Building Your Home Network - What's Smart about Smart Network Switches? Switches are central to today's LANs. Separation of switches by levels

Interlocutors. As a rule, in public networks it is impossible to provide each pair of subscribers with their own physical communication line, which they could monopoly "own" and use at any time. Therefore, the network always uses some kind of subscriber switching method, which ensures the division of the available physical channels between several communication sessions and between network subscribers.

Switching in local data transmission networks

Ethernet segment switching technology was introduced by Kalpana in 1990 in response to the growing need to increase the bandwidth of high-performance servers to workstation segments. The block diagram of the EtherSwitch offered by Kalpana is presented below. Each of the 8 10Base-T ports is served by one Ethernet Packet Processor (EPP). In addition, the switch has a system module that coordinates the work of all EPP processors. The system module maintains the general address table of the switch and provides SNMP management of the switch. To transfer frames between ports, a switching fabric is used, similar to those found in telephone switches or multiprocessor computers, connecting multiple processors with multiple memory modules. Switching matrix works on the principle of switching channels. For 8 ports, the matrix can provide 8 simultaneous internal channels at half duplex port operation and 16 at full duplex, when the transmitter and receiver of each port operate independently of each other.

When a frame arrives at a port, the EPP processor buffers the first few bytes of the frame to read the destination address. After receiving the destination address, the processor immediately decides to transfer the packet, without waiting for the remaining bytes of the frame to arrive. To do this, he looks through his own cache of the address table, and if he does not find the required address there, he turns to the system module, which works in multitasking mode, in parallel serving the requests of all EPP processors. The system module scans the general address table and returns the found row to the processor, which it buffers in its cache for later use. After finding the destination address, the EPP processor knows what to do next with the incoming frame (while viewing the address table, the processor continued buffering the frame bytes arriving at the port). If a frame needs to be filtered, the processor simply stops writing the bytes of the frame to the buffer, flushes the buffer, and waits for a new frame to arrive. If the frame needs to be transmitted to another port, the processor turns to the switching matrix and tries to establish a path in it that connects its port with the port through which the route to the destination address goes. The switching fabric can do this only when the destination port is free at that moment, that is, it is not connected to another port. If the port is busy, then, as with any circuit-switched device, the matrix fails the connection. In this case, the frame is fully buffered by the processor of the input port, after which the processor waits for the release of the output port and the formation of the desired path by the switching matrix.

Once the correct path is set, buffered frame bytes are sent to it and received by the processor of the output port. As soon as the downstream processor accesses the attached Ethernet segment using the CSMA / CD algorithm, the frame bytes are immediately transferred to the network. The input port processor permanently stores a few bytes of the received frame in its buffer, which allows it to independently and asynchronously receive and transmit frame bytes.

Switching in city telephone networks

The city telephone network is a collection of line and station structures. A network that has one PBX is called non-zoned. Line structures of such a network consist only of subscriber lines. The typical value of the capacity of such a network is 8-10 thousand subscribers. With large capacities, due to a sharp increase in the length of the AL, it is advisable to switch to a zoned network construction. In this case, the territory of the city is divided into districts, in each of which one district automatic telephone exchange (RATS) is being built, to which the subscribers of this district are connected. Subscribers of one region are connected through one RATS, subscribers of different PATC - through two. RATS are connected with each other by connecting lines in the general case according to the principle "each to each". The total number of beams between RATS is equal to the number of RATS / 2. With an increase in the network capacity, the number of trunk lines connecting PATCs with each other according to the principle "each to each" begins to grow sharply, which leads to an excessive increase in cable consumption and communication costs. Therefore, with network capacities of more than 80 thousand subscribers, an additional switching node is used. On such a network, communication between automatic telephone exchanges of different regions is carried out through the nodes of the incoming message (UMC), and communication within its nodal area (UR is carried out according to the principle "each with each" or through its own UMC.

The organization of a computer network is impossible without such a device as a switch or similar network equipment. There are various network devices with which it becomes possible to form a local network, organize Internet access for several computers and other network switching tasks. The most popular of these devices are the hub, router, and switch. Not everyone knows how to set up such devices to make their work more comfortable.

A network switch is required to create a computer network.

If a router (router) is used to connect and route various networks, then a hub and a switch are used to combine various nodes into a single network. The advantageous difference between a switch (switch) and a hub (hub) is that in the first, data packets are transmitted strictly to the address to the specified node, and are not broadcast to all network devices. Thus, by means of a switch, direct address data transfer between two network nodes is realized, while the network resource is used as efficiently as possible. For this reason, at the moment, hubs are practically not used anywhere; they have been supplanted by more efficient and secure switches.

