Basic Network concept 3

2.2.5 maximum transmission unit of MTU

According to the definition of the above Mac encapsulation, we now know that the maximum data volume that can be transferred in the standard Ethernet frame can reach 1500 bytes. This value is called MTU (maximum transmission unit, maximum Transmission Unit ). Note that the MTU of each network interface is different. Therefore, you may see 1492 bytes MTU in some network articles. However, in the Ethernet, the standard definition is 1500 bytes.

In the IP packets that will be introduced later, the maximum size of this IP packet can be 65535 bytes, which is larger than that of MTU! Since the gift (IP) is bigger than the box (MAC), how can it be put in? So, IP packets can be split before they can be placed on Mac! When the data is uploaded to the destination, the host at the destination assembles the data. So, if MTU can be larger, the splitting of IP packets will be reduced, and the waiting time between packets and packet transmission will be reduced (96-bit time mentioned in the previous section) it will also be reduced to increase the network bandwidth usage!

For this purpose, the Gigabit Ethernet Media supports jumbo frame! This Jumbo frame is usually defined as 9000 bytes. So, can we change the MTU to 9000 bytes? In this way, can the data packet splitting be reduced to increase network usage? Yes. You can change MTU on Linux! However, if you consider the entire network, we do not recommend that you modify this value. Why?

Do our packets always need to run on the Internet? You cannot confirm that all network media support such a large MTU, right! If your 9000 bytes packet passes through a network media that does not support jumbo frame, the better thing is that the network media (such as switch/router) will take the initiative to help you reorganize and transfer it, if the packet is almost lost, the packet will be directly returned ~ At this time, it will be too big ~ Therefore, MTU is set to 9000, which can only be used in the internal network environment ~ For example, many internal cluster systems set their internal network environment MTU to 9000, but the external adapter is still the original standard of 1500! Pai_^

That is to say, no matter how large your network media supports MTU, you must consider all the network media that need to pass your packets to the destination, then you can determine your MTU settings. In this case, we do not recommend that you modify the MTU of the standard Ethernet network!

 

TIPS:
In the early days, some network media (such as the IP address sharer) supported 802.2. the MTU of the 802.3 standard is 1492, and these devices may not reassemble packets, as a result, some friends on the early network often asked, when they connect to some websites, they will always be online disconnected. However, after the MTU of the client is changed to 1492, there is no problem with accessing the Internet. Why? After reading the data above, Do you understand it? Pai_^

 

2.2.6 hubs, switches and related mechanisms

· Sharing is important. Is it a hub or a switch? (Note 11)

As we mentioned above, when a very busy network is running, the shared media of the hub network may collide, because of CSMA/CD. Is there a way to avoid this inexplicable packet collision? Yes, you can use a non-shared media switch!

Switches have many levels. Here we only discuss switches that support the second layer of OSI. The biggest difference between a switch and a hub is that the switch has a special memory that records the MAC address of each switch port and the PC connected to it. Therefore, when the PCs at both ends of the switch need to transmit data to each other, each message frame will be directly transmitted to the target host through the memory data of the switch! Therefore, the switch is not a shared media, and each port of the switch has an independent bandwidth!

For example, if the hub of 10/100 is connected to five hosts, the entire 10/100 Mbps is allocated to these five hosts. Therefore, these five hosts can only use 10/100 Mbps in total. What if it is a switch? Because "each port has a bandwidth of 10/100 Mbps", it depends on your transmission behavior at that time! For example, in the lower case, each connection is 10/100 Mbps.

 

Figure 2.2-5. bandwidth usage of each port of the switch

Both A and B send data to C with a bandwidth of 10/100 Mbps, which does not affect each other! However, if both A and D are transmitted to C, since C port is only 10/100 Mbps, it means that A and D both need to grab the C node's 10/100 Mbps. In short, you must remember that the switch has overcome the packet collision problem because it has a switch port corresponding to Mac functions, so the switch is not a shared media! At the same time, you must remember that there are many switch specifications. During the purchase, you must remember that the purchase can support full/half duplex and jumbo frame!

· What is full-duplex/half-duplex (full-duplex, half-duplex)

When talking about network lines, we know that only two pairs of network lines are used, one for transmission, and the other for receiving. If both PCs support full dual-work hours at the same time, the input/output can reach 10/100 Mbps, that is, the data transmission and receipt can reach 10/100 BPS at the same time, the total bandwidth can reach 20/200 mbps (in fact, it is a bit language disease, because the input can reach 10/100 Mbps, the output can reach 10/100 Mbps, rather than the input can directly reach 20/200 mbps !) If you want to achieve full dual-work hours in your network environment, it is impossible to use the hub of the shared media. Because of the relationship between the network pin, you cannot use the shared media to achieve full duplex! If your switch also supports full duplex mode, the PC at both ends of the switch can achieve full duplex!

· Auto-negotiation ):

We all know that the current ethernet card can be supported downward, that is, the gigabit network card can be connected to the early 10/100 Mbps network card without any problem. However, how can we determine the network speed at this time? Early switch/hub had to be manually switched, because the new hub/switch supports auto-negotiation, also known as N-way, he can automatically coordinate the highest transmission speed to communicate! If Gigabit and 10/100 Mbps are on the switch, the N-way will first use the highest speed (Gigabit) to test whether all data can be supported. If not, the speed is reduced to the next level, that is, the speed of 100 Mbps!

· Auto MDI/mdix ):

So do we need to identify the parallel line and jumper on our own? No! If the switch contains the auto MDI/mdix function, it will automatically identify the foot of the network line to adjust the online, so you do not need to worry about whether your network line is a jumper or a parallel line! Convenient! Pai_^

· Problems caused by Signal Attenuation

Because the electronic signal will decay, when the network line is too long, leading to serious electronic signal attenuation, it will lead to poor online quality. Therefore, the length of the network lines connecting each node is limited! However, in general, the cat5 network lines of the current Ethernet network can support a length of about 100, so it should be no worries!

However, the signal attenuation is not only the length of the network line! If your network cable is broken too seriously (for example, when the door side is often pressed by the door panel, resulting in deformation), or if you manually press the network cable joint, however, insufficient winding of the eight cores of the joint may cause serious electromagnetic interference, or the situation where the network cables are placed outdoors and exposed to the sun may lead to poor electronic signal transmission and lead to poor online quality, at this time, you will often find the problem of occasional online and sometimes online! Therefore, it is important to pay attention to structured cabling when you need to build an overall network for the enterprise!

· Structured cabling

The so-called structured cabling refers to separating components of each network and installing and arranging them separately within the enterprise. In the future, if you want to increase the network hardware level or move some network devices, you only need to change the Cabinet location similar to the wiring board, and the wall reserved holes at the end can be connected to the host equipment. Example:

Figure 2.2-6 Structured Cabling icons

Pay attention to the cabling in the wall, because it may take more than 5 to 10 years after the cabling is completed! Note only the reserved holes on the end wall and the distribution end. In fact, the level of network media and network lines required for structured cabling, as well as the selection of cabinets, racks, and materials for Beautifying and hiding network lines, as well as the actual construction precautions, as well as all the hardware, construction standards and specifications that need to be paid attention to, and so on, can already be filled with a thick book, the article here aims to introduce an environment with a small number of hosts in a small and medium-sized enterprise, so we only mention one or two switches (swtich) connect all the network devices to the online status of a small star.

If you need information about structured hardware cabling, refer to the second half of the book swtich and Fast Ethernet translated by Feng Xin! As for the experts on the network? You can go to cool school to ask Zman (http: // http://wordpress.morezman.com/) brother oh!