LAN architectures or technologies can be divided into two generations. The first generation includes architectures that provide low and medium data transfer rates: Ethernet 10 Mbps, Token Ring (16 Mbps) and ARC net (2.5 Mbps).
For data transmission, these technologies use cables with a copper core. The second generation of technologies includes modern high-speed architectures: FDDI (100 Mbps), ATM (155 Mbps) and upgraded versions of the first generation architectures (Ethernet): Fast Ethernet (100 Mbps) and Gigabit Ethernet (1000 Mbps) ). Enhanced first-generation architectures are designed for both copper and fiber optic cables. New technologies (FDDI and ATM) are focused on the use of fiber-optic data transmission lines and can be used for the simultaneous transmission of various types of information (video, voice and data). Network technology is a minimum set of standard protocols and software and hardware that implement them, sufficient to build a computer network. Network technologies are called basic technologies. Currently, there are a huge number of networks with different levels of standardization, but such well-known technologies as Ethernet, Token-Ring, Arcnet, FDDI are widely used.
Network access methods
ethernet is a multiple access method with listening to the carrier and resolving collisions (conflicts). Before starting a transmission, each workstation determines whether the channel is free or busy. If the channel is free, the station starts transmitting data. In reality, conflicts lead to a decrease in network performance only when 80–100 stations are working. Access Method Arcnet. This access method has become widespread mainly due to the fact that Arcnet equipment is cheaper than Ethernet or Token-Ring equipment. Arcnet is used in local area networks with a star topology. One of the computers creates a special token (special message) that is passed sequentially from one computer to another. If the station needs to send a message, after receiving the token, it forms a packet, complete with the sender and destination addresses. When the packet reaches the destination station, the message is unhooked from the token and passed to the station. Access Method token ring. This method was developed by IBM; it is designed for the ring topology of the network. This method is similar to Arcnet in that it also uses a token passed from one station to another. Unlike Arcnet, the Token Ring access method provides the ability to assign different priorities to different workstations.
Basic LAN technologies
Ethernet technology is now the most popular in the world. In a classic Ethernet network, a standard coaxial cable of two types (thick and thin) is used. However, a twisted-pair version of Ethernet is becoming more common, as it is much easier to install and maintain. Bus topologies and passive star topologies are used. The standard defines four basic media types.
10BASE5 (thick coaxial cable);
10BASE2 (thin coaxial cable);
10BASE-T (twisted pair);
10BASE-F (fiber optic cable).
Fast Ethernet is a high-speed version of the Ethernet network that provides a transmission rate of 100 Mbps. Fast Ethernet networks are compatible with networks based on the Ethernet standard. The basic topology of a Fast Ethernet network is a passive star.
The standard defines three media types for Fast Ethernet:
100BASE-T4 (quad twisted pair);
100BASE-TX (dual twisted pair);
100BASE-FX (fiber optic cable).
Gigabit Ethernet is a high-speed version of the Ethernet network that provides a transmission rate of 1000 Mbps. The Gigabit Ethernet network standard currently includes the following media types:
1000BASE-SX - a segment on a multimode fiber optic cable with a light signal wavelength of 850 nm.
1000BASE-LX – a segment on a multimode and single-mode fiber optic cable with a light signal wavelength of 1300 nm.
1000BASE-CX - segment on an electrical cable (shielded twisted pair).
1000BASE-T - a segment on an electrical cable (quadruple unshielded twisted pair).
Due to the fact that the networks are compatible, it is easy and simple to connect Ethernet, Fast Ethernet and Gigabit Ethernet segments into a single network.
The Token-Ring network is proposed by IBM. Token-Ring was intended to network all types of computers manufactured by IBM (from personal to large). The Token-Ring network has a star-ring topology. The Arcnet network is one of the oldest networks. The Arcnet network uses a "bus" and a "passive star" as its topology. The Arcnet network was very popular. Among the main advantages of the Arcnet network are high reliability, low cost of adapters and flexibility. The main disadvantage of the network is the low data transfer rate (2.5 Mbit/s). FDDI (Fiber Distributed Data Interface) - a standardized specification for a network architecture for high-speed data transmission over fiber optics. Transfer rate - 100 Mbps. The main technical characteristics of the FDDI network are as follows:
The maximum number of network subscribers is 1000.
The maximum length of the network ring is 20 km
The maximum distance between network subscribers is 2 km.
Transmission medium - fiber optic cable
Access method – marker.
Information transfer rate – 100 Mbps.
Topic 1.3: Open Systems and the OSI Model
Topic 1.4: Basics of local networks
Topic 1.5: Basic LAN technologies
Topic 1.6: Basic software and hardware components of a LAN
Local networks
1.5. Core technologies or networking technologies of local area networks
1.5.3. Network technologies of local networks
In local networks, as a rule, a shared data transmission medium (monochannel) is used and the main role is assigned to the protocols of the physical and link layers, since these levels reflect the specifics of local networks to the greatest extent.
Network technology is an agreed set of standard protocols and software and hardware that implement them, sufficient to build a local area network. Network technologies are called basic technologies or network architectures of local area networks.
Network technology or architecture determines the topology and method of access to the data transmission medium, the cable system or data transmission medium, the network frame format, the type of signal coding, the transmission rate in the local network. In modern local area networks, technologies or network architectures such as Ethernet, Token-Ring, ArcNet, FDDI are widely used.