Switch basics

Figure 1. Scheme of the switch.

So, a network switch, also known as a switch or switch ("switch" - switch), is a type of network equipment that connects a certain number of nodes (computers) into a single segment of a computer network and carries out packet transfer of information and data between individual elements of this network.

The switch has at its disposal several ports - connectors into which computers and other network nodes, equipment, etc. are connected. Communication between a port and a node is made using a crimped cable called a twisted pair cable.

For a device such as a switch 8 ports is the norm, but there are also more impressive numbers up to 48 and even 96. (FIG. 1) Within the OSI model, this device operates at the channel level, therefore, as a rule, it only combines other devices into one segment of the network based on their identification MAC addresses.

A standard switch cannot combine several separate networks. For routing at the network level, for example, for organizing Internet access on several computers, which is an example of connecting a local network to a global network, a router or a switch router is needed.

Thus, in the OSI network hierarchy, the switch occupies an intermediate link between the hub and the router:

1. Hub - Physical layer. Broadcasts incoming data, duplicating them to all used interfaces.
2. Switch - Link Layer. Distributes data to highly targeted recipients.
3. Router - Network Layer. Connects various network segments.

The switch is structured as follows. A virtual table of correspondences between MAC addresses and switch ports is stored in the device memory.

MAC-address (“Media Access Control”), also known as Hardware Address, is a special identifier that is assigned to each active element or node in the network, and for each of them it is unique.

At the moment immediately after turning on the switch, its MAC table is still empty and it must be filled in, so the switch enters the primary learning mode.

The peculiarity of this mode is that the data received on any of the ports, as in the hub, is transmitted to all nodes connected to the device in total.

By analyzing data packets, the MAC address of the sending device is determined, then this address is tied to the number of a specific port from which this data was sent. Thus, it turns out to which port a particular network element is connected, then these data are entered into the table.

Now, when data arrives at any of the switch ports, packets addressed to a node in this table will be directed to a specific port corresponding to this node, and not broadcast to all interfaces at once, as it happens in a hub.

If the sent data contains an unknown recipient address that is not in the table, duplicate packets are created and sent to all interfaces.

In parallel, new unknown sender addresses continue to be written to the table.

Subsequently, the switch gradually fills its routing table, including all connections between external computers and its own interfaces, thereby localizing traffic.

The main types of switches

Figure 2. An approximate diagram of connecting a switch via a modem.

The simplest network switch is unmanaged. Although such a switch can be configured directly, it does not have support for network management protocols. The difference between a managed and an unmanaged switch is that, thanks to the support of a simple network management protocol SNMP, a managed switch allows you to remotely configure itself and manage its work via the network using specialized programs.

A managed switch is most often installed in areas of a network with complex topologies where particularly careful control is required. The most typical tasks performed by such devices are:

• monitoring of network traffic;
• configuration management of interfaces (ports);
• organization of virtual networks (VLAN);
• merging a group of channels.

Managed switches are special in that they are able to provide a wide range of functioning both at the channel and at the network level. Access to the management of such a switch can be obtained through a special Web interface, as well as through the command line or various protocols (SNMP, Telnet). Among other things, the switch can use various switching methods, the difference between which is due to the time and reliability of information transfer:

The order of the arrangement of the wires when "crimping" the "twisted pair" cable.

1. Store and Forward - when the switch reads all information in the data frame in order to check for errors, and only then the packet is transmitted to the selected port.
2. Cut-through - the switching process occurs immediately after reading the header of the data frame, where the recipient's address is stored. This reduces the transmission time delay, but it becomes impossible to detect errors, which reduces reliability.
3. Fragment-free is an advanced cut-through mode, in which packets are transmitted after they have been pre-filtered.

This type of switch is rarely used at home, because is intended primarily for switching large and complex structures such as networks of Internet providers, corporate local networks, customer support centers, etc.

An example of such a device is the TP-Link TL-SG2424 24-port gigabit switch, which has a lot of useful functions, including: protection against network storms and distributed attacks, advanced QoS data prioritization, the highest port speed up to 1 Gbps, and others.

How to configure a switch and create your own network

Let's say you decided to create a local area network of several computers in your home and for this purpose you have chosen a network switch. Before setting up the switch and configuring the network, it must be deployed at the physical level, i.e. provide communication between each computer and the switch via a network cable. All connections between nodes are made using a patch cord - a twisted pair network patch cable.

Figure 3. An approximate diagram of a switch connection without a modem.

