3.ETHERNET
3.1 Ethernet fundamentals
Ethernet is now the dominant LAN technology in the world. Ethernet is a family of LAN technologies that may be best understood with the OSI reference model. All LANs must deal with the basic issue of how individual stations, or nodes, are named. Ethernet specifications support different media, bandwidths, and other Layer 1 and 2 variations. However, the basic frame format and address scheme is the same for all varieties of Ethernet.
The success of Ethernet is due to the following factors:
- Simplicity and ease of maintenance
- Ability to incorporate new technologies
- Reliability
- Low cost of installation and upgrade
The introduction of Gigabit Ethernet has extended the original LAN technology to distances that make Ethernet a MAN and WAN standard.
The original idea for Ethernet was to allow two or more hosts to use the same medium with no interference between the signals. This problem of multiple user access to a shared medium was studied in the early 1970s at the University of Hawaii..
3.2 IEEE Ethernet:Naming Rules
Ethernet is not one networking technology, but a family of networking technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet. Ethernet speeds can be 10, 100, 1000, or 10,000 Mbps. The basic frame format and the IEEE sublayers of OSI Layers 1 and 2 remain consistent across all forms of Ethernet.
When Ethernet needs to be expanded to add a new medium or capability, the IEEE issues a new supplement to the 802.3 standard. The new supplements are given a one or two letter designation such as 802.3u. An abbreviated description, called an identifier, is also assigned to the supplement.
The abbreviated description consists of the following elements:
A number that indicates the number of Mbps transmitted
The word base to indicate that baseband signaling is used
One or more letters of the alphabet indicating the type of medium used. For example, F = fiber optical cable and T = copper unshielded twisted pair
Ethernet relies on baseband signaling, which uses the entire bandwidth of the transmission medium. The data signal is transmitted directly over the transmission medium.
In broadband signaling, the data signal is no longer placed directly on the transmission medium. Ethernet used broadband signaling in the 10BROAD36 standard. 10BROAD36 is the IEEE standard for an 802.3 Ethernet network using broadband transmission with thick coaxial cable running at 10 Mbps. 10BROAD36 is now considered obsolete. An analog or carrier signal is modulated by the data signal and then transmitted. Radio broadcasts and cable TV use broadband signaling.
3.3 Ethernet and OSI model
Ethernet operates in two areas of the OSI model. These are the lower half of the data link layer, which is known as the MAC sublayer, and the physical layer.
Data that moves from one Ethernet station to another often passes through a repeater. All stations in the same collision domain see traffic that passes through a repeater.
A collision domain is a shared resource. Problems that originate in one part of a collision domain will usually impact the entire collision domain.
A repeater forwards traffic to all other ports. A repeater never sends traffic out the same port from which it was received. Any signal detected by a repeater will be forwarded. If the signal is degraded through attenuation or noise, the repeater will attempt to reconstruct and regenerate the signal.
To guarantee minimum bandwidth and operability, standards specify the maximum number of stations per segment, maximum segment length, and maximum number of repeaters between stations. Stations separated by bridges or routers are in different collision domains.
Data link sublayers contribute significantly to technological compatibility and computer communications. The MAC sublayer is concerned with the physical components that will be used to communicate the information. The Logical Link Control (LLC) sublayer remains relatively independent of the physical equipment that will be used for the communication process.
3.4 Naming
An address system is required to uniquely identify computers and interfaces to allow for local delivery of frames on the Ethernet.
Ethernet uses MAC addresses that are 48 bits in length and expressed as 12 hexadecimal digits. The first six hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor. This portion of the MAC address is known as the Organizational Unique Identifier (OUI). The remaining six hexadecimal digits represent the interface serial number or another value administered by the manufacturer.
MAC addresses are sometimes referred to as burned-in MAC addresses (BIAs) because they are burned into ROM and are copied into RAM when the NIC initializes.
At the data link layer MAC headers and trailers are added to upper layer data. The header and trailer contain control information intended for the data link layer in the destination system. The data from upper layers is encapsulated within the data link frame, between the header and trailer, and then sent out on the network.
3.5 Layer 2 framing
Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit.
There are many different types of frames described by various standards. A single generic frame has sections called fields. Each field is composed of bytes.
The names of the fields are as follows:
Start Frame field
Address field
Length/Type field
Data field
Frame Check Sequence (FCS) field
(Generic frame format)
When computers are connected to a physical medium, there must be a way to inform other computers when they are about to transmit a frame. Various technologies do this in different ways. Regardless of the technology, all frames begin with a sequence of bytes to signal the data transmission.
All frames contain naming information, such as the name of the source node, or source MAC address, and the name of the destination node, or destination MAC address.
Most frames have some specialized fields. In some technologies, a Length field specifies the exact length of a frame in bytes. Some frames have a Type field, which specifies the Layer 3 protocol used by the device that wants to send data.
Frames are used to send upper-layer data and ultimately the user application data from a source to a destination. The data package includes the message to be sent, or user application data. Extra bytes may be added so frames have a minimum length for timing purposes. LLC bytes are also included with the Data field in the IEEE standard frames. The LLC sublayer takes the network protocol data, which is an IP packet, and adds control information to help deliver the packet to the destination node. Layer 2 communicates with the upper layers through LLC.