IEEE802.3/Ethernet LAN Networking Technologies
Currently, this network technology is the most popular in the world. Popularity is ensured by simple, reliable and inexpensive technologies. In a classic Ethernet local area network, a standard coaxial cable of two types (thick and thin) is used.
However, a twisted-pair version of Ethernet is becoming more common, as it is much easier to install and maintain. Ethernet LANs use bus and passive star topologies, and the access method is CSMA/CD.
The IEEE802.3 standard, depending on the type of data transmission medium, has modifications:
- 10BASE5 (thick coaxial cable) - provides a data transfer rate of 10 Mbps and a segment length of up to 500m.
- 10BASE2 (thin coaxial cable) - provides a data transfer rate of 10 Mbps and a segment length of up to 200m.
- 10BASE-T (Unshielded Twisted Pair) - allows you to create a network in star topology. The distance from the concentrator to the end node is up to 100m. The total number of nodes must not exceed 1024.
- 10BASE-F (fiber optic cable) - allows you to create a network in star topology. The distance from the concentrator to the end node is up to 2000m.
In the development of Ethernet network technology, high-speed options have been created: IEEE802.3u/Fast Ethernet and IEEE802.3z/Gigabit Ethernet. The main topology used in Fast Ethernet and Gigabit Ethernet LANs is the passive star.
Fast Ethernet network technology provides a transmission rate of 100 Mbps and has three modifications:
- 100BASE-T4 - uses unshielded twisted pair (quad twisted pair). The distance from the concentrator to the end node is up to 100m.
- 100BASE-TX - uses two twisted pairs (unshielded and shielded). The distance from the concentrator to the end node is up to 100m.
- 100BASE-FX - uses fiber optic cable (two fibers per cable). The distance from the concentrator to the end node is up to 2000m.
Network technology of Gigabit Ethernet local area networks - provides a transfer rate of 1000 Mbps.
There are the following modifications of the standard:
- 1000BASE-SX - uses fiber optic cable with a light wavelength of 850 nm.
- 1000BASE-LX - uses fiber optic cable with a light wavelength of 1300 nm.
- 1000BASE-CX - Uses shielded twisted pair.
- 1000BASE-T - uses quad unshielded twisted pair.
Fast Ethernet and Gigabit Ethernet local networks are compatible with local networks made according to the Ethernet technology (standard), so it is easy and simple to connect Ethernet, Fast Ethernet and Gigabit Ethernet segments into a single computer network.
IEEE802.5/Token-Ring LAN Networking Technologies
The Token-Ring network involves the use of a shared data transmission medium, which is formed by combining all nodes into a ring.
The Token-Ring network has a star-ring topology (main ring and star supplementary topology). The marker method (deterministic marker method) is used to access the data transfer medium.
The standard supports twisted pair (shielded and unshielded) and fiber optic cable. The maximum number of nodes on the ring is 260, the maximum length of the ring is 4000 m. The data transfer rate is up to 16 Mbps.
IEEE802.4/ArcNet LAN Networking Technologies
The ArcNet LAN uses a "bus" and a "passive star" as its topology. Supports shielded and unshielded twisted pair and fiber optic cable.
ArcNet uses a delegation of authority method to access the media. The ArcNet LAN is one of the oldest networks and has been very popular. Among the main advantages of the ArcNet local area network are high reliability, low cost of adapters, and flexibility.
The main disadvantage of the network is the low data transfer rate (2.5 Mbit/s). The maximum number of subscribers is 255. The maximum network length is 6000 meters.
FDDI (Fiber Distributed Data Interface) LAN Networking Technologies
FDDI- a standardized specification for a network architecture for high-speed data transmission over fiber optics. Transfer rate - 100 Mbps. This technology is largely based on the Token-Ring architecture and uses deterministic token access to the data transfer medium.
The maximum length of the network ring is 100 km. The maximum number of network subscribers is 500. The FDDI network is a very highly reliable network, which is created on the basis of two fiber optic rings that form the main and backup data transmission paths between nodes.
Computer networks are divided into three main classes:
1. Local computer networks (LAN - LocalAreaNetwork) are networks that unite computers located geographically in one place. A local area network combines computers located physically close to each other (in the same room or building).
2. Regional computer networks (MAN - MetropolitanAreaNetwork) are networks that combine several local computer networks located within the same territory (city, region or region, for example, the Far East).
3. Wide area networks (WAN - Wide Area Network) are networks that combine many local, regional networks and
computers of individual users located at any distance from each other (Internet, FIDO).
At the moment, the following standards for building local area networks are used:
Arcnet;(IEEE 802.4)
Token Ring;(802,5)
Ethernet.(802,3)
Let's consider each of them in more detail.
IEEE 802.4 ARCNET technology (or ARCnet, from Attached Resource Computer NETwork) is a LAN technology, the purpose of which is similar to that of Ethernet or Token ring. ARCNET was the first microcomputer networking technology and became very popular in the 1980s for office automation. It is intended for the organization of a LAN in a network topology "star".
The basis of communication equipment is:
switch
passive/active hub
Switching equipment has an advantage, as it allows the formation of network domains. Active hubs are used when the workstation is far away (they restore the signal shape and amplify it). Passive - with a small. The network uses the assigned access principle of workstations, that is, the station that has received the so-called software token from the server has the right to transmit. That is, deterministic network traffic is implemented.
