Introduction to Ethernet Cable Standards

The following Ethernet cabling standards exist:

• 10BASE-2

• 10BASE-5

• 10BASE-T

• 10BASE-F

• 100BASE-T4

• 100BASE-TX

• 100BASE-FX

• 1000BASE-SX

• 1000BASE-LX

• 1000BASE-TX

In these cabling standards, 10, 100, and 1000 represent the transmission rate (in Mbit/s), and BASE represents baseband.

• 10M Ethernet cable standard

  Table 1-332 lists the 10M Ethernet cabling standard specifications defined in IEEE 802.3.

  Table 1-332 10M Ethernet cable standard

  Name	Cable	Maximum Transmission Distance
  10BASE-5	Thick coaxial cable	500 m
  10BASE-2	Thin coaxial cable	200 m
  10BASE-T	Twisted pair cable	100 m
  10BASE-F	Fiber	2000 m

  The greatest limitation of coaxial cable is that devices on the cable are connected in series, so a single point of failure (SPOF) may cause a breakdown of the entire network. As a result, the physical standards of coaxial cables, 10BASE-2 and 10BASE-5, have fallen into disuse.

• 100M Ethernet cable standard

  100M Ethernet is also called Fast Ethernet (FE). Compared with 10M Ethernet, 100M Ethernet has a faster transmission rate at the physical layer, but has the same rate at the data link layer.

  Table 1-333 lists the 100M Ethernet cable standard specifications.

  Table 1-333 100M Ethernet cable standard

  Name	Cable	Maximum Transmission Distance
  100Base-T4	Four pairs of Category 3 twisted pair cables	100 m
  100Base-Tx	Two pairs of Category 5 twisted pair cables	100 m
  100Base-Fx	Single-mode or multi-mode fiber	2000 m

  10Base-T and 100Base-TX have different transmission rates, but both apply to Category 5 twisted pair cables. 10Base-T transmits data at 10 Mbit/s, while 100Base-TX transmits data at 100 Mbit/s.

  100Base-T4 is now rarely used.

• Gigabit Ethernet cable standard

  Gigabit Ethernet developed from the Ethernet standard defined in IEEE 802.3. Based on the Ethernet protocol, Gigabit Ethernet increases the transmission rate to 10 times the FE transmission rate, reaching 1 Gbit/s. Table 1-334 lists the Gigabit Ethernet cable standard specifications.

  Table 1-334 Gigabit Ethernet cable standard

  Name	Cable	Maximum Transmission Distance
  1000Base-LX	Single-mode or multi-mode fiber	316 m
  1000Base-SX	Multi-mode fiber	316 m
  1000Base-TX	Category 5 twisted pair cable	100 m

  Using Gigabit Ethernet technology, you can upgrade an existing Fast Ethernet network from 100 Mbit/s to 1000 Mbit/s.

  Gigabit Ethernet uses 8B10B coding at the physical layer. In traditional Ethernet transmission technologies, the data link layer delivers 8-bit data sets to the physical layer, where they are processed and sent still as 8 bits to the physical link for transmission.

  In contrast, on the optical fiber-based Gigabit Ethernet, the physical layer maps the 8-bit data sets transmitted from the data link layer to 10-bit data sets before sending them out.

• 10GE cable standards

  10GE cable standards are numerous and continuously evolving, and include IEEE802.3ae, IEEE802.3an, IEEE 802.3aq, and IEEE 802.3ap. 10GE provides a 10 Gbit/s transmission rate, which overcomes bandwidth and transmission distance problems and enables Ethernet technology to be applied to the backbone and aggregation layers of metro networks. 10GE only supports the full-duplex mode. Table 1-335 lists the related cable standards.

  Table 1-335 10GE cable standards

  Name	Cable	Maximum Transmission Distance
  10GBase-SR	Multi-mode fiber	300 m
  10GBase-LR	Single-mode fiber	10 km
  10GBase-LRM	Multi-mode fiber	260 m
  10GBase-ER	Single-mode fiber	40 km
  10GBase-ZR	Single-mode fiber	80 km
  10GBase-LX4	Single-mode or multi-mode fiber	10 km
  10GBase-CX4	Shielded twisted pair	15 m
  10GBase-T	Category 6 twisted pair	55 m
  10GBase-KX4	Copper line	1 m
  10GBase-KR	Copper line	1 m

  The development of 10GE is well under way, and will be widely deployed in future.