All frames and the bits, bytes, and fields contained within them, are susceptible to errors from a variety of sources. The FCS field contains a number that is calculated by the source node based on the data in the frame. This number is added to the end of a frame that is sent. When the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed, the frame is discarded.
Because the source cannot detect that the frame has been discarded, retransmission has to be initiated by higher layer connection-oriented protocols providing data flow control. Because these protocols, such as TCP, expect frame acknowledgment, ACK, to be sent by the peer station within a certain time, retransmission usually occurs.
The node that transmits data must get the attention of other devices to start and end a frame. The Length field indicates where the frame ends. The frame ends after the FCS. Sometimes there is a formal byte sequence referred to as an end-frame delimiter.
3.6 Ethernet Frame Structure
At the data link layer the frame structure is nearly identical for all speeds of Ethernet from 10 Mbps to 10,000 Mbps.
However, at the physical layer almost all versions of Ethernet are very different. Each speed has a distinct set of architecture design rules.
In the version of Ethernet that was developed by DIX prior to the adoption of the IEEE 802.3 version of Ethernet, the Preamble and Start-of-Frame (SOF) Delimiter were combined into a single field. The binary pattern was identical. The field labeled Length/Type was only listed as Length in the early IEEE versions and only as Type in the DIX version. These two uses of the field were officially combined in a later IEEE version since both uses were common.
The Ethernet II Type field is incorporated into the current 802.3 frame definition. When a node receives a frame it must examine the Length/Type field to determine which higher-layer protocol is present. If the two-octet value is equal to or greater than 0x0600 hexadecimal, 1536 decimal, then the contents of the Data Field are decoded according to the protocol indicated.
Ethernet II is the Ethernet frame format that is used in TCP/IP networks.
3.7 Ethernet Frame Fields
Some of the fields permitted or required in an 802.3 Ethernet frame are as follows:
3.8 MAC
MAC refers to protocols that determine which computer in a shared-media environment, or collision domain, is allowed to transmit data. MAC and LLC comprise the IEEE version of the OSI Layer 2. MAC and LLC are sublayers of Layer 2. The two broad categories of MAC are deterministic and non-deterministic.
Examples of deterministic protocols include Token Ring and FDDI. In a Token Ring network, hosts are arranged in a ring and a special data token travels around the ring to each host in sequence. When a host wants to transmit, it seizes the token, transmits the data for a limited time, and then forwards the token to the next host in the ring. Token Ring is a collisionless environment since only one host can transmit at a time.
Non-deterministic MAC protocols use a first-come, first-served approach. Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is a simple system. The NIC
listens for the absence of a signal on the media and begins to transmit. If two nodes transmit at the same time a collision occurs and none of the nodes are able to transmit.
Three common Layer 2 technologies are Token Ring, FDDI, and Ethernet. All three specify Layer 2 issues, LLC, naming, framing, and MAC, as well as Layer 1 signaling components and media issues. The specific technologies for each are as follows:
Ethernet – uses a logical bus topology to control information flow on a linear bus and a physical star or extended star topology for the cables
Token Ring – uses a logical ring topology to control information flow and a physical star topology
FDDI – uses a logical ring topology to control information flow and a physical dual-ring topology
MAC rules and collision detection/backoff -Ethernet is a shared-media broadcast technology.The access method CSMA/CD used in Ethernet performs three functions:
- Transmitting and receiving data frames
- Decoding data frames and checking them for valid addresses before passing them to the upper layers of the OSI model
- Detecting errors within data frames or on the network
(CSMA/CD)
In the CSMA/CD access method, networking devices with data to transmit work in a listen-before-transmit mode. This means when a node wants to send data, it must first check to see whether the networking media is busy. If the node determines the network is busy, the node will wait a random amount of time before retrying. If the node determines the networking media is not busy, the node will begin transmitting and listening. The node listens to ensure no other stations are transmitting at the same time. After completing data transmission the device will return to listening mode.
Networking devices detect a collision has occurred when the amplitude of the signal on the networking media increases. When a collision occurs, each node that is transmitting will continue to transmit for a short time to ensure that all nodes detect the collision. When all nodes have detected the collision, the backoff algorithm is invoked and transmission stops. The nodes stop transmitting for a random period of time, determined by the backoff algorithm. When the delay periods expire, each node can attempt to access the networking media. The devices that were involved in the collision do not have transmission priority.
3.9 Ethernet timing
The basic rules and specifications for proper operation of Ethernet are not particularly complicated, though some of the faster physical layer implementations are becoming so. Despite the basic simplicity, when a problem occurs in Ethernet it is often quite difficult to isolate the source. Because of the common bus architecture of Ethernet, also described as a distributed single point of failure, the scope of the problem usually encompasses all devices within the collision domain. In situations where repeaters are used, this can include devices up to four segments away.
Any station on an Ethernet network wishing to transmit a message first “listens” to ensure that no other station is currently transmitting. If the cable is quiet, the station will begin transmitting immediately. The electrical signal takes time to travel down the cable (delay), and each subsequent repeater introduces a small amount of latency in forwarding the frame from one port to the next. Because of the delay and latency, it is possible for more than one station to begin transmitting at or near the same time. This results in a collision.