Advantages of the approach:
Remarks: messages transmitted by workstations form a queue on the server. If the queue service time is significantly (more than 2 times) greater than the maximum packet delivery time between the two most remote stations, then the network bandwidth is considered to have reached its maximum limit. In this case, further expansion of the network is impossible, and installation of a second server is required.
Limit specifications:
The minimum distance between workstations connected to the same cable is 0.9 m.
The maximum network length along the longest route is 6 km.
The limitations are related to the hardware delay in the transmission of information with a large number of switching elements.
The maximum distance between the passive hub and workstation is 30 m.
The maximum distance between the active and passive hub is 30 m.
Between the active hub and the active hub - 600 m.
Advantages:
Low cost of network equipment and the possibility of creating extended networks.
Flaws:
Low data transfer rate. After the spread of Ethernet as a technology for creating a LAN, ARCNET found its way into embedded systems.
Support for ARCNET technology (in particular, distribution of specifications) is handled by the non-profit organization ARCNET Trade Association (ATA).
Technology - ArcNET architecture is represented by two main topologies: bus and star. The transmission medium is an RG-62 coaxial cable with a wave impedance of 93 Ohm, crimped onto BNC plugs with the appropriate termination diameter (different from 10Base-2 (“thin” Ethernet) plugs).
Network equipment consists of network adapters and hubs. Network adapters can be for bus topology, for star and universal. Hubs can be active or passive. Passive hubs are used to create stellar sections of the network. Active hubs can be for bus, star and mixed topologies. Ports for bus topology are not physically compatible with ports for star topology, although they have the same physical connection (BNC socket).
In the case of a bus topology, workstations and servers are connected to each other using T-connectors (the same as in 10Base-2 ("thin" Ethernet)), connected to network adapters and hubs and connected by a coaxial cable. The extreme points of the segment are terminated by tips with a resistance of 93 Ohm. The number of devices on one bus is limited. The minimum distance between connectors is 0.9 meters and must be a multiple of this value. To facilitate cutting, marks can be applied to the cable. Individual buses can be combined with bus hubs.
When using a star topology, active and passive hubs are used. The passive hub is a resistive splitter-matcher that allows you to connect four cables. All cables in this
In this case, they are connected according to the “point-to-point” principle, without the formation of buses. No more than two passive hubs should be connected between two active devices. The minimum length of any network cable is 0.9 meters and must be a multiple of this value. There is a cable length limit between active and passive ports, between two passive ports, between two active ports.
In a mixed topology, active hubs are used that support both types of connection.
On network adapters of workstations and servers using jumpers or DIP switches, a unique network address is set, permission to use a BIOS expansion chip that allows remote boot of a workstation (can be diskless), connection type (bus or star topology), connection of a built-in terminator ( the last two paragraphs are optional). The limit on the number of workstations is 255 (according to the bit depth of the network address register). In the event that two devices have the same network address, both lose their functionality, but this collision does not affect the operation of the network as a whole.
In a bus topology, a break in a cable or terminator causes the network to be inoperable for all devices connected to the segment that this cable belongs to (that is, from terminator to terminator). With a star topology, a break in any cable leads to a failure of the segment that is disconnected by this cable from the file server.
The logical architecture of ArcNET is a ring with token access. Since such an architecture does not allow collisions in principle, with a relatively large number of hosts (in practice, 25-30 workstations were tested), the performance of the ArcNET network turned out to be higher than 10Base-2, with a fourfold lower speed in the environment (2.5 versus 10 Mbps ).
802.5 Token Ring technology is a local area network (LAN) technology of a ring with "token access" - a local area network protocol that is located at the data link layer (DLL) of the OSI model. It uses a special three-byte frame called a marker that moves around the ring. Possession of a marker grants the holder the right to transmit information on the carrier. Token ring frames move around in a loop. Stations on a local area network (LAN) Token ring are logically organized into a ring topology with data being sent sequentially from one ring station to another with a control token circulating around the control ring. This token passing mechanism is shared by ARCNET, the token bus, and FDDI, and has theoretical advantages over stochastic CSMA/CD Ethernet.
Token Ring Token Passing and IEEE 802.5 are prime examples of token passing networks. Token passing networks move a small block of data called a token along the network. Ownership of this token guarantees the right to transfer. If the host receiving the token has no information to send, it simply forwards the token to the next end station. Each station can hold the marker for a certain maximum time (default is 10ms).
This technology offers a solution to the problem of collisions that occurs during the operation of a local network. In Ethernet technology, such collisions occur during the simultaneous transmission of information by several workstations located within the same segment, that is, using a common physical data channel.
If the station that owns the token has information to transmit, it grabs the token, changes one bit of it (as a result of which the token turns into the “beginning of data block” sequence), supplements the information that it wants to transmit and sends this information to the next station ring network. When an information block circulates around the ring, there is no token on the network (unless the ring provides an "early token release"), so other stations wishing to transmit information must wait. Therefore, collisions cannot occur in Token Ring networks. If early release of the token is provided, then a new token can be released after the transmission of the data block is completed.
Information block circulates around the ring until it reaches the intended destination station, which copies the information for further processing. The information block continues to circulate around the ring; it is finally removed after reaching the station that sent the block. The sending station can check the returned block to ensure that it has been viewed and then copied by the destination station.
