Ethernet frame formats. Ethernet frame formats Ethernet 1000base t networking

The development of multimedia technologies has led to the need to increase the capacity of communication lines. In this regard, the Gigabit Ethernet technology was developed, providing data transmission at a speed of 1 Gbit / s. In this technology, as well as in Fast Ethernet, the continuity with the Ethernet technology has been preserved: frame formats have practically not changed, survived access method CSMA/ CD in half duplex mode. At the logical level, coding is used 8 B/10 B... Since the transmission speed increased 10 times compared to Fast Ethernet, it was necessary or reduce the network diameter to 20 - 25 m, or increase the minimum frame length... In Gigabit Ethernet technology, they chose the second path, increasing the minimum frame length to 512 bytes instead of 64 bytes in Ethernet and Fast Ethernet technology. The diameter of the net is 200 m, as in Fast Ethernet. Increasing the frame length can be done in two ways. The first method involves filling the data field of a short frame with symbols of forbidden code combinations, and there will be network overhead. According to the second method, it is allowed to transmit several short frames in a row with a total length of up to 8192 byte.

Today's Gigabit Ethernet networks are typically switch-based and operate in full duplex mode. In this case, one speaks not about the diameter of the network, but about the length of the segment, which is determined by the technical means of the physical layer, first of all, by the physical medium of data transmission. Gigabit Ethernet provides for the use of:

single mode fiber optic cable; 802.3 z

multimode fiber optic cable; 802.3 z

balanced UTP cable category 5; 802.3 ab

coaxial cable.

When transmitting data over a fiber optic cable, either LEDs operating at a wavelength are used as emitters 830 nm, or lasers - at a wavelength 1300 nm. According to this standard 802.3 z defined two specifications 1000 Base- SX and 1000 Base- LX... The maximum segment length implemented on a multimode 62.5 / 125 cable of the 1000Base-SX specification is 220 m, and on a 50/125 cable - no more than 500 m. The maximum segment length implemented on a single-mode 1000Base-LX specification is 5000 m. The segment length on a coaxial cable does not exceed 25 m.

To use existing Category 5 balanced UTP cables, a standard has been developed 802.3 ab... Since in Gigabit Ethernet technology data must be transmitted at a speed of 1000 Mbit / s, and twisted pair of category 5 has a bandwidth of 100 MHz, it was decided to transmit data in parallel over 4 twisted pairs and use UTP category 5 or 5e with a bandwidth of 125 MHz. Thus, for each twisted pair, it is necessary to transfer data at a speed of 250 Mbit / s, which is 2 times higher than the capabilities of UTP category 5e. To eliminate this contradiction, the 4D-PAM5 code with five potential levels (-2, -1, 0, +1, +2) is used. Each pair of wires simultaneously transmits and receives data at a speed of 125 Mbit / s in each direction. In this case, collisions occur, in which signals of a complex shape of five levels are formed. The separation of the input and output streams is carried out using hybrid decoupling schemes H(Figure 5.4). As such schemes are used signal processors... To extract the received signal, the receiver subtracts its own transmitted signal from the total (transmitted and received) signal.

Thus, Gigabit Ethernet technology provides high-speed data exchange and is mainly used for data transfer between subnets, as well as for the exchange of multimedia information.

Rice. 5.4. Data transmission over 4 pairs of UTP category 5

The IEEE 802.3 standard recommends that Gigabit Ethernet with fiber transmission should be backbone. Timeslots, frame format, and transmission are common to all 1000 Mbps versions. The physical layer is determined by two signal coding schemes (Figure 5.5). Scheme 8 B/10 B used by for optical fiber and copper shielded cables. For balanced cables UTP pulse amplitude modulation is used (code PAM5 ). Technology 1000 BASE- X uses boolean coding 8 B/10 B and line coding ( NRZ).

Figure 5.5. Gigabit Ethernet Technology Specifications

Signals NRZ transmitted over fiber using either shortwave ( short- wavelength), or long-wave ( long- wavelength) sources of light. LEDs with a wavelength of 850 nm for transmission over multimode optical fiber (1000BASE-SX). This less expensive option is used for short distance transmission. Long-wave laser sources ( 1310 nm) use single-mode or multi-mode optical fiber (1000BASE-LX). Single-mode fiber laser sources are capable of transmitting information over a distance of up to 5000 m.

In point-to-point connections ( point- to- point) for transmission ( Tx) and reception ( Rx), separate fibers are used, therefore full duplex connection. Gigabit Ethernet technology allows you to install only single repeater between the two stations. Below are the parameters of 1000BASE technologies (Table 5.2).

Table 5.2

Comparative characteristics of Gigabit Ethernet specifications

Gigabit Ethernet networks are built around switches where the full duplex distance is limited only by the environment and not the round trip time. In this case, as a rule, the topology " star" or " extended star”And problems are determined by logical topology and data flow.

The 1000BASE-T standard uses almost the same UTP cable as the 100BASE-T and 10BASE-T standards. A 1000BASE-T UTP cable is the same as a 10BASE-T and 100BASE-TX cable, except that a Category 5e cable is recommended. With a cable length of 100 m, 1000BASE-T equipment is operating at the limit of its capabilities.