If the attached station is operating in full duplex then the station may send and receive simultaneously and collisions should not occur. Full-duplex operation also changes the timing considerations and eliminates the concept of slot time. Full-duplex operation allows for larger network architecture designs since the timing restriction for collision detection is removed.
In half duplex, assuming that a collision does not occur, the sending station will transmit 64 bits of timing synchronization information that is known as the preamble. The sending station will then transmit the following information:
Destination and source MAC addressing information
Certain other header information
The actual data payload
3.10 Error handling
The most common error condition on Ethernet networks are collisions.
Collisions are the mechanism for resolving contention for network access. A few collisions provide a smooth, simple, low overhead way for network nodes to arbitrate contention for the network resource. When network contention becomes too great, collisions can become a significant impediment to useful network operation.
Collisions result in network bandwidth loss that is equal to the initial transmission and the collision jam signal. This is consumption delay and affects all network nodes possibly causing significant reduction in network throughput.
The considerable majority of collisions occur very early in the frame, often before the SFD. Collisions occurring before the SFD are usually not reported to the higher layers, as if the collision did not occur. As soon as a collision is detected, the sending stations transmit a 32-bit “jam” signal that will enforce the collision. This is done so that any data being transmitted is thoroughly corrupted and all stations have a chance to detect the collision.
3.11 Types of collisions
Collisions typically take place when two or more Ethernet stations transmit simultaneously within a collision domain. A single collision is a collision that was detected while trying to transmit a frame, but on the next attempt the frame was transmitted successfully. Multiple collisions indicate that the same frame collided repeatedly before being successfully transmitted. The results of collisions, collision fragments, are partial or corrupted frames that are less than 64 octets and have an invalid FCS. Three types of collisions are:
To create a local collision on coax cable (10BASE2 and 10BASE5), the signal travels down the cable until it encounters a signal from the other station. The waveforms then overlap, canceling some parts of the signal out and reinforcing or doubling other parts. The doubling of the signal pushes the voltage level of the signal beyond the allowed maximum. This over-voltage condition is then sensed by all of the stations on the local cable segment as a collision.
The characteristics of a remote collision are a frame that is less than the minimum length, has an invalid FCS checksum, but does not exhibit the local collision symptom of over-voltage or simultaneous RX/TX activity. This sort of collision usually results from collisions occurring on the far side of a repeated connection. A repeater will not forward an over-voltage state, and cannot cause a station to have both the TX and RX pairs active at the same time. The station would have to be transmitting to have both pairs active, and that would constitute a local collision. On UTP networks this is the most common sort of collision observed.
There is no possibility remaining for a normal or legal collision after the first 64 octets of data has been transmitted by the sending stations. Collisions occurring after the first 64 octets are called “late collisions". The most significant difference between late collisions and collisions occurring before the first 64 octets is that the Ethernet NIC will retransmit a normally collided frame automatically, but will not automatically retransmit a frame that was collided late.
3.12 Ethernet errors
Knowledge of typical errors is invaluable for understanding both the operation and troubleshooting of Ethernet networks.
The following are the sources of Ethernet error:
Collision or runt – Simultaneous transmission occurring before slot time has elapsed
Late collision – Simultaneous transmission occurring after slot time has elapsed
Jabber, long frame and range errors – Excessively or illegally long transmission
Short frame, collision fragment or runt – Illegally short transmission
FCS error – Corrupted transmission
Alignment error – Insufficient or excessive number of bits transmitted
Range error – Actual and reported number of octets in frame do not match
Ghost or jabber – Unusually long Preamble or Jam event
3.13 Ethernet auto-negotiation
As Ethernet grew from 10 to 100 and 1000 Mbps, one requirement was to make each technology interoperable, even to the point that 10, 100, and 1000 interfaces could be directly connected. A process called Auto-Negotiation of speeds at half or full duplex was developed. Specifically, at the time that Fast Ethernet was introduced, the standard included a method of automatically configuring a given interface to match the speed and capabilities of the link partner. This process defines how two link partners may automatically negotiate a configuration offering the best common performance level. It has the additional advantage of only involving the lowest part of the physical layer.
10BASE-T required each station to transmit a link pulse about every 16 milliseconds, whenever the station was not engaged in transmitting a message. Auto-Negotiation adopted this signal and renamed it a Normal Link Pulse (NLP). When a series of NLPs are sent in a group for the purpose of Auto-Negotiation, the group is called a Fast Link Pulse (FLP) burst. Each FLP burst is sent at the same timing interval as an NLP, and is intended to allow older 10BASE-T devices to operate normally in the event they should receive an FLP burst.
Auto-Negotiation is accomplished by transmitting a burst of 10BASE-T Link Pulses from each of the two link partners. The burst communicates the capabilities of the transmitting station to its link partner. After both stations have interpreted what the other partner is offering, both switch to the highest performance common configuration and establish a link at that speed. If anything interrupts communications and the link is lost, the two link partners first attempt to link again at the last negotiated speed. If that fails, or if it has been too long since the link was lost, the Auto-Negotiation process starts over. The link may be lost due to external influences, such as a cable fault, or due to one of the partners issuing a reset.
