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1. The collision domain refers to the minimum range that will cause conflicts. When the computer and the computer are interconnected through the device, a channel is established. If the channel only allows one data message to pass at a time, then if there are two or more When more data messages are wanted to pass through here, there will be conflicts. The size of the collision domain can measure the performance of the device. There is only one collision domain of the multi-port hub, that is, all the data packets on the port are queued for waiting. The switch significantly reduces the size of the collision domain, so that each port is a collision domain, that is, high-speed transmission of one or more ports will not affect the transmission of other ports, because all data packets are in order. Queued through, but only the data to the same port is queued.
2. If the destination address of a data message is the broadcast address of this network segment or the MAC address of the target computer is FF-FF-FF-FF-FF-FF, then this data message will be used by all computers in this network segment. Receive and respond, this is called broadcast. The broadcast is usually used for ARP addressing, but the uncontrollable broadcast domain can also have a serious impact on network health, mainly bandwidth and network latency. The range that this kind of broadcast can cover is called the broadcast domain. The switch of the second layer is broadcasted, so the broadcast domain cannot be divided, and the router generally does not forward the broadcast, so the broadcast domain can be divided or defined.
Network interconnection devices can divide the network into different collision domains and broadcast domains. However, because different network interconnection devices may work at different levels of the OSI model. Therefore, their effects on dividing the collision domain and the broadcast domain are different. For example, the repeater works at the physical layer, the bridge, and switch work at the data link layer, the router works at the network layer, and the gateway works on the upper three layers of the OSI model. And each layer of network interconnection equipment should complete different tasks according to different levels of characteristics.
Below we discuss the working principles of common network interconnection devices and their respective characteristics when dividing conflict domains and broadcast domains.
1, traditional Ethernet operation
A typical representation of traditional shared Ethernet is bus-type Ethernet. In this type of Ethernet, there is only one communication channel, and the access method of media sharing (media contention) (the CSMA/CD medium access method described in Chapter 1) is adopted. Each site first listens to whether the network is idle before sending data, and sends data if it is idle. Otherwise, continue listening until the network is idle. If two stations simultaneously detect that the medium is idle and send out one frame of data at the same time, the data frame will collide and both data frames will be destroyed. At this time, the two sites will use the "binary exponential backoff" method to wait for a random time to listen and send.
In Figure 1, host A just wants to send a unicast packet to host B. However, due to the broadcast nature of traditional shared Ethernet, all hosts accessing the bus will receive this unicast packet. At the same time, if any second party, including host B, also sends data to the bus, it will conflict, causing the data transmission of both parties to fail. We call all the hosts connected to the bus together to form a collision domain.
When Host A sends a broadcast type packet destined for all hosts, all hosts on the bus receive the broadcast packet and check the contents of the broadcast packet for further processing if needed. We call all the hosts connected to the bus together to form a broadcast domain.
2, repeater (Repeater)
There are two main reasons why a repeater appears as an actual product:
First, the network distance is extended and the attenuation signal is regenerated.
Second, the interconnection of thick coaxial cable Ethernet and thin coaxial cable Ethernet is realized.
Although the repeater can extend the distance of signal transmission and realize the interconnection of two network segments. But it did not increase the available bandwidth of the network. As shown in Figure 2, network segment 1 and network segment 2 are connected by a repeater to form a single collision domain and broadcast domain.
3, hub (HUB)
A hub is actually equivalent to a multiport repeater. Hubs typically have 8, 16, or 24 equal numbers of interfaces.
Hubs can also extend the communication distance of the network or connect to networks with different physical structures, but mainly serve as a point of convergence for a host site, connecting hosts connected to each interface on the hub to communicate with each other.
As shown in Figure 3, all hosts are connected to the hub of the central node to form a physical star connection. But in fact, inside the hub, each interface is connected through the backplane bus, which still logically constitutes a shared bus. Therefore, the hub and the hosts connected to all its interfaces form a collision domain and a broadcast domain.
4, bridge (Bridge)
A bridge is also known as a bridge. Similar to the repeater, the traditional bridge has only two ports for connecting different network segments. Different from the repeater, the bridge has a certain "smart" nature, can "learn" the address of the host on the network, and has the function of signal filtering.
As shown in FIG. 4, the data packet sent by the host A of the network segment 1 to the host B is not forwarded to the network segment 2 by the bridge. Because the bridge can recognize that this is the communication data flow inside the network segment 1. Similarly, the data packet sent by the host X of the network segment 2 to the host Y is not forwarded to the network segment 1 by the bridge. As you can see, the bridge can split a collision domain into two. Among them, each collision domain shares its own bus channel bandwidth.
However, if Host C sends a broadcast type packet whose destination is all hosts, the bridge will forward such a packet. All hosts on the two-segment buses on both sides of the bridge receive the broadcast packet. Therefore, network segment 1 and network segment 2 still belong to the same broadcast domain.
5, the switch (Switch)
A switch is also known as a switching hub. It emerged to address the flaw in sharing the available bandwidth for all hosts connected to the hub.
The switch increases the available bandwidth by establishing a dedicated communication channel directly for the two hosts that need to communicate. From this perspective, the switch is equivalent to a multi-port bridge.
For example, the switch establishes a dedicated channel for Host A and Host B, and also establishes a dedicated channel for Host C and Host D. Only when an interface is directly connected to a hub, and the hub is connected to multiple hosts, the interface on the switch and all the hosts connected to the hub may collide, forming a collision domain. In other words, each interface on the switch is its own collision domain.
However, the switch also does not have the function of filtering broadcast communications. If the switch receives a broadcast packet, it forwards the broadcast packet to all its ports. Therefore, the switch and the hosts connected to all its interfaces together form a broadcast domain.
The local area network where we will use switches as interconnected devices is called a switched LAN.
6, the router (Router)
The router works at the network layer and can identify the address-IP address of the network layer and has the ability to filter Layer 3 broadcast messages. In fact, unless specifically configured, the router never forwards broadcast-type packets. Therefore, the network to which each port of the router is connected constitutes a broadcast domain by itself. As shown in Figure 6, if each network segment is a shared LAN, each segment itself constitutes an independent collision domain.
7, Gateway (Gateway)
The gateway works on the upper three layers of the OSI reference model, so the concept of collision domain and the broadcast domain is not used. The gateway is mainly used to convert between high-level protocols. For example, act as a mail gateway between the LOTUS 1-2-3 mail service and the Microsoft Exchange mail service.
Note that the concept of a gateway here is completely different from the PC host and the default gateway configured on the router.