The Preamble (7 bytes) and Initial Frame Delimiter (SFD) (1 byte) frame fields in Ethernet are used for synchronization between the sending and receiving devices. These first eight bytes of the frame are used to draw the attention of the receiving nodes. Essentially, the first few bytes tell receivers to prepare to receive a new frame.

Destination MAC address field

The Destination MAC Address field (6 bytes) is the identifier for the intended recipient. As you may recall, this address is used by Layer 2 to help devices determine if a given frame is being addressed to them. The address in the frame is compared with the MAC address of the device. If the addresses match, the device receives the frame.

Source MAC address field

The Destination MAC Address field (6 bytes) identifies the sending NIC or frame interface. Switches also use this address to add it to their mapping tables. The role of switches will be discussed later in this section.

Field Length / Type

For any IEEE 802.3 standard earlier than 1997, the Length field specifies the exact length of the frame data field. This is later used later as part of the FCS to ensure that the message was received correctly. If the purpose of the field is to define a type, as in Ethernet II, the Type field describes which protocol is being implemented.

These two uses of the field were formally combined in 1997 in the IEEE 802.3x standard because both applications were common. The Ethernet II Type field is included in the current 802.3 frame definition. When a node receives a frame, it must examine the Length field to determine which higher layer protocol is present in it. If the value of two octets is greater than or equal to the hexadecimal number 0x0600 or decimal number 1536, then the contents of the Data field are decoded according to the indicated protocol type. If the field value is less than or equal to 0x05DC hexadecimal or 1500 decimal, the Length field is used to indicate the use of the IEEE 802.3 frame format. This differentiates Ethernet II and 802.3 frames.

Fields Data and Padding

The Data and Padding fields (46 - 1500 bytes) contain encapsulated data from a higher layer, which is a typical Layer 3 PDU, usually an IPv4 packet. All frames must be at least 64 bytes long. If a smaller packet is encapsulated, Padding is used to increase the frame size to this minimum size.

The IEEE maintains a list of general purpose Ethernet II types.

EtherNet standard IEEE 802.3

It is the most widely used networking technology standard today.

Peculiarities:

works with coaxial cable, twisted pair, optical cables;
topology - bus, star;
access method - CSMA / CD.

The architecture of Ethernet networking technology actually brings together a set of standards that have both common features and differences.

Ethernet technology was developed in conjunction with many of Xerox PARC's early projects. It is generally accepted that Ethernet was invented on May 22, 1973, when Robert Metcalfe wrote a memo for the head of PARC on the potential of Ethernet technology. But Metcalfe acquired the legal right to the technology a few years later. In 1976, he and his assistant David Boggs published a brochure entitled Ethernet: Distributed Packet Switching For Local Computer Networks. Metcalfe left Xerox in 1979 and founded 3Com to promote computers and local area networks. He managed to convince DEC, Intel and Xerox to work together and develop the Ethernet standard (DIX). This standard was first published September 30, 1980.

Further development of EtherNet technology:

1982-1993 development of 10Mbps EtherNet;
1995-1998 Fast EtherNet development;
1998-2002 development of GigaBit EtherNet;
2003-2007 development of 10GigaBit EtherNet;
2007-2010 development of 40 and 100GigaBit EtherNet;
2010 to date Terabit Ethernet development.

At the MAC layer, which provides access to the medium and the transmission of the frame, unique 6-byte addresses, called MAC addresses, regulated by the standard, are used to identify the network interfaces of network nodes. Typically, the MAC address is written as six pairs of hexadecimal digits separated by dashes or colons, such as 00-29-5E-3C-5B-88. Each network adapter has a MAC address.

Ethernet MAC address structure:

the first bit of the destination MAC address is called the I / G (individual / group or broadcast) bit. In the source address, it is called the Source Route Indicator;
the second bit determines how the address is assigned;
The three most significant bytes of the address are called the Burned In Address (BIA) or Organizationally UniqueIdentifier (OUI);
the manufacturer is responsible for the uniqueness of the lower three bytes of the address.

Some network programs, in particular wireshark, can immediately display the name of the manufacturer of the given network card instead of the manufacturer code.

EtherNet technology frame format

There are 4 types of frames (frames) in Ethernet networks:

802.3 / LLC frame (or Novell 802.2 frame),
Raw 802.3 frame (or Novell 802.3 frame)
Ethernet DIX frame (or Ethernet II frame),
Ethernet SNAP frame.

In practice, EtherNet equipment uses only one frame format, namely the EtherNet DIX frame, sometimes referred to as the latest DIX frame number.

The first two header fields are for addresses:

DA (Destination Address) - MAC address of the destination node;
SA (Source Address) - MAC address of the sender node. To deliver a frame, one address is enough - the destination address, the source address is placed in the frame so that the host that received the frame knows who the frame came from and who needs to respond to it.