3.14 Link establishment & full and half duplex
Link partners are allowed to skip offering configurations of which they are capable. This allows the network administrator to force ports to a selected speed and duplex setting, without disabling Auto-Negotiation.
Auto-Negotiation is optional for most Ethernet implementations. Gigabit Ethernet requires its implementation, though the user may disable it. Auto-Negotiation was originally defined for UTP implementations of Ethernet and has been extended to work with other fiber optic implementations.When an Auto-Negotiating station first attempts to link it is supposed to enable 100BASE-TX to attempt to immediately establish a link. If 100BASE-TX signaling is present, and the station supports 100BASE-TX, it will attempt to establish a link without negotiating. If either signaling produces a link or FLP bursts are received, the station will proceed with that technology. If a link partner does not offer an FLP burst, but instead offers NLPs, then that device is automatically assumed to be a 10BASE-T station. During this initial interval of testing for other technologies, the transmit path is sending FLP bursts. The standard does not permit parallel detection of any other technologies.If a link is established through parallel detection, it is required to be half duplex. There are only two methods of achieving a full-duplex link. One method is through a completed cycle of Auto-Negotiation, and the other is to administratively force both link partners to full duplex. If one link partner is forced to full duplex, but the other partner attempts to Auto-Negotiate, then there is certain to be a duplex mismatch. This will result in collisions and errors on that link. Additionally if one end is forced to full duplex the other must also be forced. The exception to this is 10-Gigabit Ethernet, which does not support half duplex.
There are two duplex modes, half and full. For shared media, the half-duplex mode is mandatory.All coaxial implementations are half duplex in nature and cannot operate in full duplex. UTP and fiber implementations may be operated in half duplex. 10-Gbps implementations are specified for full duplex only.In half duplex only one station may transmit at a time. For the coaxial implementations a second station transmitting will cause the signals to overlap and become corrupted. Since UTP and fiber generally transmit on separate pairs the signals have no opportunity to overlap and become corrupted. Ethernet has established arbitration rules for resolving conflicts arising from instances when more than one station attempts to transmit at the same time. Both stations in a point-to-point full-duplex link are permitted to transmit at any time, regardless of whether the other station is transmitting.
3.15 10-Mbps Ethernet
10BASE5, 10BASE2, and 10BASE-T Ethernet are considered Legacy Ethernet.
The four common features of Legacy Ethernet are timing parameters, the frame format, transmission processes, and a basic design rule.10-Mbps Ethernet and slower versions are asynchronous. Each receiving station uses eight octets of timing information to synchronize its receive circuit to the incoming data. 10BASE5, 10BASE2, and 10BASE-T all share the same timing parameters.
10BASE5, 10BASE2, and 10BASE-T also have a common frame format.
The Legacy Ethernet transmission process is identical until the lower part of the OSI physical layer. As the frame passes from the MAC sublayer to the physical layer, other processes occur before the bits move from the physical layer onto the medium. One important process is the signal quality error (SQE) signal. The SQE is a transmission sent by a transceiver back to the controller to let the controller know whether the collision circuitry is functional. The SQE is also called a heartbeat. The SQE signal is designed to fix the problem in earlier versions of Ethernet where a host does not know if a transceiver is connected. SQE is always used in half-duplex. SQE can be used in full-duplex operation but is not required
Legacy Ethernet has common architectural features. Networks usually contain multiple types of media. The standard ensures that interoperability is maintained. The overall architectural design is most important in mixed-media networks. It becomes easier to violate maximum delay limits as the network grows. The timing limits are based on the following types of parameters:
- Cable length and propagation delay
- Delay of repeaters
- Delay of transceivers
- Interframe gap shrinkage
- Delays within the station
10-Mbps Ethernet operates within the timing limits for a series of up to five segments separated by up to four repeaters. This is known as the 5-4-3 rule. No more than four repeaters can be used in series between any two stations. There can also be no more than three populated segments between any two stations.
10BASE5
10BASE5 transmitted 10 Mbps over a single thin coaxial cable bus.The primary benefit of 10BASE5 was length. 10BASE5 may be found in legacy installations. It is not recommended for new installations. 10BASE5 systems are inexpensive and require no configuration. Two disadvantages are that basic components like NICs are very difficult to find and it is sensitive to signal reflections on the cable. 10BASE5 systems also represent a single point of failure. 10BASE5 uses Manchester encoding.
10BASE2
Installation was easier because of its smaller size, lighter weight, and greater flexibility. 10BASE2 still exists in legacy networks. Like 10BASE5, it is no longer recommended for network installations. It has a low cost and does not require hubs.
10BASE2 also uses Manchester encoding. Only one station can transmit at a time or a collision will occur. 10BASE2 also uses half-duplex. The maximum transmission rate of 10BASE2 is 10 Mbps.There may be up to 30 stations on a 10BASE2 segment. Only three out of five consecutive segments between any two stations can be populated.
