Getting started with bridges and switches

Getting started with bridges and switches

Bridges and switches are both data communication equipment and work at the second layer of the OSI reference model, so they are generally referred to as data link layer equipment. They were first used in the 1980s and were mainly used to connect The same type of network realizes data forwarding between networks. With the continuous development of network technology, today's bridges have been able to connect different types of networks, and their intelligence has also been improved.
Bridges as network interconnection devices can be divided into several different types. Among them, the transparent bridge is mainly used in the Ethernet environment, the source routing bridge is mainly used in the token ring network, and the conversion bridge can perform data format between different network media types (usually Ethernet and token ring) And transfer protocol conversion.
Today, switches have replaced traditional bridges and become the most important network interconnection technology. Compared with the bridge, the data throughput of the switch is better, the port integration is higher, the cost per port is lower, and the use is more flexible and convenient.
Introduction to Link Layer Devices The functions of bridges and switches are implemented at the data link layer. The role of the data link layer is mainly to control data flow, handle transmission errors, provide physical addresses (without logical addresses), and manage access to physical media. By using different link layer protocols, the bridge can achieve all the above functions. The currently popular link layer protocols include: Ethernet, Token Ring, and FDDI.
The function realization principle of bridges and switches is not complicated, mainly by analyzing the incoming data frames, making forwarding decisions based on the information contained in the frames, and then forwarding the data frames to the destination. The forwarding of data frames by the bridge is divided into two forms. If you use the source path bridge technology, then each data frame already contains the complete path to the destination; if you use the transparent bridge technology, then Each data frame will be forwarded to the next node and finally reach the destination.
Transparency to upper layer protocols is a very important advantage of bridges and switches. Because both devices work at the link layer, there is no need to check the information of the upper layer protocol. This can speed up the forwarding of data traffic using any network layer protocol. Generally speaking, the bridge can support the following network layer protocols: AppleTalk, DECnet, TCP / IP and XNS.
The bridge can filter data frames based on any domain established at the second layer. For example, we can set the bridge to refuse to forward all data frames from a particular network. Because the data link layer information contained in the data frame will often mark the type of upper layer protocol used, we can also use this as a parameter to set the filtering mechanism. In addition, the effective filtering of data frames can greatly reduce unnecessary data traffic caused by broadcast or multicast in the network.
The use of bridges and switches can divide a type of network into several small subnets to provide other functions. Because only a small part of the traffic needs to be forwarded after subnetting, either the bridge or the switch can reduce the data traffic flowing through the devices on all connected network segments. Bridges or switches can also act as firewalls, reducing possible network errors. Finally, bridges and switches can extend the effective connection range of the local area network and allow access to more remote devices.
From the above introduction, we can see that bridges and switches have many similarities, but there are still some significant differences between the two technologies. In terms of speed, the switch is significantly faster than the bridge, this is because the switch mainly uses hardware for switching, and the bridge needs to use software to achieve switching. In addition, the switch can connect different bandwidth networks. For example, a 10 Mbps Ethernet LAN and a 100 Mbps Ethernet LAN can be interconnected through a switch. In terms of switching technology, the switch supports cut-through, which can reduce network latency; while the bridge can only use store-and-forward technology. Finally, because the switch can provide dedicated bandwidth for each network segment, it can reduce the collision rate on the network segment.
Types of bridges We can divide the types of bridges in many ways according to the different characteristics of the product. One of the more common division methods is to divide the bridges into two categories: local bridges and remote bridges. Local bridges are mainly used to provide direct connections between multiple LAN segments in the same geographic area. Remote bridges are used to connect LAN segments in different areas, and generally require the use of telephone lines. The schematic diagram of the two types of bridges is as follows:

The remote bridge technology faces some challenges in achieving network interconnection. One of them is how to solve the speed difference between LAN and WAN. Although some high-speed WAN technologies have been adopted, generally speaking, the speed of LAN is still significantly faster than WAN. This huge difference between the speed of the local area network and the wide area network prevents users from running applications that require high latency across the wide area network.
Although the remote switch cannot increase the speed of the WAN, it can compensate for the difference in speed by providing sufficient buffering functions. For example, if a LAN device with a transmission rate of 3 Mbps wants to communicate with another device on the remote LAN, the bridge must adjust the 3 Mbps data flow to avoid blocking the WAN connection with only 64-kbps bandwidth. To solve this problem, the bridge uses an internal cache to store the received data, and then sends it to the WAN at a rate that the WAN can withstand.
Types of switches Switches, like bridges, are also data link layer devices that can interconnect multiple physical network segments into a unified large network. There are many different types of switches, including ATM switches, LAN switches, and various types of WAN switches.
ATM switches ATM switches can provide high-speed switching capabilities and scalable bandwidth for different types of networks such as workgroups, enterprise backbone networks, and wide area networks. ATM switches support voice, video and data applications and are designed to exchange fixed-size information units, namely ATM cells. The figure below shows an enterprise network that connects multiple LANs through an ATM backbone.

LAN switches LAN switches can be used to connect multiple LAN segments to achieve conflict-free dedicated communication between network devices. LAN switches can store and forward data frames at high speed.

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