The T (Type) field contains the conditional code of the upper layer protocol, the data of which is in the data field of the frame, for example, the hexadecimal value 08-00 corresponds to an IP puncture. This field is required to support the interface functions of multiplexing and demultiplexing frames when interworking with upper layer protocols.
Data field. If the length of user data is less than 46 bytes, then this field is padded to the minimum size with padding bytes.
The Frame Check Sequence (FCS) field consists of a 4-byte checksum. This value is calculated using the CRC-32 algorithm.

The EtherNet DIX (II) frame does not reflect the division of the EtherNet link layer into the MAC layer and the LLC layer: its fields support the functions of both layers, for example, the interface functions of the T field belong to the functions of the LLC layer, while all other fields support the functions of the MAC layer.

Consider the EtherNet II frame format using the example of an intercepted packet using a Wireshark network analyzer

Please note that since the MAC address consists of a manufacturer code and an interface number, the network analyzer immediately converts the manufacturer code into the manufacturer's name.

Thus, in EtherNet technology, the MAC addresses act as the destination and destination addresses.

Ethernet technology standards

Physical specifications for Ethernet technology include the following transmission media.

l0Base-5 - coaxial cable with a diameter of 0.5 "(1dm = 2.54cm), called" thick "coaxial cable, with a characteristic impedance of 50Ω.
l0Base-2 - Coaxial cable with a diameter of 0.25 ", called" thin "coaxial cable, with a characteristic impedance of 50Ω.
l0Base-T is an Unshielded Twisted Pair (UTP) cable, category 3,4,5.
l0Base-F - fiber optic cable.

The number 10 denotes the nominal bit rate of the standard data, that is, 10Mbit / s, and the word “Base” is the transmission method at one base frequency. The last character indicates the type of cable.

The cable is used as a mono channel for all stations, the maximum segment length is 500m. The station is connected to the cable through a transceiver - transceiver. The transceiver is connected to the DB-15 connector with an AUI interface cable. Terminators are required at each end to absorb signals propagating through the cable.

Rules "5-4-3" for coaxial networks:

The standard for coaxial cable networks allows the use of no more than 4 repeaters in the network and, accordingly, no more than 5 cable segments. With a maximum cable segment length of 500 m, this gives a maximum network length of 500 * 5 = 2500 m. Only 3 out of 5 segments can be loaded, that is, those to which end nodes are connected. There must be unloaded segments between loaded segments.

l0Base-2

The cable is used as a mono channel for all stations, the maximum segment length is 185 m. To connect the cable to the network card, you need a T-connector, and the cable must have a BNC connector.

The 5-4-3 rule is also used.

l0Base-T

It forms a star topology based on a hub, the hub acts as a repeater and forms a single mono-channel, the maximum segment length is 100m. The end nodes are connected using two twisted pairs. One pair for transferring data from node to hub is Tx, and the other for transferring data from hub to node is Rx.
4-Hub Rules for Twisted Pair Networks:
The twisted pair standard defines the maximum number of hubs between any two stations on the network, namely 4. This rule is called the "4-hub rule". Obviously, if there should not be more than 4 repeaters between any two network nodes, then the maximum diameter of a twisted pair network is 5 * 100 = 500 m (maximum segment length 100 m).

10Base-F

Functionally, an Ethernet over an optical cable consists of the same elements as a 10Base-T network

The FOIRL (Fiber Optic Inter-Repeater Link) standard is the committee's first 802.3 standard for the use of fiber over Ethernet. Max segment length 1000m, max number of hubs 4, with a total network length of not more than 2500 m.

10Base-FL is a minor improvement on the FOIRL standard. Max segment length 2000 m. The maximum number of hubs is 4, and the maximum network length is 2500 m.

The 10Base-FB standard is intended only for connecting repeaters. End nodes cannot use this standard to connect to hub ports. Max number of hubs 5, max length of one segment 2000 m and maximum network length 2740 m.

Table. Ethernet Physical Layer Specification Parameters

When considering the rule "5-4-3" or "4-hubs", if an imaginary signal from a device of the "switch" type appears on the path of propagation through the cables, the calculation of topological constraints starts from zero.

Ethernet bandwidth

Bandwidth is measured in terms of the number of frames or the number of bytes of data transmitted over the network per unit of time. If no collisions occur on the network, the maximum frame rate for the smallest frame size (64 bytes) is 14881 frames per second. At the same time, the useful bandwidth for Ethernet II frames is 5.48 Mbps.

The maximum frame rate for the maximum frame size (1500 bytes) is 813 frames per second. The useful bandwidth will be 9.76 Mbps.

History

Ethernet technology was developed in conjunction with many of the early projects of Xerox PARC Corporation. It is generally accepted that Ethernet was invented on May 22, 1973, when Robert Metcalfe ( Robert Metcalfe) wrote a memo for the head of PARC on the potential of Ethernet technology. But Metcalfe acquired the legal right to the technology a few years later. In 1976, he and his assistant David Boggs published a brochure entitled "Ethernet: Distributed Packet-Switching For Local Computer Networks." R. M. Metcalfe and D. R. Boggs... Ethernet: Distributed Packet Switching for Local Computer Networks. // ACM Communications, 19 (5): 395-404, July 1976.