CSMA/CD

• Concept of CSMA/CD

  Ethernet was originally designed to connect stations, such as computers and peripherals, on a shared physical line. However, the stations can only access the shared line in half-duplex mode. Therefore, a mechanism of collision detection and avoidance is required to enable multiple devices to share the same line in way that gives each device fair access. Carrier Sense Multiple Access with Collision Detection (CSMA/CD) was therefore introduced.

  The concept of CSMA/CD is as follows:

  • CS: carrier sense

    Before transmitting data, a station checks to see if the line is idle. In this manner, chances of collision are decreased.

  • MA: multiple access

    The data sent by a station can be received by other stations.

  • CD: collision detection

    If two stations transmit electrical signals at the same time, the signals are superimposed, doubling the normal voltage amplitude. This situation results in collision.

    The stations stop transmitting after sensing the conflict, and then resume transmission after a random delay time.

• Working process of CSMA/CD

  CSMA/CD works as follows:

  1. A station continuously checks whether the shared line is idle.

     • If the line is idle, the station sends data.

     • If the line is in use, the station waits until the line is idle.

  2. If two stations send data at the same time, a conflict occurs on the line, and the signal becomes unstable.

  3. After detecting an instability, the station immediately stops sending data.

  4. The station sends a series of pulses.

     The pulses inform other stations that a conflict has occurred on the line.

     After detecting a conflict, the station waits for a random period of time, and then resumes the data transmission.

Minimum Frame Length and Maximum Transmission Distance

• Minimum frame length

  Due to the CSMA/CD algorithm limitation, an Ethernet frame cannot be shorter than a certain length. The minimum frame length is 64 bytes. This length was determined based on Ethernet maximum transmission distance and the collision detection mechanism.

  The use of a minimum frame length prevents situations in which station A finishes sending the last bit of a frame, but the first bit has not arrived at station B. Station B senses that the line is idle and begins to send data, leading to a conflict.

  The upper layer protocol must ensure that each frame's Data field contains at least 46 bytes. As such, a Data field with a 14-byte Ethernet frame header and a 4-byte check code at the end of the frame equals the minimum frame length of 64 bytes. If the Data field is less than 46 bytes, the upper layer protocol must make up the difference.

  The maximum length of the Data field is arbitrary, but it has been set to 1500 bytes as required by the memory cost and buffer of low-cost LAN controllers in 1979.

• Maximum transmission distance

  The maximum transmission distance depends on factors such as line quality and signal attenuation.

Ethernet Duplex Modes

The Ethernet physical layer can work in either half- or full-duplex mode.

• Half-duplex mode

  Half-duplex mode has the following features:

  • Sending and receiving data takes place in one direction at a time.

  • The CSMA/CD mechanism is used.

  • The transmission distance is limited.

  Hubs work in half-duplex mode.

• Full-duplex mode

  After Layer 2 switches replace hubs, the shared Ethernet changes to the switched Ethernet, and the half-duplex mode is replaced by the full-duplex mode. As a result, the transmission rate of data frames increases significantly, with the maximum throughput doubled.

  The full-duplex mode fundamentally solves the problem of collisions on Ethernets and eliminates the need for CSMA/CD.

  Full-duplex mode has the following features:

  • Transmitting and receiving data can take place simultaneously.

  • The maximum throughput is theoretically twice that of half-duplex mode.

  • This mode extends the transmission distance of half-duplex mode.

  Except for hubs, all network cards, Layer 2 switches, and routers produced in the past 10 years support full-duplex mode.

  Full-duplex mode has the following requirements:

  • Full-duplex network cards and modules

  • Physical media over which sending and receiving frames are separated

  • Point-to-point connection

Ethernet Auto-Negotiation

• Purpose of auto-negotiation

  As the earlier Ethernet utilizes a 10 Mbit/s half-duplex mode, mechanisms such as CSMA/CD are required to guarantee system stability. As technology developed, the full-duplex mode and 100M Ethernet have emerged in succession, both of which have significantly improved Ethernet performance. However, they have also introduced an entirely new problem: achieving compatibility between older and newer Ethernet networks.

