PC and Industrial Networks

What is a network bridge? A network bridge is a device that divides a network into segments. Each segment represent a separate collision domain, so the number of collisions on the network is reduced. Each collision domain has its own separate bandwidth, so a bridge also improves the network performance.

A bridge works at the Data link layer (Layer 2) of the OSI model. It inspects incoming traffic and decides whether to forward it or filter it. Each incoming Ethernet frame is inspected for destination MAC address. If the bridge determines that the destination host is on another segment of the network, it forwards the frame to that segment.

Consider the following example network:

In the picture above we have a network of four computers. The network is divided into segments by a bridge. Each segment is a separate collision domain with its own bandwidth. Let’s say that Host A wants to communicate with Host C. Host A will send the frame with the Host C’s destination MAC address to the bridge. The bridge will inspect the frame and forward it to the segment of the network Host C is on.

Network bridges offer substantial improvements over network hubs, but they are not widely used anymore in modern LANs.

What is a network hub? A network hub serves as a connection point for all devices in a LAN. It is basically a multiple-port repeater because it repeats an electrical signal that comes in one port out all other ports (except the incoming port). Here is an example 4-port Ethernet hub (source: Wikipedia):

Hubs are OSI Layer 1 devices and have no concept of Ethernet frames or addressing. They have no way of distinguishing which port a signal should be sent to; instead, an electrical signal is sent to every port. All nodes on the network will receive data, and the data will eventually reach the correct destination.

In the picture above you can see that the hub has sent out the receiving signal out all other ports, except the incoming port.

Modern LANs rarely use hubs; switches are used instead. Hubs have many disadvantages, including:

 they operate in half-duplex.  they are prone to collisions.  each port on a hub is in the same collision domain.  data is forwarded out all ports and can be captured with a network sniffer.

What is a network switch?

 A network switch is a device that connects devices together on a LAN. A switch is essentially a multiport network bridge and performs the same basic functions as a bridge, but at much faster speeds and with many additional features. Each port on a switch is in a separate collision domain and can run in the full duplex mode, which means that hosts connected to a switch port can transmit to the switch at the same time that the switch transmits to them.

 An Ethernet switch usually works at the Data link layer of the OSI model (Layer 2). It manages the flow of data across a network by inspecting the incoming frame’s destination MAC address and forwarding the frame only to the host for which the message was intended. Each switch has a dynamic table (called the MAC address table) that maps MAC addresses to ports. With this information, a switch can identify which system is sitting on which port and where to send the received frame.

 To better understand how a switch works, consider the following example:

  Host A is trying to communicate with Host C and sends a frame with the Host C’s

destination MAC address. The frame arrives at the switch, which looks at the destination MAC address. The switch then searches that MAC address in its MAC address table. If the MAC address is found, the switch then forwards the frame only out the port connected to the frame’s destination. Hosts connected to other ports will not receive the frame.

What is a router? A router is a network device that connects different computer networks by routing packets from one network to the other. This device is usually connected to two or more different networks. When a data packet comes to a router port, the router reads the address information in the packet to determine out which port the packet will be sent. For example, a router provides you with the internet access by connecting your LAN with the Internet.

A router is considered a Layer 3 device of the OSI model because its primary forwarding decision is based on the information of the OSI Layer 3 (the destination IP address). If two hosts from different networks want to communicate with each other, they will need a router between them. Consider the following example:

We have a network of three computers. Note that each computer is on a different network. Host A wants to communicate with Host B and sends a packet with Host B’s IP address (10.0.0.20) to the default gateway (the router). The router receives the packet, compares the packet’s destination IP address to the entries in its routing table and finds a match. It then sends the packet out the interface associated with that network. Only Host B will receive the packet. In fact, Host C will not even be aware that the communication took place.

Each port on a router is in a separate collision and broadcast domain and can run in the full duplex mode.

Collision domain explained The term collision domain is used to describe a part of a network where frame collisions can occur. Frame collisions occur when two devices on a shared network segment send frames simultaneously. The colliding frames must be discarded and sent again, which reduces network efficiency.

Collisions occur often in a hub environment because all devices connected to the hub are in the same collision domain. Only one device may transmit at time, and all the other devices connected to the hub must listen to the network in order to avoid collisions. Total network bandwidth is shared among all devices.

In contrast to hubs, every port on a bridge, switch, or a router is in a separate collision domain. This eliminates the possibility of collisions and enables the devices to use the full-duplex mode of communication, which effectively doubles the maximum data capacity.

To better understand the concept of collision domains, consider the following example:

In the picture above you can see a network of seven computers, two hubs, a bridge, a switch, and a router. The collision domains created by these devices are marked in red. Remember, all devices connected to the hub are in the same collision domain. Each port on a bridge, a switch or router is in a separate collision domain. That is why there are seven collision domains in the network pictured above.

Broadcast domain explained The term broadcast domain is used to describe a group of devices on a specific network segment that can reach each other with Ethernet broadcasts. Broadcasts sent by a device in one broadcast domain are not forwarded to devices in another broadcast domain. This improves the performance of the network because not all devices on a network will receive and process broadcasts.

Routers separate a LAN into multiple broadcast domains (every port on a router is in a different broadcast domain). Switches (by default) flood Ethernet broadcast frames out all ports, just like bridges and hubs. All ports on these devices are in the same broadcast domain.

To better understand the concept of broadcast domains, consider the following example:

In the picture above we have a network of six computers, two hubs, a bridge, a switch, and a router. The broadcast domains are marked in red. Remember, all devices connected to a hub, a bridge, and a switch are in the same broadcast domain. Only routers separate the LAN into multiple broadcast domains. That is why we have four broadcast domains in the network pictured above.

Ethernet broadcasts are usually used by Address Resolution Protocol (ARP) to translate IP addresses to MAC addresses.

What is a Modem A modem is a hardware device that allows a computer to send and receive data over a

telephone line or a cable or satellite connection. In the case of transmission over an analog

telephone line, which was once the most popular way to access the internet, the modem

converts data between analog and digital formats in real time for two-way network

communication. In the case of the high-speed digital modems popular today, the signal is much

simpler and doesn't require the analog-to-digital conversion.

Broadband Modems

A broadband modem like those used for DSL or cable internet access uses advanced signaling techniques to achieve dramatically higher network speeds than traditional dial- up modems. Broadband modems are often referred to as high-speed modems. Cellular modems are a type of digital modem that establishes internet connectivity between a mobile device and a cell phone network.

External broadband modems plug into a home broadband router or other home gateway device on one end and the external internet interface such as a cable line on the other. The router or gateway directs the signal to all the devices in the business or home as needed. Some broadband routers include an integrated modem as a single hardware unit.