In this section we're going to discuss the common devices that will be found in our networks today. These devices include bridges, switches, hubs, and routers. Each of these devices serve there own purpose and we will be learning when to use each device to accomplish our goals. Before we get into each device though we should discuss the different types of traffic on a network.
Internetworks are comprised of various networking devices. The most common ones found in networks today are bridges, switches, hubs, and routers. These devices are responsible for taking data from point a source to a destination. We're going to learn how each of these devices assists in this process. Before we talk about the devices themselves we need to talk about the different types of traffic that can be found on IPv4 networks.
Unicast, Multicast, and Broadcast
There are 3 different types of traffic in IPv4 and they are unicast, multicast, and broadcast. The following images will explain the differences.
Unicast is a simple 1 to 1 communication. In the following figure you can see R1 is communicating directly to R2.
Multicast refers to communication between one device to a group of other devices. For example, in the following picture R1 sends a multicast to R2 and R4. R3 would not recieve any traffic in this scenario.
The last type of communication is a broadcast. Broadcasts are traffic from one device to every other device in the network. Broadcasts can be limited by placing routers to break up broadcast domains. Broadcasts are never forwarded past a router so whenever you place a router in your network you are segmenting the broadcast domain. Switches can also be used to segment broadcast domains but only with the use of VLANs.
Segmenting broadcast domains is an important part of designing a network but you also need to be aware of collision domains. Collisions occur when 2 or more devices send traffic onto a link at the same time. Ethernet uses a protocol called carrier sense multi-access / collision detection or CSMA/CD to determine whether or not a line is being used. If the line is being used CSMA/CD will wait until it is unused before it sends traffic out onto the line. If a collision occurs CSMA/CD will issue a back-off algorithm that will cause each device on that segment wait a random period of time before re-sending the packets. These collisions can cause latency issues and can be addressed by segmenting collision domains with switches and routers.
Broadcast and Collision Domains
Routers and switches both break up collision domains by default.
The above network has 3 broadcast domains and 6 collision domains. As you can see the collision domains are segmented very well and it is unlikely that any collisions will occur. Bridges also break up collision domains however hubs do not break up broadcast or collision domains.