Metcalfe left Xerox in 1979 and founded 3Com to promote computers and local area networks (LANs). He managed to convince DEC, Intel and Xerox to work together and develop the Ethernet standard (DIX). This standard was first published on September 30, 1980. It began a rivalry with two major patented technologies, Token Ring and Arcnet, which were soon buried under the rolling waves of Ethernet products. In the process, 3Com became the main company in the industry.

Technology

The standard of the first versions (Ethernet v1.0 and Ethernet v2.0) specifies that a coaxial cable is used as the transmission medium, later it became possible to use twisted pair and optical cable.

Popular types of Ethernet are designated as 10Base2, 100BaseTX, etc. Here, the first element denotes the transmission speed, Mbps. Second element:

Base - direct (unmodulated) transmission,
Broad - Uses broadband cable with frequency division multiplexing.

Third element: rounded cable length in hundreds of meters (10Base2 - 185 m, 10Base5 - 500 m) or transmission medium (T, TX, T2, T4 - twisted pairs, FX, FL, FB, SX and LX - fiber, CX - twinax cable for Gigabit Ethernet).

The reasons for the transition to twisted pair were:

the ability to work in duplex mode;
low cost of "twisted pair" cable;
higher reliability of networks in case of cable faults;
high noise immunity when using a differential signal;
Possibility of power supply by cable to low-power nodes, for example, IP phones (Power over Ethernet, POE);
lack of galvanic connection (current flow) between network nodes. When using a coaxial cable in Russian conditions, where, as a rule, there is no grounding of computers, the use of a coaxial cable was often accompanied by a breakdown of network cards, and sometimes even a complete "burnout" of the system unit.

The reason for the transition to optical cable was the need to increase the segment length without repeaters.

Access control method (for network on coaxial cable) - multiple access with carrier sense and collision detection (CSMA / CD, Carrier Sense Multiple Access with Collision Detection), data rate 10 Mbps, packet size from 72 to 1526 bytes, described data encoding methods. The operating mode is half-duplex, that is, the node cannot simultaneously transmit and receive information. The number of nodes in one shared network segment is limited by the limit value of 1024 workstations (physical layer specifications can set stricter limits, for example, no more than 30 workstations can be connected to a thin coaxial segment, and no more than 100 to a thick coaxial segment). However, a network built on a single shared segment becomes ineffective long before the limit of the number of nodes is reached, mainly due to half-duplex operation.

Most Ethernet cards and other devices support multiple baud rates using autonegotiation of speed and duplex to achieve the best possible connection between the two devices. If auto-sensing does not work, the speed adjusts to the partner, and the half-duplex transmission mode is activated. For example, the presence of an Ethernet 10/100 port in a device means that it can operate using 10BASE-T and 100BASE-TX technologies, and the Ethernet 10/100/1000 port supports 10BASE-T, 100BASE-TX and 1000BASE- T.

Early Ethernet modifications

Xerox Ethernet- the original technology, the speed of 3Mbps, existed in two versions Version 1 and Version 2, the frame format of the latest version is still widely used.
10BROAD36- did not receive wide distribution. One of the first standards to allow long distance work. Used broadband modulation technology similar to that used in cable modems. A coaxial cable was used as a data transmission medium.
1BASE5- also known as StarLAN, was the first modification of Ethernet technology to use a twisted pair cable. It worked at a speed of 1 Mbit / s, but did not find commercial use.

10 Mbps Ethernet

10BASE5, IEEE 802.3 (also called "Thick Ethernet") was the original development of a technology with a data transfer rate of 10 Mbps. Following an early IEEE standard, it uses a 50 ohm coaxial cable (RG-8), with a maximum segment length of 500 meters.
10BASE2, IEEE 802.3a (called "Thin Ethernet") - an RG-58 cable is used, with a maximum segment length of 200 meters, computers are connected to one another, a T-connector is needed to connect the cable to a network card, and the cable must have a BNC connector ... Terminators are required at each end. For many years this standard has been the main standard for Ethernet technology.
StarLAN 10- The first development using twisted pair cable for data transmission at a speed of 10 Mbit / s. Later it evolved into the 10BASE-T standard.

Although it is theoretically possible to connect more than two devices operating in simplex mode to one twisted pair cable (segment), such a scheme is never used for Ethernet, as opposed to working with coaxial cable. Therefore, all twisted pair networks use a star topology, while coax networks use a bus topology. Twisted-pair terminators are built into each device, and there is no need to use additional external terminators on the line.

10BASE-T, IEEE 802.3i - for data transmission, 4 wires of a twisted pair cable (two twisted pairs) of category-3 or category-5 are used. The maximum segment length is 100 meters.
FOIRL- (acronym for Fiber-optic inter-repeater link). Basic standard for Ethernet technology using optical cable for data transmission. The maximum data transmission distance without repeater is 1 km.
10BASE-F, IEEE 802.3j - The primary term for a family of 10 Mbps ethernet standards using fiber optic cables up to 2 kilometers away: 10BASE-FL, 10BASE-FB, and 10BASE-FP. Of the above, only 10BASE-FL is widely used.
10BASE-FL(Fiber Link) - Improved version of the FOIRL standard. The improvement concerned an increase in the segment length up to 2 km.
10BASE-FB(Fiber Backbone) - Now an unused standard, it was intended for combining repeaters into a backbone.
10BASE-FP(Fiber Passive) - Passive star topology that does not require repeaters - never used.