  The auto-negotiation technology has been introduced to solve this problem. With auto-negotiation, the device at each end of a physical link chooses the same operation parameters by exchanging information. The main parameters to be automatically negotiated include mode (half-duplex or full-duplex), rate, and flow control. Once negotiation completes, the devices operate in the agreed mode and rate.

• Principle of auto-negotiation

  Auto-negotiation is based on a bottom-layer mechanism of twisted-pair Ethernets, and applies only to such Ethernets.

  When data is not transmitted over a twisted pair cable, the cable does not remain idle. Instead, it continues transmitting low frequency pulse signals, and any Ethernet adapter with interfaces for twisted pair cables can identify these pulses. The device at each end can also identify lower frequency pulses — referred to as fast link pulses (FLPs) — after such pulses have been inserted. In this way, the devices achieve auto-negotiation by using FLPs to transmit a small amount of data. Figure 1-585 shows the pulse insertion process.

  Figure 1-585 Pulse insertion
  

  Auto-negotiation priorities of the Ethernet duplex link are listed as follows in descending order:

  • 1000M full-duplex

  • 1000M half-duplex

  • 100M full-duplex

  • 100M half-duplex

  • 10M full-duplex

  • 10M half-duplex

  If auto-negotiation succeeds, the Ethernet card activates the link. Then, data can be transmitted over it. If auto-negotiation fails, the link is inaccessible.

  Auto-negotiation is implemented at the physical layer and does not require any data packets or have impact on upper-layer protocols.

• Auto-negotiation rules for interfaces

  Two connected interfaces can communicate with each other only when they are in the same working mode.

  • If both interfaces work in the same non-auto-negotiation mode, the interfaces can communicate.
  • If both interfaces work in auto-negotiation mode, the interfaces can communicate through negotiation. The negotiated working mode depends on the interface with lower capability. Specifically, if one interface works in full-duplex mode and the other interface works in half-duplex mode, the negotiated working mode is half-duplex. The auto-negotiation function also allows the interfaces to negotiate the use of the traffic control function.

  • If a local interface works in auto-negotiation mode and the remote interface works in a non-auto-negotiation mode, the negotiated working mode of the local interface depends on the working mode of the remote interface.

    Table 1-336 describes the auto-negotiation rules for interfaces of the same type.

    Table 1-336 Auto-negotiation rules for interfaces of the same type (local interface working in auto-negotiation mode)

    Interface Type	Working Mode of the Remote Interface	Auto-negotiation Result	Description
    FE electrical interface	10M half-duplex	10M half-duplex	If the remote interface works in 10M full-duplex or 100M full-duplex mode, the working modes of the two interfaces are different after auto-negotiation, and packets may be dropped. Therefore, if the remote interface works in 10M full-duplex or 100M full-duplex mode, configure the local interface to work in the same mode.
    	10M full-duplex	10M half-duplex
    	100M half-duplex	100M half-duplex
    	100M full-duplex	100M half-duplex
    GE electrical interface	FE auto-negotiation	100M full-duplex	If the remote interface works in 10M full-duplex or 100M full-duplex mode, the working modes of the two interfaces are different after auto-negotiation, and packets may be dropped. Therefore, if the remote interface works in 10M full-duplex or 100M full-duplex mode, configure the local interface to work in the same mode.
    	10M half-duplex	10M half-duplex
    	10M full-duplex	10M half-duplex
    	100M half-duplex	100M half-duplex
    	100M full-duplex	100M half-duplex
    	1000M full-duplex	1000M full-duplex

    Table 1-337 describes the auto-negotiation rules for interfaces of different types.

    Table 1-337 Auto-negotiation rules for interfaces of different types

    Interface Type	Working Mode of an FE Electrical Interface	Working Mode of a GE Electrical Interface	Auto-negotiation Result	Description
    An FE electrical interface connecting to a GE electrical interface	10M half-duplex	Auto-negotiation	10M half-duplex	If the FE electrical interface works in 10M full-duplex or 100M full-duplex mode and the GE electrical interface works in auto-negotiation mode, the working modes of the two interfaces are different after auto-negotiation and packets may be dropped. Therefore, if the FE electrical interface works in 10M full-duplex or 100M full-duplex mode, configure the GE electrical interface to work in the same mode.
    	10M full-duplex		10M half-duplex
    	100M half-duplex		100M half-duplex
    	100M full-duplex		100M half-duplex
    	Auto-negotiation	10M half-duplex	10M half-duplex	If the FE electrical interface works in auto-negotiation mode and the GE electrical interface works in 10M full-duplex or 100M full-duplex mode, the working modes of the two interfaces are different after auto-negotiation, and packets may be dropped. Therefore, if the GE electrical interface works in 10M full-duplex or 100M full-duplex mode, configure the FE electrical interface to work in the same mode. If you configure the GE electrical interface to work in 1000M full-duplex mode, auto-negotiation fails.
    		10M full-duplex	10M half-duplex
    		100M half-duplex	100M half-duplex
    		100M full-duplex	100M half-duplex
    		1000M full-duplex	Failure