Scope Unlike CSMA/CD networks (eg Ethernet), token passing networks are deterministic networks. This means that it is possible to calculate the maximum time that will pass before any end station can transmit. This characteristic, along with some reliability characteristics, makes the Token Ring network ideal for applications where latency must be predictable and network stability is important. Examples of such applications are the environment of automated stations in factories.
It is used as a cheaper technology and has become widespread wherever there are critical applications for which it is important not so much speed as reliable information delivery. Currently, Ethernet is not inferior to Token Ring in terms of reliability and is significantly higher in performance.
Token Ring modifications There are 2 transmission speed modifications: 4 Mbps and 16 Mbps. In Token Ring 16 Mbps is used
early marker release technology. The essence of this technology lies in the fact that the station that “captured” the token generates a free token at the end of the data transfer and launches it into the network. Attempts to introduce 100 Mbps technology were not commercially successful. Token Ring technology is not currently supported.
802.3 Ethernet technology ether "ether") is a packet technology for data transmission mainly of local computer networks.
Ethernet standards define wired connections and electrical signals at the physical layer, frame format and media access control protocols at the data link layer of the OSI model. Ethernet is mainly described by the IEEE 802.3 standards. Ethernet became the most common LAN technology in the mid-1990s, replacing legacy technologies such as Arcnet, FDDI, and token ring.
For the implementation of work on the creation of a local network, the following must be considered:
* Creating a local network and setting up equipment for accessing the Internet;
* The choice of equipment should be based on technical specifications that can meet the requirements for data transfer speed;
* The equipment must be safe, protected from electric shock to people;
* Each workstation must have a network cable to connect to the network;
* Possible availability of wi-fi throughout the office;
* The location of workplaces must meet the requirements of standards for the placement of equipment in educational institutions;
* The cost of creating a local network must be economically justified;
* Reliability of the local network.
Let's consider the application of the above in real network technologies. Network technology is an agreed set of standard protocols and software and hardware that implements them (for example, network adapters, drivers, cables and connectors), sufficient to build a computer network, i.e. this is the minimum set of tools with which you can build a workable network; Sometimes network technologies are called basic technologies, meaning that the basis of any network is built on their basis. Currently, there are more than 200 networks with some level of standardization, but no more than 10 of them have received wide distribution and universal recognition. This is due to the fact that these networks are supported by the most powerful firms and therefore brought to the level of international standards. Known technologies such as Ethernet, Token-Ring, Arcnet, FDDI can serve as examples of basic technologies.
NETWORK ETHERNET. The Ethernet network is the most widespread among standard networks. It appeared in 1972 (the developer was the well-known company Xerox). In 1985, the Ethernet network became an international standard, it was accepted by the largest international standards organizations: the 802 committee of the IEEE (Institute of Electrical and Electronic Engineers) and ECMA (European Computer Manufacturers Association). The standard is called IEEE 802.3. It defines multiple access to a bus-type channel with collision detection and transmission control, i.e. with the already mentioned CSMA/CD access method.
The main characteristics of the IEEE 802.3 standard are as follows: topology - "bus", transmission medium - coaxial cable, transmission rate - 10 Mbps, maximum number of subscribers - up to 1024, network segment length - up to 500 m, number of subscribers on one segment - up to 100 .
In a classic Ethernet network, a standard coaxial cable of two types (thick and thin) is used. However, in Lately The version of Ethernet that uses twisted pairs as the transmission medium is becoming more common, as installation and maintenance are much easier. IN last years a faster version of Ethernet has appeared, operating at a speed of 100 Mbps (Fast Ethernet). A standard has also been defined for use in a fiber optic cable network. In addition to the standard bus topology, a passive star topology is also used. The main thing is that there are no closed paths (loops) in the resulting topology. In fact, it turns out that the subscribers are all connected to the same "bus", since the signal from each of them propagates in all directions at once and does not return back. The maximum cable length of the entire network as a whole (maximum signal path) can theoretically reach 6.5 km, but practically does not exceed 2.5 km.
FAST ETHERNET NETWORK. The Fast Ethernet network is component IEEE 802.3 standard, which appeared as recently as 1995. It is a faster version of a standard Ethernet network, operating at 100 Mbps. In order to maintain compatibility with earlier versions of Ethernet, the standard defines a special mechanism for Fast Ethernet to automatically detect the transmission speed in auto-dialog mode, which allows Fast Ethernet network adapters to automatically switch from 10 Mbps to 100 Mbps and vice versa.
The basic topology of a Fast Ethernet network is a passive star. Fast Ethernet requires the mandatory use of more expensive hubs than with Ethernet. Hubs in this case can be interconnected by connected segments, which allows you to build complex configurations.
Local area networks of all other types, except for Ethernet, are much less common.
FDDI NETWORK. The FDDI network (from the English Fiber Distributed Data Interface) is one of the latest developments in local area network standards. The FDDI standard, proposed by the American National Standards Institute (ANSI), was originally focused on high transmission speed (100 Mbps) and on the use of advanced fiber optic cable (light wavelength - 850 nm). Therefore, in this case, the developers were not constrained by the framework of standards that focused on low speeds and electric cable.