Fast Ethernet (Fast Ethernet, 100 Mbps)

100BASE-T- a general term for designating standards using twisted pair as a data transmission medium. Segment length up to 100 meters. Includes 100BASE-TX, 100BASE-T4 and 100BASE-T2 standards.
100BASE-TX IEEE 802.3u is an evolution of the 10BASE-T standard for use in star networks. A twisted pair of category 5 is used, in fact, only two unshielded pairs of conductors are used, full-duplex data transmission is supported, a distance of up to 100 m.
100BASE-T4- a standard using a twisted pair of category 3. All four pairs of conductors are involved, data transmission is in half duplex. Practically not used.
100BASE-T2- a standard using a twisted pair of category 3. Only two pairs of conductors are involved. Full duplex is supported, where signals propagate in opposite directions on each pair. The transmission rate in one direction is 50 Mbps. Practically not used.
100BASE-SX is a standard using multimode fiber. The maximum segment length is 400 meters in half duplex (for guaranteed collision detection) or 2 kilometers in full duplex.
100BASE-FX is a standard using single-mode fiber. The maximum length is limited only by the amount of attenuation in the fiber optic cable and the power of the transmitters.
100BASE-FX WDM is a standard using single-mode fiber. The maximum length is limited only by the amount of attenuation in the fiber optic cable and the power of the transmitters. Interfaces are of two types, differ in the transmitter wavelength and are marked with either numbers (wavelength) or one Latin letter A (1310) or B (1550). Only paired interfaces can work in pairs: on the one hand, the transmitter is at 1310 nm, and on the other, at 1550 nm.

Fast Ethernet

Fast Ethernet (IEEE802.3u, 100BASE-X) is a set of standards for data transmission in computer networks, at speeds up to 100 Mbit / s, as opposed to conventional Ethernet (10 Mbit / s).

Gigabit Ethernet (Gigabit Ethernet, 1 Gbps)

1000BASE-T IEEE 802.3ab is a standard using Category 5e twisted pair cable. All 4 pairs are involved in data transfer. The data rate is 250 Mbps over one pair, using the PAM5 coding method, the fundamental frequency is 62.5 MHz.
1000BASE-TX was created by the Telecommunications Industry Association (eng. Telecommunications Industry Association, TIA) and published in March 2001 as the Physical Layer Specification of Duplex Ethernet 1000 Mb / s (1000BASE-TX) Symmetrical Cat 6 Cabling Systems (ANSI / TIA / EIA-854-2001). "A Full Duplex Ethernet Specification for 1000 Mbis / s (1000BASE-TX) Operating Over Category 6 Balanced Twisted-Pair Cabling (ANSI / TIA / EIA-854-2001)"). The standard uses separate transmission and reception (1 pair for transmission, 1 pair for reception, data for each pair is transmitted at a speed of 500 Mbit / s), which greatly simplifies the design of transceiver devices. But, as a consequence, for stable operation with this technology, a high quality cable system is required, so 1000BASE-TX can only use Category 6 cable. Another significant difference of 1000BASE-TX is the absence of a digital compensation circuit for pickups and return noise, as a result of which the complexity, power consumption level and the price of processors becomes lower than that of processors of the 1000BASE-T standard. There are practically no products based on this standard, although 1000BASE-TX uses a simpler protocol than the 1000BASE-T standard and therefore can use simpler electronics.
1000BASE-X is a general term for standards with pluggable GBIC or SFP transceivers.
1000BASE-SX, IEEE 802.3z is a standard using multimode fiber. Signal transmission range without repeater is up to 550 meters.
1000BASE-LX IEEE 802.3z is a standard using single-mode fiber. Signal transmission range without repeater is up to 80 kilometers.
1000BASE-CX- the standard for short distances (up to 25 meters) using a twinax cable with a characteristic impedance of 150 ohms. Replaced by 1000BASE-T standard and is not used now.
1000BASE-LH(Long Haul) is a standard using single-mode fiber. Signal transmission range without repeater is up to 100 kilometers.

10 Gigabit Ethernet

The new 10 Gigabit Ethernet standard includes seven physical media standards for LAN, MAN and WAN. It is currently covered by the IEEE 802.3ae amendment and should be included in the next revision of the IEEE 802.3 standard.