    According to the auto-negotiation rules described in Table 1-336 and Table 1-337, if an interface works in auto-negotiation mode and the connected interface works in a non-auto-negotiation mode, packets may be dropped or auto-negotiation may fail. It is recommended that you configure two connected interfaces to work in the same mode to ensure that they can communicate properly.

    FE and higher-rate optical interfaces only support full-duplex mode. Auto-negotiation is enabled on GE optical interfaces only for the negotiation of flow control only. When devices are directly connected using GE optical interfaces, auto-negotiation is enabled on the optical interfaces to detect unidirectional optical fiber faults. If one of two optical fibers is faulty, the fault information is synchronized on both ends through auto-negotiation. As a result, interfaces on both ends go Down. After the fault is rectified, the interfaces go Up again through auto-negotiation.

HUB

• Hub principle

  When terminals are connected through twisted pair cables, a convergence device called a hub is required. Hubs operate at the physical layer. Figure 1-586 shows a hub operation model.

  Figure 1-586 Hub operation mode
  

  A hub is configured as a box with multiple interfaces, each of which can connect to a terminal. Therefore, multiple devices can be connected through a hub to form a star topology.

  Note that although the physical topology is a star, the hub uses bus and CSMA/CD technologies.

  Figure 1-587 Hub operation principle
  
• Two types of hubs are possible, distinguished by their interfaces:

  • Category-I hub: provides a single type of physical interfaces.

    For example, a Category-I hub can accommodate either Category-5 twisted pair interfaces, Category-3 twisted pair interfaces, or optical fiber interfaces.

  • Category-II hub: provides interfaces of different types. For example, a Category-II hub can provide both Category-5 twisted pair interfaces and optical fiber interfaces.

    Aside from the interface provision, these hub types have no differences in their internal operation. In practice, Category-I hubs are commonly used.

Hierarchical Structure of the Data Link Layer

In Ethernet, the following access modes are used according to different duplex modes:

• CSMA/CD is used in half-duplex mode.

• Data is sent in full-duplex mode without having to detect if the line is idle.

Duplex mode, either half or full, refers to the operation mode of the physical layer. Access mode refers to the access of the data link layer. Therefore, in the Ethernet, the data link layer and physical layer are associated.

Therefore, different access modes are required for different operation modes. This brings about some inconvenience to the design and application of the Ethernet.

Some organizations and vendors have proposed dividing the data link layer into two sub-layers: the Logical Link Control (LLC) sub-layer and the Media Access Control (MAC) sub-layer. Then, different physical layers correspond to different MAC sub-layers, and the LLC sub-layer becomes totally independent, as shown in Figure 1-588.

Figure 1-588 Hierarchical structure of the Ethernet data link layer

MAC Sub-layer

• Functions of the MAC sub-layer

  The MAC sub-layer is responsible for the following:

  • Accessing physical links

  • Identifying stations at the data link layer

    The MAC sub-layer reserves a unique MAC address to identify each station.

  • Transmitting data over the data link layer. After receiving data from the LLC sub-layer, the MAC sub-layer adds the MAC address and control information to the data, and then transfers the data to the physical link. During this process, the MAC sub-layer provides other functions, such as the check function.

• Accessing physical links

  The MAC sub-layer is associated with the physical layer so that different MAC sub-layers provide access to different physical layers.

  Ethernet has two types of MAC sub-layers:

  • Half-duplex MAC: provides access to the physical layer in half-duplex mode.

  • Full-duplex MAC: provides access to the physical layer in full-duplex mode.

  The two types of MAC are integrated in a network interface card. After the network interface card is initialized, auto-negotiation is performed to choose an operation mode, and then a MAC is chosen according to the operation mode.