The choice of optical fiber as a transmission medium immediately determined the advantages of the new network: high noise immunity and secrecy of information transmission. The high transmission speed, which is much easier to achieve with fiber optic cable, allows many tasks that are not possible with slower networks, such as real-time image transmission. In addition, fiber optic cable easily solves the problem of transmitting data over a distance of several kilometers without relaying, which allows you to build much larger networks, even covering entire cities, while having all the advantages of local networks (in particular, low error rate). And although FDDI equipment has not yet received wide distribution, it is very promising.
The FDDI standard was based on the token access method provided for by the international standard IEEE 802.5 Token-Ring. Slight differences from this standard are determined by the need to ensure a high speed of information transmission over long distances. The topology of the FDDI network is a ring, using two multi-directional fiber optic cables, which allows information to be transmitted at twice the effective speed of 200 Mbps (with each of the two channels operating at a speed of 100 Mbps).
The main technical characteristics of the FDDI network are as follows: The maximum number of network subscribers is 1000. The maximum length of the network ring is 20 km. The maximum distance between network subscribers is 2 km. Transmission medium - fiber optic cable (it is possible to use an electric twisted pair).
Access method - marker.
Information transfer rate - 100 Mbps (200 Mbps for duplex transmission mode).
As you can see, FDDI has great advantages over all previously discussed networks. Even a Fast Ethernet network with the same bandwidth of 100 Mbps cannot match FDDI in terms of allowed network size and allowed number of subscribers. signal passing around the ring to ensure the maximum allowable access time.
The FDDI standard for achieving high network flexibility provides for the inclusion of two types of network adapters in the ring:
1. Class A adapters are connected to the inner and outer rings of the network. In this case, the possibility of exchanging at speeds up to 200 Mbps or the possibility of redundant network cable is realized (if the main cable is damaged, a backup cable is used). Equipment of this class is used in the most critical parts of the network.
2. Class B adapters connect only to the outer ring of the network. They may be simpler and cheaper than Class A adapters, but will not have the same capabilities.
The FDDI standard provides for the possibility of reconfiguring the network in order to maintain its operability in the event of a cable failure. The damaged section of the cable is removed from the ring, but the integrity of the network is not violated due to the transition to one ring instead of two (i.e., class A adapters begin to work as class B adapters).
Despite the obvious advantages, the FDDI network has not yet become widespread, this is mainly due to the high cost of its equipment. However, the situation may change in the near future.
GIGABIT ETHERNET NETWORK. The speed of the Fast Ethernet network, other networks operating at a speed of 100 Mbps, currently meets the requirements of most tasks, but in some cases even it is not enough. This is especially true in situations where it is necessary to connect modern high-performance servers to the network or build networks with a large number of subscribers that require high traffic intensity.
Maintaining continuity makes it easy and simple to connect Ethernet, Fast Ethernet and Gigabit Ethernet segments into a single network and move to new speeds gradually, introducing gigabit segments only in the most stressed sections of the network. In addition, such a high throughput is not really needed everywhere.
The rapid development of local area networks, which is now further embodied in the 10 Gigabit Ethernet standard and IEEE 802.11b/a wireless networking technologies, is attracting more and more attention. For cable networks, Ethernet technology has now become the de facto standard. And although Ethernet technology has not been found in its classical form for a long time, the ideas that were originally laid down in the IEEE 802.3 protocol have received their logical continuation both in Fast Ethernet technology and in Gigabit Ethernet. For the sake of historical fairness, we note that technologies such as Token Ring, ARCNET, 100VG-AnyLAN, FDDI and Apple Talk also deserve attention. Well. Let's restore historical justice and remember the technologies of bygone days.
I think I can't tell you about the rapid progress in the semiconductor industry over the past decade. Network equipment suffered the fate of the entire industry: an avalanche of production growth, high speeds and minimum prices. In 1995, which is considered a turning point in history Internet development, about 50 million new Ethernet ports have been sold. Not a bad start for market dominance, which became overwhelming over the next five years.
For specialized telecommunications equipment, this price level is not available. The complexity of the device does not play a special role - the question is rather in quantity. Now it seems quite natural, but ten years ago, the absolute dominance of Ethernet was far from obvious (for example, there is still no clear leader in industrial networks).
However, only in comparison with other methods of building networks can the advantages (or disadvantages) of today's leader be revealed.
The main ways to access the medium to the transmission medium
The physical principles according to which the equipment operates are not too complicated. According to the method of obtaining access to the transmission medium, they can be divided into two classes: deterministic and non-deterministic.
With deterministic access methods, the transmission medium is distributed among the nodes using a special control mechanism that guarantees the transmission of node data for a certain period of time.
The most common (but by no means the only) deterministic access methods are the polling method and the transfer method. The polling method is of little use in local networks, but is widely used in industry for process control.
The transfer method, on the other hand, is convenient for transferring data between computers. The principle of operation consists in the transmission of a service message - a token - over a network with a ring logical topology.
Obtaining a token entitles the device to access the shared resource. The choice at the workstation in this case is limited to only two options. In either case, it must send the token to the next device in the queue. Moreover, this can be done after the delivery of data to the addressee (if any) or immediately (in the absence of information that needs to be transmitted). There is no token in the network for the duration of the data passage, other stations are unable to transmit, and collisions are impossible in principle. To handle possible errors, as a result of which the token may be lost, there is a mechanism for regenerating it.
Random access methods are called non-deterministic. They provide for the competition of all network nodes for the right to transmit. It is possible for multiple nodes to transmit at the same time, resulting in collisions.