10GBASE-CX4- 10 Gigabit Ethernet technology for short distances (up to 15 meters) using CX4 copper cable and InfiniBand connectors.
10GBASE-SR- 10 Gigabit Ethernet technology for short distances (up to 26 or 82 meters, depending on the type of cable), using multimode fiber. It also supports distances up to 300 meters using new multimode fiber (2000 MHz / km).
10GBASE-LX4- Uses wavelength division multiplexing to support distances from 240 to 300 meters over multimode fiber. Also supports distances up to 10 kilometers when using single mode fiber.
10GBASE-LR and 10GBASE-ER- these standards support distances of up to 10 and 40 kilometers, respectively.
10GBASE-SW, 10GBASE-LW and 10GBASE-EW- These standards use a physical interface that is speed and data format compatible with the OC-192 / STM-64 SONET / SDH interface. They are similar to the 10GBASE-SR, 10GBASE-LR and 10GBASE-ER standards respectively, as they use the same cable types and transmission distances.
10GBASE-T, IEEE 802.3an-2006 - adopted in June 2006 after 4 years of development. Uses shielded twisted pair cable. Distances - up to 100 meters.

The 10 Gigabit Ethernet standard is still too young, so it will take time to understand which of the above standards for transmission media will actually be in demand on the market. 10 Gigabits / second is not the limit yet. Development of 1000 G Ethernet and higher is already underway.

Multiple computers on a network must share the transmission medium. However, if two computers try to transmit data at the same time, a collision will occur and the data will be lost.

All networked computers must use the same access method, otherwise the network will fail. Individual computers, whose methods will dominate, will prevent others from transferring. Access methods are used to prevent multiple computers from accessing the cable at the same time by sequencing the transmission and reception of data over the network and ensuring that only one computer can transmit at a time.

In Carrier Sense Multiple Access with Collision Detection (abbreviated CSMA / CD), all computers on the network — both clients and servers — are listening on the cable to detect transmissions (that is, traffic).

1) The computer "understands" that the cable is free (that is, there is no traffic).

2) The computer can start transferring data.

3) Until the cable is free (during data transfer), none of the network computers can transmit.

When more than one network device tries to access the transmission medium at the same time, a collision occurs. Computers register the occurrence of a collision, release the transmission line for some randomly specified (within the limits defined by the standard) time interval, after which the transmission attempt is repeated. The first computer to seize the transmission line starts transmitting data.

CSMA / CD is known as adversarial because network computers "compete" (compete) with each other for the right to transfer data.

Collision detection is a reason that limits the scope of the CSMA / CD itself. Due to the finite speed of propagation of the signal in the wires at distances over 2,500 m (1.5 miles), the collision detection mechanism is not effective. If the distance to the transmitting computer exceeds this limit, some computers do not have time to detect the load on the cable and start transmitting data, which leads to collisions and destruction of data packets.

Examples of CDSMA / CD protocols are DEC's Ethernet version 2 and IEEE 802.3.

Ethernet Physical Media Specification

For Ethernet technology, various physical layer options have been developed, differing not only in the type of cable and electrical parameters of the pulses, as is done in 10 Mb / s Ethernet technology, but also in the method of signal coding and the number of conductors used in the cable. Therefore, the physical layer of Ethernet has a more complex structure than classic Ethernet.

Ethernet technology specifications today include the following transmission media.

10Base-2- coaxial cable with a diameter of 0.25 inches, called thin coaxial. Has a wave impedance of 50 ohms. The maximum segment length is 185 meters (without repeaters).
10Base-5- coaxial cable with a diameter of 0.5 "is called" thick "coaxial. Has a wave impedance of 50 Ohm. The maximum segment length without repeater is 500 meters.
10Base-T- cable based on unshielded twisted pair (UTP). Forms a hub-based star topology. The distance between the hub and the end node is no more than 100 meters.
10Base-F- fiber optic cable. The topology is similar to that of the 10Base-T standard. There are several options for this specification - FOIRL (distance up to 1000 m), 10Base-FL (distance up to 2000 m).

Ethernet frame formats

As in production, frames are everything on Ethernet. They serve as a container for all high-level packets, so to understand each other, the sender and receiver must use the same Ethernet frame type. The Ethernet technology standard defined in IEEE802.3 provides a description of a single MAC-layer frame format. Frames can be only four different formats, and also not very different from each other. Moreover, there are only two basic frame formats (in English terminology they are called "raw formats") - Ethernet_II and Ethernet_802.3, and they differ in the purpose of only one field.

Frame Ethernet DIX (Ethernet II). It emerged as a result of the work of a consortium of three firms Digital, Intel and Xerox in 1980, which submitted its proprietary version of the Ethernet standard to the 802.3 committee as a draft international standard.
802.3 / LLC, 802.3 / 802.2 or Novell 802.2... Adopted by committee 802.3 adopted a standard that differs in some details from Ethernet DIX.
Raw 802.3 frame, or Novell 802.3- emerged as a result of Novell's efforts to accelerate its protocol stack over Ethernet

Each frame begins with a 7 byte Preamble filled with the 0b10101010 pattern (to synchronize the source and destination). After the preamble comes the Start of Frame Delimiter (SFD) byte, which contains the sequence 0b10101011 and indicates the beginning of its own frame. Next are the Destination Address (DA) and Source Address (SA) fields. Ethernet uses 48-bit IEEE MAC layer addresses.

The next field has different meanings and different lengths depending on the type of frame.