• Identifying stations at the data link layer

  The MAC sub-layer uses a MAC address to uniquely identify a station.

  MAC addresses are managed by the Institute of Electrical and Electronics Engineers (IEEE) and allocated in blocks. An organization, generally a vendor, obtains a unique address block from the IEEE. The address block is called the Organizationally Unique Identifier (OUI), and can be used by the organization to allocate addresses to 16,777,216 devices.

  A MAC address consists of 48 bits, generally represented in dotted hexadecimal notation. For example, the 48-bit MAC address 000000001110000011111100001110011000000000110100 is generally represented as 00e0.fc39.8034.

  The first 24 bits stand for the OUI; the last 24 bits are allocated by the vendor. For example, in 00e0.fc39.8034, 00e0.fc is the OUI allocated by the IEEE to Huawei; 39.8034 is the address number allocated by Huawei.

  The second bit of a MAC address indicates whether the address is globally or locally unique. The Ethernet uses globally unique MAC addresses.

  Ethernet uses the following types of MAC addresses:

  • Physical MAC address

    A physical MAC address is permanently stored in network interface hardware (such as a network interface card) and is used to uniquely identify a terminal on an Ethernet.

  • Broadcast MAC address

    A broadcast MAC address indicates all the terminals on a network.

    The 48 bits of a broadcast MAC address are all 1s. In hexadecimal notation, this address is ffff.ffff.ffff.

  • Multicast MAC address

    A multicast MAC address indicates a group of terminals on a network.

    The eighth bit of a multicast MAC address is 1, such as 000000011011101100111010101110101011111010101000.

• Transmitting data at the data link layer

  Data transmission at the data link layer is as follows:

  1. The upper layer delivers data to the MAC sub-layer.

  2. The MAC sub-layer stores the data in a buffer.

  3. The MAC sub-layer adds the destination and source MAC addresses to the data, calculates the length of the data frame, and forms Ethernet frames.

  4. The Ethernet frame is sent to the peer according to the destination MAC address.

  5. The peer compares the destination MAC address with entries in the MAC address table.

     • If there is a matching entry, the frame is accepted.

     • If there is no matching entry, the frame is discarded.

  The preceding describes frame transmission in unicast mode. After an upper-layer application is added to a multicast group, the data link layer generates a multicast MAC address according to the application, and then adds the multicast MAC address to the MAC address table. The MAC sub-layer then receives frames with the multicast MAC address and transmits the frames to the upper layer.

Ethernet Frame Structure

• Format of an Ethernet_II frame

  Figure 1-589 Format of an Ethernet_II frame
  
  

  An Ethernet_II frame has the following fields:

  • DMAC

    Indicates the destination MAC address, which specifies the receiver of the frame.

  • SMAC

    Indicates the source MAC address, which specifies the sender of the frame.

  • Type

    The 2-byte Type field identifies the upper layer protocol of the Data field. The receiver can interpret the meaning of the Data field according to the Type field.

    Multiple protocols can coexist on a local area network (LAN). The hexadecimal values in the Type field of an Ethernet_II frame specify different protocols.

    • Frames with the Type field value 0800 are IP frames.

    • Frames with the Type field value 0806 are Address Resolution Protocol (ARP) frames.

    • Frames with the Type field value 0835 are Reverse Address Resolution Protocol (RARP) frames.

    • Frames with the Type field value 8137 are Internetwork Packet Exchange (IPx) and Sequenced Packet Exchange (SPx) frames.

  • Data

    The minimum length of the Data field is 46 bytes, which ensures that the frame is at least 64 bytes in length. A 46-byte Data field is required even if a station transmits 1 byte of data.

    If the payload of the Data field is less than 46 bytes, the Data field must be padded to 46 bytes.

    The maximum length of the Data field is 1500 bytes.

  • CRC

    The Cyclic Redundancy Check (CRC) field provides an error detection mechanism.

    Each sending device calculates a CRC code from the DMAC, SMAC, Type, and Data fields. Then the CRC code is filled into the 4-byte CRC field.