The most common method of this type is CSMA / CD (carrier-sense multiple access / collision detection) - multiple access with carrier sense / collision detection. Before starting data transfer, the device “listens” on the network to make sure that no one else is using it. If the transmission medium is being used by someone at that moment, the adapter delays the transmission, if not, it starts transmitting data.
In the case when two adapters, having detected a free line, start transmitting at the same time, a collision occurs. When it is detected, both transmissions are interrupted and the devices repeat the transmission after some arbitrary time (of course, having previously “listened” to the channel again for busyness). To obtain information, a device must receive all packets on the network to determine if it is the destination.
From the history of Ethernet
If we started our discussion of local area networks with any other technology, we would not take into account the real importance that Ethernet currently has in this area. Whether by the will of the circumstances or due to technical advantages, but today it has no competition, occupying about 95% of the market.
Ethernet's birthday is May 22, 1973. It was on this day that Robert Metcalfe and David Boggs published a description of an experimental network they had built at the Xerox Research Center. It was based on a thick coaxial cable and provided a data transfer rate of 2.94 Mbps. The new technology was named Ethernet (ethernet), after the University of Hawaii's ALOHA radio network, which used a similar mechanism for dividing the transmission medium (radio).
By the end of the 1970s, a solid theoretical foundation had been laid for Ethernet. And in February 1980, Xerox, together with DEC and Intel, presented the IEEE development, which three years later was approved as the 802.3 standard.
The Ethernet media access method is non-deterministic and is Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Simply put, devices share the transmission medium randomly. In this case, the algorithm can lead to far from equal resolution of the rivalry between stations for access to the medium. This, in turn, can generate long access delays, especially under congestion conditions. In extreme cases, the transmission speed may drop to zero.
Because of this unordered approach, it has long been believed (and still is) that Ethernet does not provide high-quality data transmission. It was predicted that it would be replaced first by marker Token Ring, then by ATM, but in reality everything happened the other way around.
The fact that Ethernet still dominates the market is due to the great changes it has undergone since its 20-year existence. That "gigabit" in full duplex, which we now see already in entry-level networks, bears little resemblance to the ancestor of the 10Base 5 family. At the same time, after the introduction of 10Base-T, compatibility is maintained both at the level of device interaction and at the level of cable infrastructure.
Evolving from simple to complex, growing along with user needs, is the key to the technology's incredible success. Judge for yourself:
- March 1981 - 3Com introduces an Ethernet transceiver;
- September 1982 - the first network adapter for a personal computer was created;
- 1983 - the IEEE 802.3 specification appeared, the bus topology of the 10Base 5 (thick Ethernet) and 10Base 2 (thin Ethernet) network was defined. Transfer rate - 10 Mbps. The maximum distance between the points of one segment is set - 2.5 km;
- 1985 - The second version of the IEEE 802.3 (Ethernet II) specification was released, in which minor changes were made to the structure of the packet header. A hard identification of Ethernet devices (MAC addresses) has been formed. An address list has been created where any manufacturer can register a unique range (currently only $1,250);
- September 1990 - IEEE approves 10Base-T (twisted-pair) technology with star physical topology and hubs. The logical topology of CSMA/CD has not changed. The standard was based on the development of SynOptics Communications under the general name LattisNet;
- 1990 - Kalpana (subsequently it was quickly bought together with the future giant Cisco developed the CPW16 switch) offers a switching technology based on the rejection of the use of shared communication lines between all segment nodes;
- 1992 - the beginning of the use of switches (swich). Using the address information contained in the packet (MAC address), the switch organizes independent virtual channels between pairs of nodes. Switching virtually imperceptibly to the user transforms a non-deterministic Ethernet model (with competition for bandwidth) into a system with data transfer;
- 1993 IEEE 802.3x specification introduces full duplex and link control for 10Base-T, IEEE 802.1p specification adds multicast and 8-level priority system. Fast Ethernet proposed;
- in June 1995, Fast Ethernet was introduced, the IEEE 802.3u (100Base-T) standard.
On this a brief history we can finish: Ethernet has taken quite modern shape, but the development of technology, of course, has not stopped - we will talk about this a little later.
Undeservedly forgotten ARCNET
ttached Resource Computing Network (ARCNET) is a network architecture developed by Datapoint in the mid 70s. ARCNET has not been adopted as an IEEE standard, but partially complies with IEEE 802.4 as a token passing network (logical ring). The data packet can be any size between 1 and 507 bytes.
Of all local area networks, ARCNET has the most extensive topology capabilities. Ring, common bus, "star", "tree" can be applied in the same network. In addition, very long segments (up to several kilometers) can be used. The same broad possibilities apply to the transmission medium - both coaxial and fiber optic cables, as well as twisted pair, are suitable.
This inexpensive standard was prevented from dominating the market by low speed - only 2.5 Mbps. When Datapoint developed ARCNET PLUS with transfer rates up to 20 Mbps in the early 90s, time was already lost. Fast Ethernet did not leave ARCNET the slightest chance for widespread use.
Nevertheless, in favor of the large (but never realized) potential of this technology, we can say that in some industries (usually process control systems) these networks still exist. Deterministic access, auto-configuration capabilities, exchange rate negotiation in the range from 120 Kbps to 10 Mbps in difficult real-world conditions make ARCNET simply indispensable.