At the end of the frame is a 32-bit Frame Check Sequence (FCS) field. The checksum is calculated using the CRC-32 algorithm. Ethernet frame size from 64 to 1518 bytes (excluding the preamble, but including the checksum field)

Ethernet DIX frame type

An Ethernet DIX frame, also called an Ethernet II frame, is similar to an 802.3 Raw frame in that it also does not use LLC sublayer headers, but differs in that it defines a protocol type field (Type field) in place of the length field. This field serves the same purpose as the DSAP and SSAP fields of the LLC frame — to indicate the type of upper layer protocol that nested its packet in the data field of this frame. Values exceeding the maximum data field length of 1500 are used to encode the protocol type, so Ethernet II and 802.3 frames are easily distinguishable.

Frame type Raw 802.3.

Following the source address, it contains a 16-bit length (L) field that specifies the number of bytes following the length field (excluding the checksum field). An IPX packet is always embedded in this frame type. The first two bytes of the IPX header contain the checksum of the IPX datagram. However, by default this field is not used and has a value of 0xFFFF.

Frame type 802.3.LLC

The source address field is followed by a 16-bit length field specifying the number of bytes following this field (excluding the checksum field) followed by the LLC header. The 802.3 / LLC frame header is the result of the concatenation of the frame header fields defined in the 802.3 and 802.2 standards.

The 802.3 standard defines eight header fields:

Preamble field consists of seven bytes of sync data. Each byte contains the same sequence of bits - 10101010. In Manchester coding, this combination is represented in the physical medium as a periodic waveform. The preamble is used to allow time and opportunity for the transceiver circuits to come into sync with the received clock signals.

Initial delimiter the frame consists of one byte with the bit set 10101011. The appearance of this combination is an indication of the imminent reception of the frame.

Address of the recipient- can be 2 or 6 bytes long (destination MAC address). The first bit of the recipient's address is an indication of whether the address is individual or group: if 0, then the address indicates a specific station, if 1, then this is the group address of several (possibly all) stations on the network. For broadcast addressing, all bits of the address field are set to 1. It is common practice to use 6-byte addresses.

Sender address- 2 or 6 byte field containing the address of the sender station. The first bit is always 0.

Double byte length field defines the length of the data field in the frame.

Data field can contain from 0 to 1500 bytes. But if the length of the field is less than 46 bytes, then the next field is used - the padding field to complete the frame to the minimum allowable length.

Fill field consists of as many padding bytes as the specified minimum data field length (46 bytes). This ensures that the collision detection mechanism works correctly. If the length of the data field is sufficient, then the padding field does not appear in the frame.

Checksum field- 4 bytes containing a value that is calculated according to a certain algorithm (polynomial CRC-32). After receiving a frame, the workstation performs its own checksum calculation for this frame, compares the received value with the value of the checksum field, and thus determines whether the received frame is corrupted.

The 802.3 frame is a MAC sublayer frame; in accordance with the 802.2 standard, an LLC sublayer frame is embedded in its data field with the start and end flags removed.

The resulting 802.3 / LLC frame is depicted at the bottom. Since the LLC frame has a 3 byte header, the maximum data field size is reduced to 1497 bytes.

Ethernet SNAP frame type

The Ethernet SNAP (SubNetwork Access Protocol) frame is an extension of the 802.3 / LLC frame by introducing an additional SNAP header. The header consists of a 3-byte organization identifier (OUI) field and a 2-byte type (Type, Ethertype) field. The type identifies the upper layer protocol, and the OUI field specifies the identifier of the organization that controls the assignment of the protocol type codes. The protocol codes for the IEEE 802 standards are controlled by the IEEE, which has an OUI code of 0x000000. For this OUI, the Ethernet SNAP type field is the same as the Ethernet DIX type value.

Summary table on the use of different types of frames by higher-level protocols.

Type offrame	Ethernet II	Ethernet Raw 802.3	Ethernet 802.3 / LLC	Ethernet SNAP
Networkprotocols	IPX, IP, AppleTalk Phase I			IPX, IP, AppleTalk Phase II

Fast Ethernet

The difference between Fast Ethernet technology and Ethernet

All the differences between Ethernet and Fast Ethernet technology are concentrated on the physical layer. The goal of Fast Ethernet technology is to obtain significantly, an order of magnitude higher, speed compared to 10 Base T Ethernet - IEEE 802.3, while maintaining the same access method, frame format and recording system. MAC and LLC levels in Fast Ethernet remained absolutely the same the same.

The organization of the physical layer of Fast Ethernet technology is more complex, since it uses three options for cable systems:

Fiber optic multimode cable (two fibers)
Category 5 twisted pair (two pairs)
Category 3 twisted pair (four pairs)

Coaxial cable is not used in Fast Ethernet. Shared Fast Ethernet networks, like 10Base-T / 10Base-F networks, have a hierarchical tree structure based on hubs. The main difference in the configuration of Fast Ethernet networks is the reduction in diameter to 200 meters, which is explained by the reduction in the transmission time of a minimum frame length by 10 times compared to a 10-megabyte Ethernet network.

But when using switches, the Fast Ethernet protocol can work in full duplex mode, in which there is no limit on the total length of the network, but only on individual physical segments.