10BASE-T
10BASE-T used cheaper and easier to install Category 3 UTP copper cable instead of coax cable. The cable plugged into a central connection device that contained the shared bus. This device was a hub. It was at the center of a set of cables that radiated out to the PCs like the spokes on a wheel. This is referred to as a star topology. As additional stars were added and the cable distances grew, this formed an extended star topology. Originally 10BASE-T was a half-duplex protocol, but full-duplex features were added later..
10BASE-T also uses Manchester encoding. A 10BASE-T UTP cable has a solid conductor for each wire. The maximum cable length is 90 m (295 ft). UTP cable uses eight-pin RJ-45 connectors. Though Category 3 cable is adequate for 10BASE-T networks, new cable installations should be made with Category 5e or better. All four pairs of wires should be used either with the T568-A or T568-B cable pinout arrangement. This type of cable installation supports the use of multiple protocols without the need to rewire.
10BASE-T wiring and architecture
A 10BASE-T link generally connects a station to a hub or switch. Hubs are multi-port repeaters and count toward the limit on repeaters between distant stations. Hubs do not divide network segments into separate collision domains. Bridges and switches divide segments into separate collision domains. The maximum distance between bridges and switches is based on media limitations.
Although hubs may be linked, it is best to avoid this arrangement. A network with linked hubs may exceed the limit for maximum delay between stations. Multiple hubs should be arranged in hierarchical order like a tree structure. Performance is better if fewer repeaters are used between stations.
The distance from one end of the network to the other places the architecture at its limit. The most important aspect to consider is how to keep the delay between distant stations to a minimum, regardless of the architecture and media types involved. A shorter maximum delay will provide better overall performance.
10BASE-T links can have unrepeated distances of up to 100 m (328 ft). While this may seem like a long distance, it is typically maximized when wiring an actual building. Hubs can solve the distance issue but will allow collisions to propagate. The widespread introduction of switches has made the distance limitation less important. If workstations are located within 100 m (328 ft) of a switch, the 100-m distance starts over at the switch.
3.16 100-Mbps Ethernet
100-Mbps Ethernet, which is also known as Fast Ethernet. The two technologies that have become important are 100BASE-TX, which is a copper UTP medium and 100BASE-FX, which is a multimode optical fiber medium.
Three characteristics common to 100BASE-TX and 100BASE-FX are the timing parameters, the frame format, and parts of the transmission process. 100BASE-TX and 100BASE-FX both share timing parameters.
Note that one bit time at 100-Mbps = 10 ns =.01 microseconds = 1 100-millionth of a second.
The 100-Mbps frame format is the same as the 10-Mbps frame.
Fast Ethernet is ten times faster than 10BASE-T. The bits that are sent are shorter in duration and occur more frequently. These higher frequency signals are more susceptible to noise. In response to these issues, two separate encoding steps are used by 100-Mbps Ethernet. The first part of the encoding uses a technique called 4B/5B, the second part of the encoding is the actual line encoding specific to copper or fiber.
100BASE-TX
This page will describe 100BASE-TX.
In 1995, 100BASE-TX was the standard, using Category 5 UTP cable, which became commercially successful.
The original coaxial Ethernet used half-duplex transmission so only one device could transmit at a time. In 1997, Ethernet was expanded to include a full-duplex capability that allowed more than one PC on a network to transmit at the same time. Switches replaced hubs in many networks. These switches had full-duplex capabilities and could handle Ethernet frames quickly.
100BASE-TX uses 4B/5B encoding, which is then scrambled and converted to Multi-Level Transmit (MLT-3) encoding.
Notice that the two separate transmit-receive paths exist. This is identical to the 10BASE-T configuration.
100BASE-TX carries 100 Mbps of traffic in half-duplex mode. In full-duplex mode, 100BASE-TX can exchange 200 Mbps of traffic. The concept of full duplex will become more important as Ethernet speeds increase
100BASE-FX
When copper-based Fast Ethernet was introduced, a fiber version was also desired. A fiber version could be used for backbone applications, connections between floors, buildings where copper is less desirable, and also in high-noise environments. 100BASE-FX was introduced to satisfy this desire. However, 100BASE-FX was never adopted successfully. This was due to the introduction of Gigabit Ethernet copper and fiber standards. Gigabit Ethernet standards are now the dominant technology for backbone installations, high-speed cross-connects, and general infrastructure needs.
The timing, frame format, and transmission are the same in both copper and fiber versions of 100-Mbps Fast Ethernet. A fiber pair with either ST or SC connectors is most commonly used.
The separate Transmit (Tx) and Receive (Rx) paths in 100BASE-FX optical fiber allow for 200-Mbps transmission.
3.17 Fast Ethernet architecture
This page describes the architecture of Fast Ethernet.
Fast Ethernet links generally consist of a connection between a station and a hub or switch. Hubs are considered multi-port repeaters and switches are considered multi-port bridges.
A Class I repeater may introduce up to 140 bit-times latency. Any repeater that changes between one Ethernet implementation and another is a Class I repeater. A Class II repeater is restricted to smaller timing delays, 92 bit times, because it immediately repeats the incoming signal to all other ports without a translation process. To achieve a smaller timing delay, Class II repeaters can only connect to segment types that use the same signaling technique.