In addition, ARCNET provides the ability for control systems to accurately determine the maximum access time to any device on the network under any load using a simple formula: T = (TDP + TOBSNb)SND, where TDP and TOB are the transmission time of a data packet and one byte, respectively, depending on the selected transmission rate, Nb is the number of data bytes, ND is the number of devices in the network.
Token Ring - a classic example of passing a token
oken ring is another technology that has its origins in the 70s. This development of the blue giant - IBM, which is the basis of the IEEE 802.5 standard, had a better chance of success than many other local networks. Token Ring is a classic token passing network. The logical topology (and the physical one in the first versions of the network) is a ring. More modern modifications are built on twisted pair in a star topology, and with some reservations are compatible with Ethernet.
The original bit rate described in IEEE 802.5 was 4 Mbps, but there is a later implementation of 16 Mbps. Due to the more ordered (deterministic) method of accessing the media, Token Ring was often promoted in the early stages of development as a better replacement for Ethernet.
Despite the existence of a priority access scheme (which was assigned to each station separately), it was not possible to provide a constant bit rate (Constant Bit Rate, CBR) for a very simple reason: applications that could take advantage of these schemes did not exist then. And now there are not much more of them.
Given this circumstance, it could only be guaranteed that the performance for all stations in the network would decrease equally. But this was not enough to win the competition, and now it is almost impossible to find a really working Token Ring network.
FDDI - the first fiber-optic LAN
The Fiber Distributed Data Interface (FDDI) technology was developed in 1980 by an ANSI committee. It was the first computer network to use only fiber optic cable as a transmission medium. The reasons that prompted manufacturers to create FDDI were the insufficient speed (no more than 10 Mbit / s) and reliability (lack of redundancy schemes) of local networks at that time. In addition, it was the first (and not very successful) attempt to bring data transmission networks to the "transport" level, competing with SDH.
The FDDI standard stipulates data transmission over a double ring of fiber optic cable at a speed of 100 Mbps, which allows you to get a reliable (reserved) and fast channel. The distances are quite significant - up to 100 km along the perimeter. Logically, the operation of the network was built on passing the token.
Additionally, a developed traffic prioritization scheme was provided. At first, workstations were divided into two types: synchronous (having a constant bandwidth) and asynchronous. The latter, in turn, distributed the transmission medium using an eight-level system of priorities.
Incompatibility with SDH networks did not allow FDDI to occupy any significant niche in the field of transport networks. Today, this technology has been virtually superseded by ATM. And the high cost left no chance for FDDI in the fight against Ethernet for a local niche. Did not help the standard and attempts to switch to a cheaper copper cable. CDDI technology, based on the principles of FDDI, but using twisted pair as a transmission medium, was not popular and was preserved only in textbooks.
AT&T and HP development - 100VG-AnyLAN
that technology, like FDDI, can be attributed to the second generation of local networks. It was created in the early 90s by the joint efforts of AT&T and HP as an alternative to Fast Ethernet technology. In the summer of 1995, almost simultaneously with its competitor, it received the status of the IEEE 802.12 standard. 100VG-AnyLAN stood a good chance of winning due to its versatility, determinism, and more complete compatibility than Ethernet with existing cable networks (category 3 twisted pair).
The Quartet Coding scheme, using a 5V / 6V redundant code, allowed the use of a 4-pair Category 3 twisted pair cable, which was then almost more common than the modern Category 5. The transition period, in fact, did not affect Russia, where, due to the later start of the construction of communication systems, networks were already laid everywhere using the 5th category.
In addition to using legacy wiring, each 100VG-AnyLAN hub can be configured to support either 802.3 (Ethernet) frames or 802.5 (Token Ring) frames. The Demand Priority media access method defines a simple two-level priority system - high for multimedia applications and low for all others.
I must say, it was a serious bid for success. Summed up by the high cost, due to the greater complexity and, to a large extent, the closeness of the technology from replication by third-party manufacturers. Added to this is the already familiar lack of real applications from Token Ring that take advantage of the priority system. As a result, 100Base-T managed to permanently and finally take the lead in the industry.
Innovative technical ideas a little later found application first in 100Base-T2 (IEEE 802.3u), and then in "gigabit" Ethernet 1000Base-T.
Apple Talk, Local Talk
Apple Talk is a protocol stack proposed by Apple in the early 80s. Initially, the Apple Talk protocols were used to work with network equipment, collectively called Local Talk (adapters built into Apple computers).
The network topology was built as a common bus or "tree", its maximum length was 300 m, the transmission rate was 230.4 Kbps. Transmission medium - shielded twisted pair. The Local Talk segment could unite up to 32 nodes.
Low bandwidth quickly necessitated the development of adapters for higher bandwidth network environments: Ether Talk, Token Talk, and FDDI Talk for Ethernet, Token Ring, and FDDI, respectively. Thus, Apple Talk went the way of universality at the data link layer and can adapt to any physical implementation of the network.
Like most other Apple products, these networks live inside the "apple" world and practically do not overlap with the PC.
UltraNet - network for supercomputers
Another practically unknown type of networks in Russia is UltraNet. It was actively used to work with computing systems of the supercomputer class and mainframes, but Gigabit Ethernet is being actively supplanted at present.
