Ethernet hub

An Ethernet hub, active hub, network hub, repeater hub, hub or concentrator is a device for connecting multiple twisted pair or fiber opticEthernet devices together and making them act as a single network segment. Hubs work at the physical layer (layer 1) of the OSI model. The device is a form of multiport repeater. Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if it detects a collision.

Hubs also often come with a BNC and/or AUI connector to allow connection to legacy 10BASE2 or 10BASE5 network segments. The availability of low-priced network switches has largely rendered hubs obsolete but they are still seen in older installations and more specialized applications.

   A network hub is a fairly unsophisticated broadcast device. Hubs do not manage any of the traffic that comes through them, and any packet entering any port is broadcast out on all other ports. Since every packet is being sent out through all other ports, packet collisions result—which greatly impedes the smooth flow of traffic.

The need for hosts to be able to detect collisions limits the number of hubs and the total size of a network built using hubs (a network built using switchesFast Ethernet network is likely to require switches to avoid the chaining limits of hubs. does not have these limitations). For 10 Mbit/s networks, up to 5 segments (4 hubs) are allowed between any two end stations. For 100 Mbit/s networks, the limit is reduced to 3 segments (2 hubs) between any two end stations, and even that is only allowed if the hubs are of the low delay variety. Some hubs have special (and generally manufacturer specific) stack ports allowing them to be combined in a way that allows more hubs than simple chaining through Ethernet cables, but even so, a large

Most hubs detect typical problems, such as excessive collisions and jabbering on individual ports, and partition the port, disconnecting it from the shared medium. Thus, hub-based Ethernet is generally more robust than coaxial cable-based Ethernet (e.g. 10BASE2, thinnet), where a misbehaving device can adversely affect the entire collision domain. Even if not partitioned automatically, a hub makes troubleshooting easier because status lights can indicate the possible problem source or, as a last resort, devices can be disconnected from a hub one at a time much more easily than a coaxial cable. They also remove the need to troubleshoot faults on a huge cable with multiple taps.

Hubs are classified as Layer 1 (Physical Layer) devices in the OSI model. At the physical layer, hubs support little in the way of sophisticated networking. Hubs do not read any of the data passing through them and are not aware of their source or destination. Essentially, a hub simply receives incoming packets, regenerates the electrical signal, and broadcasts these packets out to all other devices on the network

Dual speed hubs

  In the early days of Fast Ethernet, Ethernet switches were relatively expensive devices. Hubs suffered from the problem that if there were any 10BASE-T devices connected then the whole network needed to run at 10 Mbit/s. Therefore a compromise between a hub and a switch was developed, known as a dual-speed hub. These devices consisted of an internal two-port switch, dividing the 10BASE-T (10 Mbit/s) and 100BASE-T (100 Mbit/s) segments. The device would typically consist of more than two physical ports. When a network device becomes active on any of the physical ports, the device attaches it to either the 10BASE-T segment or the 100BASE-T segment, as appropriate. This prevented the need for an all-or-nothing migration from 10BASE-T to 100BASE-T networks. These devices are hubs because the traffic between devices connected at the same speed is not switched.

     Historically, the main reason for purchasing hubs rather than switches was their price. This has largely been eliminated by reductions in the price of switches, but hubs can still be useful in special circumstances:

• For inserting a protocol analyzer into a network connection, a hub is an alternative to a network tap or port mirroring.
• Some computer clusters require each member computer to receive all of the traffic going to the cluster. A hub will do this naturally; using a switch requires special configuration.
• When a switch is accessible for end users to make connections, for example, in a conference room, an inexperienced or careless user (or saboteur) can bring down the network by connecting two ports together, causing a loop. This can be prevented by using a hub, where a loop will break other users on the hub, but not the rest of the network. (It can also be prevented by buying switches that can detect and deal with loops, for example by implementing the Spanning Tree Protocol.)
• A hub with a 10BASE2 port can be used to connect devices that only support 10BASE2 to a modern network. The same goes for linking in an old thicknet network segment using an AUI port on a hub (individual devices that were intended for thicknet can be linked to modern Ethernet by using an AUI-10BASE-T transceiver).

  

Gigabit Ethernet

   Gigabit Ethernet (GbE or 1 GigE) is a term describing various technologies for transmitting Ethernet frames at a rate of a gigabit per second, as defined by the IEEE 802.3-2008 standard. Half-duplex gigabit links connected through hubs are allowed by the specification but in the marketplace full-duplex with switches are normal.