3.18 1000-Mbps Ethernet Or Gigabit Ethernet Standards
These standards specify both fiber and copper media for data transmissions. The 1000BASE-T standard, IEEE 802.3ab, uses Category 5, or higher, balanced copper cabling. The 1000BASE-X standard, IEEE 802.3z, specifies 1 Gbps full duplex over optical fiber.
They use a 1 ns, 0.000000001 of a second, or 1 billionth of a second bit .The differences between standard Ethernet, Fast Ethernet and Gigabit Ethernet occur at the physical layer. Due to the increased speeds of these newer standards, the shorter duration bit times require special considerations. Since the bits are introduced on the medium for a shorter duration and more often, timing is critical. This high-speed transmission requires higher frequencies. This causes the bits to be more susceptible to noise on copper media.
These issues require Gigabit Ethernet to use two separate encoding steps. Data transmission is more efficient when codes are used to represent the binary bit stream. The encoded data provides synchronization, efficient usage of bandwidth, and improved signal-to-noise ratio characteristics.
At the physical layer, the bit patterns from the MAC layer are converted into symbols. The symbols may also be control information such as start frame, end frame, and idle conditions on a link. The frame is coded into control symbols and data symbols to increase in network throughput.
Fiber-based Gigabit Ethernet, or 1000BASE-X, uses 8B/10B encoding, which is similar to the 4B/5B concept. This is followed by the simple nonreturn to zero (NRZ) line encoding of light on optical fiber. This encoding process is possible because the fiber medium can carry higher bandwidth signals.
1000BASE-T
As Fast Ethernet was installed to increase bandwidth to workstations, this began to create bottlenecks upstream in the network. The 1000BASE-T standard, which is IEEE 802.3ab, was developed to provide additional bandwidth to help alleviate these bottlenecks. It provided more throughput for devices such as intra-building backbones, inter-switch links, server farms, and other wiring closet applications as well as connections for high-end workstations. Fast Ethernet was designed to function over Category 5 copper cable that passes the Category 5e test. Most installed Category 5 cable can pass the Category 5e certification if properly terminated. It is important for the 1000BASE-T standard to be interoperable with 10BASE-T and 100BASE-TX.
Since Category 5e cable can reliably carry up to 125 Mbps of traffic, 1000 Mbps or 1 Gigabit of bandwidth was a design challenge. The first step to accomplish 1000BASE-T is to use all four pairs of wires instead of the traditional two pairs of wires used by 10BASE-T and 100BASE-TX. This requires complex circuitry that allows full-duplex transmissions on the same wire pair. This provides 250 Mbps per pair. With all four-wire pairs, this provides the desired 1000 Mbps. Since the information travels simultaneously across the four paths, the circuitry has to divide frames at the transmitter and reassemble them at the receiver.
The 1000BASE-T encoding with 4D-PAM5 line encoding is used on Category 5e, or better, UTP
1000BASE-SX and LX
This page will discuss single-mode and multimode optical fiber.
The IEEE 802.3 standard recommends that Gigabit Ethernet over fiber be the preferred backbone technology.
The timing, frame format, and transmission are common to all versions of 1000 Mbps. Two signal-encoding schemes are defined at the physical layer.
The 8B/10B scheme is used for optical fiber and shielded copper media, and the pulse amplitude modulation 5 (PAM5) is used for UTP.
1000BASE-X uses 8B/10B encoding converted to non-return to zero (NRZ) line encodingThe short-wavelength uses an 850 nm laser or LED source in multimode optical fiber (1000BASE-SX). It is the lower-cost of the options but has shorter distances. The long-wavelength 1310 nm laser source uses either single-mode or multimode optical fiber (1000BASE-LX). Laser sources used with single-mode fiber can achieve distances of up to 5000 meters. Because of the length of time to completely turn the LED or laser on and off each time, the light is pulsed using low and high power. A logic zero is represented by low power, and a logic one by high power.
The Media Access Control method treats the link as point-to-point. Since separate fibers are used for transmitting (Tx) and receiving (Rx) the connection is inherently full duplex. Gigabit Ethernet permits only a single repeater between two stations
3.19 Gigabit Ethernet architecture
The distance limitations of full-duplex links are only limited by the medium, and not the round-trip delay.Daisy-chaining, star, and extended star topologies are all allowed. The issue then becomes one of logical topology and data flow, not timing or distance limitations.
A 1000BASE-T UTP cable is the same as 10BASE-T and 100BASE-TX cable, except that link performance must meet the higher quality Category 5e or ISO Class D (2000) requirements.
Modification of the architecture rules is strongly discouraged for 1000BASE-T. At 100 meters, 1000BASE-T is operating close to the edge of the ability of the hardware to recover the transmitted signal. Any cabling problems or environmental noise could render an otherwise compliant cable inoperable even at distances that are within the specification.
It is recommended that all links between a station and a hub or switch be configured for Auto-Negotiation to permit the highest common performance. This will avoid accidental misconfiguration of the other required parameters for proper Gigabit Ethernet operation.
