSMACD or CSMACA

Carrier Sense

Multiple Access

Collision Detection or Collision Avoidance

When an Ethernet card transmit data onto the media it follows these rules

Listen = Carrier Sense, the intention is that if the media is in use the Ethernet adapter will not transmit. When two or more adapters transmit simultaneously the result is a collision. When a collision occurs all of the data is corrupted and un-usable.

Transmit = Multiple Access

Because any adapter can transmit at any time the media is not busy it is probable that two or more adapters will transmit simultaneously causing a data collision as discussed above.

Collision = either detected, or not detected

In wired media such as copper UTP (unshielded twisted pair),CAT5e for example, each adapter has the ability to detect the collision due to an incorrect voltage level in the signal as a result of the collision. At this point every adapter involved in the collision will start a random timer (known as the back off timer). The random timer is supposed to keep the collision from just happening again when the adapters re-transmit their data.

There are many ways to isolate collision domains. Primarily in switched networks, every port on the switch is considered a separate collision domain. Bridges and Routers are another method for isolating collision domains. Collisions occur at the OSI (Open Systems Interconnect) layer 1 or physical layer.

Broadcast domains:

This would be a little difficult to explain without knowing your level of knowledge. I'll try to keep this fairly simple.

Broadcast domains are the logical separation between networks. Unlike collisions they occur at the OSI layer 3 level. Networks are logically segmented into subnets to isolate or consolidate traffic.

If we consider a simple IPv4 example we might have 2 networks like 192.168.1.0 / 255.255.255.0 and 192.168.2.0 / 255.255.255.0

These 2 networks are separated by a router. A layer 3 broadcast from the first network to 192.168.1.255 would not be forwarded to the 192.168.2.0 network. Obviously these two networks can communicate with each other though they are logically separate. Here is where it gets a little more difficult. Without looking at the specific broadcast, let us just say that a broadcast can be sent to all of the networks in the 192.168.X.X network asking for a response. So this would include both networks above.

Hope this is helpful...

Other subjects that would be helpful would be ARP (Address Resolution Protocol) and VLAN (Virtual LAN)