Physical environment specification Ethernet

100BASE-T- A general term for one of the three standards of 100 Mbit / s Ethernet, using twisted pair as the data transmission medium. The segment length is up to 200-250 meters. Includes 100BASE-TX, 100BASE-T4 and 100BASE-T2.
100BASE-TX, IEEE 802.3u- Development of 10BASE-T technology, a star topology is used, a twisted pair cable of category-5 is used, which actually uses 2 pairs of conductors, the maximum data transfer rate is 100 Mbit / s.
100BASE-T4- 100 Mbps Ethernet over Cat-3 cable. All 4 pairs are involved. Now it is practically not used. Data transmission is in half duplex mode.
100BASE-T2- Not used. 100 Mbps Ethernet over Category 3 cable. Only 2 pairs are used. Full duplex transmission mode is supported, when signals propagate in opposite directions on each pair. The transfer rate in one direction is 50 Mbit / s.
100BASE-FX- 100 Mbps Ethernet using fiber optic cable. The maximum segment length is 400 meters in half duplex mode (for guaranteed collision detection) or 2 kilometers in full duplex mode over multimode fiber and up to 32 kilometers over single mode.

Gigabit Ethernet

1000BASE-T, IEEE 802.3ab- Ethernet standard 1 Gbps. A twisted pair of category 5e or category 6 is used. All 4 pairs are involved in data transmission. The data transfer rate is 250 Mbps over one pair.
1000BASE-TX, - 1 Gbps Ethernet standard using Category 6 twisted pair only. Practically not used.
1000Base-X- a general term for Gigabit Ethernet technology that uses fiber optic cable as a data transmission medium, includes 1000BASE-SX, 1000BASE-LX and 1000BASE-CX.
1000BASE-SX, IEEE 802.3z- 1 Gbps Ethernet technology, uses multimode fiber with a signal transmission range without repeater up to 550 meters.
1000BASE-LX, IEEE 802.3z- 1 Gbps Ethernet technology, uses multimode fiber with a signal transmission range without repeater up to 550 meters. Optimized for long distance using single mode fiber (up to 10 kilometers).
1000BASE-CX- Gigabit Ethernet technology for short distances (up to 25 meters), using a special copper cable (Shielded Twisted Pair (STP)) with a characteristic impedance of 150 ohms. Replaced by 1000BASE-T standard, and now not used.
1000BASE-LH (Long Haul)- 1 Gbps Ethernet technology, uses a single-mode optical cable, the signal transmission range without a repeater is up to 100 kilometers.

Gigabit Ethernet Issues

Ensuring an acceptable network diameter for operation in a shared environment... Due to the limitations of the CSMA / CD method on cable lengths, a Gigabit Ethernet version for shared media would allow a segment length of only 25 meters. It was necessary to solve this problem.
Achieve 1000Mbps bit rate on optical cable... Fiber Channel technology, the physical layer of which was taken as the basis for the fiber optic version of Gigabit Ethernet, provides data transfer rates of only 800 Mbps.
Use as a twisted pair cable.

To solve these problems, it was necessary to make changes not only to the physical layer, but also to the MAC layer.

Means to ensure a network diameter of 200 m on a shared medium

To expand the maximum diameter of a Gigabit Ethernet network in half-duplex mode to 200 m, the developers of the technology took quite natural measures based on the known ratio of the transmission time of the minimum frame length and the double-turnover time.

The minimum frame size has been increased (excluding the preamble) from 64 to 512 bytes, or 4096 bt. Accordingly, the double turnover time could now also be increased to 4095 bt, which makes a network diameter of about 200 m permissible when using a single repeater. At a double signal delay of 10 bt / m, 100 m fiber optic cables contribute during a double 1000 bt turnover, and if the repeater and network adapters introduce the same delays as in Fast Ethernet technology (data for which was given in the previous section) , then a repeater delay of 1000 bt and a pair of 1000 bt network adapters will add up to a double turnover time of 4000 bt, which satisfies the collision detection condition. To increase the frame length to the value required in the new technology, the network adapter must supplement the data field to a length of 448 bytes with the so-called extention, which is a field filled with forbidden 8B / 10B code characters that cannot be mistaken for data codes.

To reduce the overhead when using too long frames to transmit short receipts, the developers of the standard allowed end nodes to transmit several frames in a row, without transmitting the medium to other stations. This mode is called Burst Mode - exclusive burst mode. A station can transmit several frames in a row with a total length of no more than 65,536 bits or 8192 bytes. If a station needs to transmit several small frames, then it may not supplement them to a size of 512 bytes, but transmit in a row until the limit of 8192 bytes is exhausted (this limit includes all bytes of the frame, including the preamble, header, data and checksum) ... The 8192 byte limit is called BurstLength. If the station starts transmitting a frame and the BurstLength limit has been reached in the middle of the frame, then the frame is allowed to be transmitted to the end.

An increase in the "shared" frame to 8192 bytes somewhat delays access to the shared medium of other stations, but at a speed of 1000 Mbit / s this delay is not so significant

Literature

V.G.Olifer, N.A.Olifer Computer networks