10-Gigabit Ethernet
IEEE 802.3ae was adapted to include 10 Gbps full-duplex transmission over fiber optic cable. The basic similarities between 802.3ae and 802.3, the original Ethernet are remarkable. This 10-Gigabit Ethernet (10GbE) is evolving for not only LANs, but also MANs, and WANs.
With the frame format and other Ethernet Layer 2 specifications compatible with previous standards, 10GbE can provide increased bandwidth needs that are interoperable with existing network infrastructure.
A major conceptual change for Ethernet is emerging with 10GbE. Ethernet is traditionally thought of as a LAN technology, but 10GbE physical layer standards allow both an extension in distance to 40 km over single-mode fiber and compatibility with synchronous optical network (SONET) and synchronous digital hierarchy (SDH) networks. Operation at 40 km distance makes 10GbE a viable MAN technology. Compatibility with SONET/SDH networks operating up to OC-192 speeds (9.584640 Gbps) make 10GbE a viable WAN technology. 10GbE may also compete with ATM for certain applications.
- Frame format is the same, allowing interoperability between all varieties of legacy, fast, gigabit, and 10 gigabit, with no reframing or protocol conversions.
- Bit time is now 0.1 nanoseconds. All other time variables scale accordingly.
- Since only full-duplex fiber connections are used, CSMA/CD is not necessary.
- The IEEE 802.3 sublayers within OSI Layers 1 and 2 are mostly preserved, with a few additions to accommodate 40 km fiber links and interoperability with SONET/SDH technologies.
- Flexible, efficient, reliable, relatively low cost end-to-end Ethernet networks become possible.
- TCP/IP can run over LANs, MANs, and WANs with one Layer 2 transport method.
The basic standard governing CSMA/CD is IEEE 802.3. An IEEE 802.3 supplement, entitled 802.3ae, governs the 10GbE family. As is typical for new technologies, a variety of implementations are being considered, including:
- 10GBASE-SR – Intended for short distances over already-installed multimode fiber, supports a range between 26 m to 82 m
- 10GBASE-LX4 – Uses wavelength division multiplexing (WDM), supports 240 m to 300 m over already-installed multimode fiber and 10 km over single-mode fiber
- 10GBASE-LR and 10GBASE-ER – Support 10 km and 40 km over single-mode fiber
- 10GBASE-SW, 10GBASE-LW, and 10GBASE-EW – Known collectively as 10GBASE-W, intended to work with OC-192 synchronous transport module SONET/SDH WAN equipment
The IEEE 802.3ae Task force and the 10-Gigabit Ethernet Alliance (10 GEA) are working to standardize these emerging technologies.
10-Gbps Ethernet (IEEE 802.3ae) was standardized in June 2002. It is a full-duplex protocol that uses only optic fiber as a transmission medium. The maximum transmission distances depend on the type of fiber being used. When using single-mode fiber as the transmission medium, the maximum transmission distance is 40 kilometers (25 miles). Some discussions between IEEE members have begun that suggest the possibility of standards for 40, 80, and even 100-Gbps Ethernet.
3.20 Shared media environments
Here are some examples of shared media and directly connected networks:
- Shared media environment – This occurs when multiple hosts have access to the same medium. For example, if several PCs are attached to the same physical wire or optical fiber, they all share the same media environment.
- Extended shared media environment – This is a special type of shared media environment in which networking devices can extend the environment so that it can accommodate multiple access or longer cable distances.
- Point-to-point network environment – This is widely used in dialup network connections and is most common for home users. It is a shared network environment in which one device is connected to only one other device. An example is a PC that is connected to an Internet service provider through a modem and a phone line.
3.21 5-4-3-2-1 Rule
The 5-4-3-2-1 rule requires that the following guidelines should not be exceeded:
- Five segments of network media
- Four repeaters or hubs
- Three host segments of the network
- Two link sections with no hosts
- One large collision domain
The 5-4-3-2-1 rule also provides guidelines to keep round-trip delay time within acceptable limits.
3.22 Segmentation
One important skill for a networking professional is the ability to recognize collision domains.
A collision domain is created when several computers are connected to a single shared-access medium that is not attached to other network devices. This situation limits the number of computers that can use the segment. Layer 1 devices extend but do not control collision domains. Layer 2 devices segment or divide collision domains.
They use the MAC address assigned to every Ethernet device to control frame propagation. Layer 2 devices are bridges and switches. They keep track of the MAC addresses and their segments. This allows these devices to control the flow of traffic at the Layer 2 level. This function makes networks more efficient. It allows data to be transmitted on different segments of the LAN at the same time without collisions. Bridges and switches divide collision domains into smaller parts. Each part becomes its own collision domain.
These smaller collision domains will have fewer hosts and less traffic than the original domain.
The fewer hosts that exist in a collision domain, the more likely the media will be available. If the traffic between bridged segments is not too heavy a bridged network works well. Otherwise, the Layer 2 device can slow down communication and become a bottleneck.
Layer 2 and 3 devices do not forward collisions. Layer 3 devices divide collision domains into smaller domains.
Layer 3 devices also perform other functions. These functions will be covered in the section on broadcast domains.