You can make such a cable yourself, but it is better to buy it in a store. There are two ways how to connect the switch to configure it, depending on the availability of the appropriate interfaces: through a special console port, through which only the initial configuration of the switch is performed, or through a more universal Ethernet port.

In the second case, to gain access to the configuration, you need to enter the IP address specified in the documentation for the device.

Connecting to the console port does not consume switch bandwidth, which has a distinct advantage. To directly configure the switch using this method, you need to start the VT100 terminal emulator (the standard HyperTerminal is also suitable).

Connection parameters are selected according to the documentation. After connecting, the username and password are entered.

Configuration is carried out by entering commands and parameters that depend on a specific device model and must be specified in the documentation.

Internet access via a switch

After creating the network and configuring the switch, the next step is to provide all computers on this network with Internet access. Having a switch available, you can do it quickly, simply and profitably, without additional connection to the ISP separately for each computer, even if the Internet is connected with just one cable. In the case when the Internet service is provided by a landline telephone provider, access to the World Wide Web is carried out via an ADSL modem, the most common models of which do not have more than one Ethernet port. Accordingly, only one computer can be connected to it. To solve this problem, it is not necessary to purchase an expensive router with a built-in switch; an ordinary switch is quite enough. An approximate connection diagram is shown in the figure. (FIG. 2)

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The diagram shows that the ADSL modem is not connected to a computer, but directly to the switch. All computers of the local network are connected to it. A very important point here is the correct configuration of the switch and the connection parameters of computers. Each device, including the modem, must have its own IP address within a single subnet, they must not be repeated.

The book "Building Switched Computer Networks" appeared thanks to the long-term cooperation of D-Link and the country's leading technical university - MSTU im. N.E.Bauman. The book is aimed at a deep presentation of theory and the formation of practical knowledge. It was based on the training materials of the D-Link company, as well as practical exercises conducted at the D-Link training center - Moscow State Technical University named after M.V. NE Bauman - D-Link and the Department of Computer Systems and Networks.

The book contains a complete description of fundamental technologies for switching local area networks, examples of their use, as well as settings on D-Link switches. It will be useful for students studying in the field of "Informatics and Computer Engineering", graduate students, network administrators, enterprise specialists who are introducing new information Technology, as well as everyone who is interested in modern network technologies and principles of building switched networks.

The authors would like to thank all the people involved in the consultation, editing and drawing preparation for the course. The authors would like to express their gratitude to the heads of the Representative Office of D-Link International PTE Ltd and MSTU im. N.E.Bauman, D-Link specialists Pavel Kozik, Ruslan Bigarov, Alexander Zaitsev, Evgeny Ryzhov and Denis Evgrafov, Alexander Schadnev for technical consultations; Olga Kuzmina for editing the book; Alesya Dunaeva for her help in preparing the illustrations. The teachers of the M.V. N.E.Bauman Mikhail Kalinov, Dmitry Chirkov.

Conventions Used in the Course

The following pictograms are used throughout the course text to denote various types of network devices:

Command syntax

The following characters are used to describe how to enter commands, expected values and arguments when configuring the switch through the command line interface (CLI).

Evolution of local area networks

The evolution of local area networks is inextricably linked with the history of the development of Ethernet technology, which to this day remains the most common technology for local area networks.

Initially, LAN technology was seen as a time-saving and cost-effective technology for sharing data, disk space, and expensive peripherals. The decline in the cost of personal computers and peripherals has led to their widespread adoption in business, and the number of network users has increased dramatically. At the same time, the architecture of applications ("client-server") and their requirements for computing resources, as well as the architecture of computing ( distributed computing). Became popular downsizing(downsizing) - transfer of information systems and applications from mainframes to network platforms. All this led to a shift in emphasis in the use of networks: they have become an indispensable tool in business, providing the most efficient processing of information.

In the first Ethernet networks (10Base-2 and 10Base-5), a bus topology was used, when each computer was connected to other devices using a single coaxial cable used as data transmission media... The network environment was shared and the devices had to make sure that it was free before starting to transmit data packets. Although these networks were easy to install, they had significant disadvantages in terms of size, functionality and scalability, lack of reliability, and inability to cope with the exponential increase in network traffic. To improve the efficiency of local networks, new solutions were required.

The next step was the development of the 10Base -T standard with a "star" topology, in which each node was connected with a separate cable to a central device - hub... The concentrator worked at the physical layer of the OSI model and repeated the signals coming from one of its ports to all other active ports, after restoring them. The use of hubs has improved the reliability of the network, since a break in any cable did not cause the entire network to malfunction. However, despite the fact that the use of hubs in the network simplified the tasks of its management and maintenance, the transmission medium remained shared (all devices were in the same collision domain). In addition, the total number of hubs and the network segments they connect was limited due to time delays and other reasons.