• Format of an IEEE 802.3 frame

  Figure 1-590 Format of an IEEE 802.3 frame
  

  As shown in Figure 1-590, the format of an IEEE 802.3 frame is similar to that of an Ethernet_II frame. In an IEEE 802.3 frame, however, the Type field is changed to the Length field, and the LLC field and Sub-Network Access Protocol (SNAP) field occupy 8 bytes of the Data field.

  • Length

    The Length field specifies the number of bytes of the Data field.

  • LLC

    The LLC field consists of three sub-fields: Destination Service Access Point (DSAP), Source Service Access Point (SSAP), and Control.

  • SNAP

    The SNAP field consists of the Org Code field and Type field. Three bytes of the Org Code field are all 0s. The Type field functions the same as that in Ethernet_II frames.

  For descriptions of other fields, see the description of Ethernet_II frames.

  Based on the values of DSAP and SSAP, IEEE 802.3 networks use the following types of frames:

  • If DSAP and SSAP are both 0xff, the IEEE 802.3 frame becomes a NetWare-Ethernet frame bearing NetWare data.

  • If DSAP and SSAP are both 0xaa, the IEEE 802.3 frame becomes an Ethernet_SNAP frame.

    Ethernet_SNAP frames can encapsulate the data of multiple protocols. The SNAP can be considered as an extension of the Ethernet protocol. SNAP allows vendors to invent their own Ethernet transmission protocols.

    The Ethernet_SNAP standard is defined by IEEE 802.1 to help ensure compatibility between the operations between IEEE 802.3 LANs and Ethernet networks.

  • Other values of DSAP and SSAP indicate IEEE 802.3 frames.

• Jumbo frames

  Jumbo frames are Ethernet frames of greater length complying with vendor standards. Such frames are dedicated to Gigabit Ethernet.

  Jumbo frames carry more than 1518 bytes of payload. Generally, Ethernet frames carry a maximum payload of 1518 bytes. Therefore, to implement transmission of large-sized datagrams at the IP layer, datagram fragmentation is required to transmit the data within an Ethernet frame. A frame header and a framer trailer are added to each frame during frame transmission. Therefore, to reduce network costs and improve network usage and transmission rate, Jumbo frames are introduced.

  The two Ethernet interfaces that need to communicate must both support jumbo frames so that NetEngine 8000 Fs can merge several standard-sized Ethernet frames into a jumbo frame to improve transmission efficiency.

  The default value of the Jumbo frame is 10000 bytes.

LLC Sub-layer

As described, the MAC sub-layer supports IEEE 802.3 frames and Ethernet_II frames. In an Ethernet_II frame, the Type field identifies the upper layer protocol. Therefore, on a device, the LLC sub-layer is not needed and only the MAC sub-layer is required.

In an IEEE 802.3 frame, useful features are defined at the LLC sub-layer in addition to the traditional services of the data link layer. These features are specified by the sub-fields of DSAP, SSAP, and Control.

Networks can support the following types of point-to-point services:

• Connection-less service

  Currently, the Ethernet implements this service.

• Connection-oriented service

  The connection is set up before data is transmitted. The reliability of the data transmission is ensured.

• Connection-less data transmission with acknowledgment

  The connection is not required before data transmission. The acknowledgment mechanism is adopted to improve reliability.

The following is an example describing the application of SSAP and DSAP with terminals A and B that use connection-oriented services. Data is transmitted using the following process:

1. A sends a frame to B to request a connection with B.

2. After receiving the frame, if B has enough resources, B returns an acknowledgment message that contains a Service Access Point (SAP). The SAP identifies the connection required by A.

3. After receiving the acknowledgment message, A knows that B has set up a local connection between them. After creating a SAP, A sends a message containing the SAP to B. The connection is set up.

4. The LLC sub-layer of A encapsulates the data into a frame. The DSAP field is filled in with the SAP sent by B; the SSAP field is filled in with that created by A. Then the LLC sub-layer of A transfers the data to its MAC sub-layer.

5. The MAC sub-layer of A adds the MAC address and Length field to the frame, and then transfers the frame to the data link layer.

6. After the frame is received at the MAC sub-layer of B, the frame is transferred to the LLC sub-layer. The LLC sub-layer identifies the connection that the frame belongs to according to the DSAP field.

7. After checking and acknowledging the frame based on the connection type, the LLC sub-layer of B transfers the frame to the upper layer.

8. After the frame reaches its destination, A sends B a frame instructing B to release the connection. At this time, the communications end.