Layer 2 broadcasts
To communicate with all collision domains, protocols use broadcast and multicast frames at Layer 2 of the OSI model.
When a node needs to communicate with all hosts on the network, it sends a broadcast frame with a destination MAC address 0xFFFFFFFFFFFF. This is an address to which the NIC of every host must respond.
Layer 2 devices must flood all broadcast and multicast traffic. The accumulation of broadcast and multicast traffic from each device in the network is referred to as broadcast radiation. In some cases, the circulation of broadcast radiation can saturate the network so that there is no bandwidth left for application data. In this case, new network connections cannot be made and established connections may be dropped. This situation is called a broadcast storm. The probability of broadcast storms increases as the switched network grows.
A NIC must rely on the CPU to process each broadcast or multicast group it belongs to. Therefore, broadcast radiation affects the performance of hosts in the network.A host does not usually benefit if it processes a broadcast when it is not the intended destination. The host is not interested in the service that is advertised. High levels of broadcast radiation can noticeably degrade host performance. The three sources of broadcasts and multicasts in IP networks are workstations, routers, and multicast applications.
Workstations broadcast an Address Resolution Protocol (ARP) request every time they need to locate a MAC address that is not in the ARP table.
Although the numbers in the figure might appear low, they represent an average, well-designed IP network. When broadcast and multicast traffic peak due to storm behavior, peak CPU loss can be much higher than average. Broadcast storms can be caused by a device that requests information from a network that has grown too large. So many responses are sent to the original request that the device cannot process them, or the first request triggers similar requests from other devices that effectively block normal traffic flow on the network.The routing protocols that are configured on a network can increase broadcast traffic significantly. IP multicast applications can adversely affect the performance of large, scaled, switched networks. Multicasting is an efficient way to send a stream of multimedia data to many users on a shared-media hub. However, it affects every user on a flat switched network. A packet video application could generate a 7-MB stream of multicast data that would be sent to every segment. This would result in severe congestion.
3.23 Broadcast domains
A broadcast domain is a group of collision domains that are connected by Layer 2 devices.
When a LAN is broken up into multiple collision domains, each host in the network has more opportunities to gain access to the media. This reduces the chance of collisions and increases available bandwidth for every host. Broadcasts are forwarded by Layer 2 devices. Excessive broadcasts can reduce the efficiency of the entire LAN. Broadcasts have to be controlled at Layer 3 since Layers 1 and 2 devices cannot control them. A broadcast domain includes all of the collision domains that process the same broadcast frame. This includes all the nodes that are part of the network segment bounded by a Layer 3 device. Broadcast domains are controlled at Layer 3 because routers do not forward broadcasts. Routers actually work at Layers 1, 2, and 3. Like all Layer 1 devices, routers have a physical connection and transmit data onto the media. Routers also have a Layer 2 encapsulation on all interfaces and perform the same functions as other Layer 2 devices. Layer 3 allows routers to segment broadcast domains.
In order for a packet to be forwarded through a router it must have already been processed by a Layer 2 device and the frame information stripped off. Layer 3 forwarding is based on the destination IP address and not the MAC address. For a packet to be forwarded it must contain an IP address that is outside of the range of addresses assigned to the LAN and the router must have a destination to send the specific packet to in its routing table
3.24 Introduction to data flow
Data flow in the context of collision and broadcast domains focuses on how data frames propagate through a network .A good rule to follow is that a Layer 1 device always forwards the frame, while a Layer 2 device wants to forward the frame. In other words, a Layer 2 device will forward the frame unless something prevents it from doing so. A Layer 3 device will not forward the frame unless it has to. Using this rule will help identify how data flows through a network.
Layer 1 devices do no filtering, so everything that is received is passed on to the next segment. The frame is simply regenerated and retimed and thus returned to its original transmission quality. Any segments connected by Layer 1 devices are part of the same domain, both collision and broadcast.
Layer 2 devices filter data frames based on the destination MAC address. A frame is forwarded if it is going to an unknown destination outside the collision domain. The frame will also be forwarded if it is a broadcast, multicast, or a unicast going outside of the local collision domain. The only time that a frame is not forwarded is when the Layer 2 device finds that the sending host and the receiving host are in the same collision domain. A Layer 2 device, such as a bridge, creates multiple collision domains but maintains only one broadcast domain.
Layer 3 devices filter data packets based on IP destination address. The only way that a packet will be forwarded is if its destination IP address is outside of the broadcast domain and the router has an identified location to send the packet. A Layer 3 device creates multiple collision and broadcast domains.
Data flow through a routed IP based network, involves data moving across traffic management devices at Layers 1, 2, and 3 of the OSI model. Layer 1 is used for transmission across the physical media, Layer 2 for collision domain management, and Layer 3 for broadcast domain management.
3.25 What is a network segment?
In the context of data communication, network segment is defined as the following:
- Section of a network that is bounded by bridges, routers, or switches.
- In a LAN using a bus topology, a segment is a continuous electrical circuit that is often connected to other such segments with repeaters.
- Term used in the TCP specification to describe a single transport layer unit of information. The terms datagram, frame, message, and packet are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.