Task network segmentation, i.e. dividing users into groups (segments) according to their physical location in order to reduce the number of clients competing for bandwidth was solved using a device called bridge... The bridge was developed by Digital Equipment Corporation (DEC) in the early 1980s and was an OSI-based data link layer device (usually two-port) for connecting network segments. Unlike a hub, a bridge did not just forward data packets from one segment to another, but analyzed and transmitted them only if such a transfer was really necessary, that is, the destination workstation address belonged to another segment. Thus, the bridge isolated the traffic on one segment from the traffic on the other, reducing the collision domain and increasing the overall performance networks.

However, bridges were effective only as long as the number of workstations in the segment remained relatively small. As soon as it increased, congestion appeared in the networks (overflow of the receive buffers of network devices), which led to packet loss.

The increase in the number of devices connected in the network, the increase in the processing power of workstations, the emergence of multimedia applications and client-server applications required more bandwidth. In response to these growing demands, Kalpana launched the first switch dubbed EtherSwitch.

The switch was a multiport bridge and also operated at the data link layer of the OSI model. The main difference between the switch and the bridge was that it could install several connections at the same time between different pairs of ports. When transmitting a packet through a switch, a separate virtual (or real, depending on the architecture) channel was created in it, through which data was sent directly from the source port to the receiving port at the highest possible speed for the technology used. This principle of work is called "micro-segmentation"... Thanks to micro-segmentation, the switches were able to operate in full duplex mode (

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It would seem that what could be easier than connecting computers in information networks? But not everything is so simple: for them to work, it is necessary that a lot of equipment is functioning. It is very diverse. This article will consider the representatives of the second level. So what is a switch? Why is it needed and how does it function?

What is it for? A network switch is a device that is used to connect multiple nodes of a computer network. It works at the data link level. The switch technology was developed using the bridging principle. A feature of this device is that it sends data exclusively to the recipient. This has a positive effect on network performance and security, because in this case, data cannot fall into the wrong hands.

How much does a switch cost? The cheapest is 800 rubles, the most expensive is 24,000.

Principle of operation

This device has a so-called associative memory, where the switching table is stored. It indicates the correspondence of a computer node to a specific port. When the network switch is just powered up, the table is empty. In this case, the device itself works only in the learning mode. So, if you transfer some data to him, then he will alternately transfer them to all his ports. During this process, the information received is analyzed, and the sender's address is entered into the table. And if data is received that must be transferred to an already identified user, then everything will come through the previously specified port. Over time, the network switch will create a table that contains information about all active addresses. It should also be noted that this device is characterized by low latency and high speed of data transfer to each port.

Switching modes

You already know what a switch is. But do they work according to the same principle or are there several approaches to their implementation? It is clear that such a complex mechanism may have several special modes of its operation. There are three of them. Each of them is a combination of two parameters: data transmission reliability and latency.

1. With intermediate storage. The device reads all the information that is in the package. Then it is checked for errors, the switching port is selected, and only after that the data is sent.
2. Through. The switch only reads the address where the data needs to be sent, and then immediately switches them. This is a very fast transmission mode, but a significant drawback is that a packet may be sent with errors.
3. Hybrid. In this mode, only the first 64 bytes of the data packet are analyzed for errors. If they are not here, then the data is sent.

Asymmetric and symmetric switching

You already know what a switch is and what functionality it performs. Let's talk about data transfer. Switching symmetry is necessary to characterize the device itself in terms of bandwidth, its capabilities for each port of the device. It allows for the same width when all ports can transfer 100 Mb / s or 10 Mb / s.

An asymmetric switch can provide connectivity if the ports have different bandwidths. So he will calmly process data that goes at a speed of 10, 100 and 1000 Mb / s. Asymmetric switching can be used when there are large streams of network data, which are arranged according to the principle of "client-server". To send data from a port, on which the amount of information is significantly larger, to a smaller one, a memory buffer is used. It is necessary so that there is no danger of overflow, and, accordingly, data loss. Asymmetric switches are also required to maintain vertical cross-connects and links between individual trunk segments.

Conclusion

Development does not stand still, and already at the time of this writing, switches are considered obsolete devices. Of course, it is still possible to apply them from a purely technical side of the issue, but now, when there are routers that have incorporated their functionality and can additionally provide data transmission over a wireless network, switches look rather pale.