UltraNet uses a star topology and is capable of providing data exchange rates up to 1 Gbps between devices. This network is distinguished by a very complex physical implementation and very high prices, comparable to supercomputers. UltraNet is controlled by PC computers that are connected to a central hub. Additionally, the network may include bridges and routers for connecting to networks built using Ethernet or Token Ring technologies.
Coaxial cable and optical fiber can be used as a transmission medium (for distances up to 30 km).
Industrial and specialized networks
It should be noted that data networks are used not only for communication between computers or for telephony. There is still a fairly large niche of industrial and specialized devices. For example, CANBUS technology is quite popular, designed to replace thick and expensive wiring harnesses in cars with one common bus. This network does not have a large selection of physical connections, segment length is limited, and low (up to 1 Mbps) transmission speed. However, CANBUS is a successful combination of quality indicators required for small and medium automation and a low price level of implementations. Such systems can also include ModBus, PROFIBUS, FieldBus.
Today, the interests of CAN controller developers are gradually shifting towards home automation.
ATM as a universal data transmission technology
The description of the ATM standard is not in vain placed at the end of the article. This is perhaps one of the last, but unsuccessful attempts to fight Ethernet in its field. These technologies are the exact opposite of each other in terms of the history of creation, the course of implementation and ideology. If Ethernet went up “from the bottom up, from the particular to the general”, increased speed and quality, following the needs of users, then ATM developed in a completely different way.
In the mid-80s, the American National Standards Institute (ANSI) and the International Advisory Committee on Telephony and Telegraphy (CCITT, CCITT) began developing ATM (Asynchronous Transfer Mode) standards as a set of recommendations for the B-ISDN (Broadband Integrated Services Digital Network). Only in 1991 did the efforts of academic science culminate in the creation of the ATM Forum, which still determines the development of technology. The very first major project made using this technology in 1994 was the backbone of the well-known NSFNET network, which previously used the T3 channel.
The essence of the work of ATM is very simple: you need to mix all types of traffic (voice, video, data), condense and transmit over one communication channel. As noted above, this is achieved not through any technical breakthroughs, but rather through numerous compromises. In some ways, this is similar to the way of solving differential equations. Continuous data is broken down into intervals that are small enough to carry out switching operations.
Naturally, such an approach greatly complicated the already difficult task of developers and manufacturers of real equipment and, unacceptably for the market, delayed the implementation time.
The size of the minimum portion of data (cells - in ATM terminology) is influenced by several factors. On the one hand, increasing the size reduces the speed requirements of the cell switch processor and improves the efficiency of channel utilization. On the other hand, the smaller the cell, the sooner transmission is possible.
Indeed, while one cell is being transmitted, the second (even the most priority) is waiting. Strong math, queuing and prioritization mechanisms can mitigate the effect a little, but not eliminate the cause. After quite a long experiment in 1989, the size of the cell was determined to be 53 bytes (5 bytes of service and 48 bytes of data). Obviously, this size can be different for different speeds. If for speeds from 25 to 155 Mbps a size of 53 bytes is suitable, then for gigabit 500 bytes will be no worse, and for 10 gigabit 5000 bytes are also suitable. But in this case, the compatibility problem becomes unsolvable. The arguments are by no means academic in nature - it was the limitation on the switching speed that set the technical limit for increasing the ATM speed to more than 622 Mbps and sharply increased the cost at lower speeds.
ATM's second compromise is connection-oriented technology. Before a transmission session, a virtual sender-receiver channel is established at the link layer, which cannot be used by other stations, while in traditional statistical multiplexing technologies, a connection is not established, and packets with a specified address are placed in the transmission medium. To do this, the port number and connection identifier, which is present in the header of each cell, are entered in the switching table. Subsequently, the switch processes incoming cells based on the connection IDs in their headers. Based on this mechanism, it is possible to regulate the throughput, delay and maximum data loss for each connection - that is, to ensure a certain quality of service.
All of these properties, plus good compatibility with the SDH hierarchy, allowed ATM to become the standard for backbone data networks relatively quickly. But with the full implementation of all the possibilities of technology, there were big problems. As happened more than once, local networks and client applications did not support ATM functions, and without it, a powerful technology with great potential turned out to be just an unnecessary transformation between the worlds of IP (essentially Ethernet) and SDH. This is a very unfortunate situation that the ATM community has tried to remedy. Unfortunately, there were some strategic miscalculations. Despite all the advantages of fiber optics over copper cabling, the high cost of interface cards and switch ports made 155 Mbps ATM extremely expensive to use in this market segment.
In its attempt to define low-speed desktop solutions, the ATM Forum has gotten itself into a devastating debate over which speed and connection type to target. Manufacturers are divided into two camps: supporters of copper cable with a speed of 25.6 Mbps and supporters of optical cable with a speed of 51.82 Mbps. After a series of high-profile conflicts (51.82 Mbps was originally chosen), the ATM Forum proclaimed 25 Mbps as the standard. But precious time was lost forever. On the technology market, we had to meet not with the “classic” Ethernet with its shared transmission medium, but with Fast Ethernet and switched 10Base-T (with the hope of the soon appearance of switched 100Base-T). High price, a small number of manufacturers, the need for more qualified service, problems with drivers, etc. only made the situation worse. Hopes for introduction into the segment of corporate networks collapsed, and the rather weak intermediate position of ATM was fixed for some time. This is its position in the industry today.
ComputerPress 10 "2002