The result of research done at Xerox Corporation in the early 1970s, Ethernetphysical and link layer protocol today. Fast Ethernet increased speed from 10 to 100 megabits per second (Mbit/s). Gigabit Ethernet was the next iteration, increasing the speed to 1000 Mbit/s. The initial standard for gigabit Ethernet was produced by the IEEE in June 1998 as IEEE 802.3z, and required optical fiber. 802.3z is commonly referred to as 1000BASE-X, where -X refers to either -CX, -SX, -LX, or (non-standard) -ZX. has evolved into the most widely implemented 

   IEEE 802.3ab, ratified in 1999, defines gigabit Ethernet transmission over unshielded twisted pair (UTP) category 5, 5e, or 6 cabling and became known as 1000BASE-T. With the ratification of 802.3ab, gigabit Ethernet became a desktop technology as organizations could use their existing copper cabling infrastructure.

   IEEE 802.3ah, ratified in 2004 added two more Gigabit fiber standards, 1000BASE-LX10 (which was already widely implemented as vendor specific extension) and 1000BASE-BX10. This was part of a larger group of protocols known as Ethernet in the First Mile.

   Initially, gigabit Ethernet was deployed in high-capacity backbone network links (for instance, on a high-capacity campus network). In 2000, Apple's Power Mac G4 and PowerBook G4 were the first mass produced personal computers featuring the 1000BASE-T connection. It quickly became a built-in feature in many other computers. As of 2009 Gigabit NICs (1000BASE-T) are included in almost all desktop and server computer systems.

 ^[update]

   Higher bandwidth 10 gigabit Ethernet standards have since become available as the IEEE ratified a fiber-based standard in 2002, and a twisted pair standard in 2006. As of 2009 10Gb Ethernet is replacing 1Gb as the backbone network and has just begun to migrate down to high-end server systems.^[update]

There are four different physical layer standards for gigabit Ethernet using optical fiber (1000BASE-X), twisted pair cable (1000BASE-T), or balanced copper cable (1000BASE-CX).

The IEEE 802.3z standard includes 1000BASE-SX for transmission over multi-mode fiber, 1000BASE-LX for transmission over single-mode fiber, and the nearly obsolete 1000BASE-CX for transmission over balanced copper cabling. These standards use 8b/10b encoding, which inflates the line rate by 25%, from 1,000–1,250 Mbit/s to ensure a DC balanced signal. The symbols 

are then sent using NRZ.

IEEE 802.3ab, which defines the widely used 1000BASE-T interface type, uses a different encoding scheme in order to keep the symbol rate as low as possible, allowing transmission over twisted pair.

Ethernet in the First Mile later added 1000BASE-LX10 and -BX10.

Name	Medium	Specified distance
1000BASE‑CX	Shielded single twisted-pair cable	25 meters
1000BASE‑SX	Multi-mode fiber	220 to 550 meters dependent on fiber diameter and bandwidth
1000BASE‑LX	Multi-mode fiber	550 meters
1000BASE‑LX	Single-mode fiber	5 km
1000BASE‑LX10	Single-mode fiber using 1,310 nm wavelength	10 km
1000BASE‑ZX	Single-mode fiber at 1,550 nm wavelength	~ 70 km
1000BASE‑BX10	Single-mode fiber, over single-strand fiber: 1,490 nm downstream 1,310 nm upstream	10 km
1000BASE‑T	Twisted-pair cabling (Cat‑5, Cat‑5e, Cat‑6, or Cat‑7)	100 meters
1000BASE‑TX	Twisted-pair cabling (Cat‑6, Cat‑7)	100 meters

1000BASE-T

1000BASE-T (also known as IEEE 802.3ab) is a standard for gigabit Ethernet over copper wiring.

Each 1000BASE-T network segment can be a maximum length of 100 meters (328 feet), and must use Category 5 cable at a minimum. Category 5e cable or Category 6 cable may also be used.

Autonegotiation is a requirement for using 1000BASE-T according to Section 28D.5 Extensions required for Clause40 (1000BASE-T). At least the clock source has to be negotiated, as one has to be master and the other slave.

1000BASE-T requires all four pairs to be present. If two gigabit devices are connected through a non-compliant Cat-5 cable with two pairs only, negotiation takes place on two pairs only, so the devices successfully choose 'gigabit' as the highest common denominator (HCD), but the link never comes up. Most gigabit physical devices have a specific register to diagnose this behaviour. Some drivers offer an "Ethernet@Wirespeed" option where this situation leads to a slower yet functional connection.

1000BASE-T details

In a departure from both 10BASE-T and 100BASE-TX, 1000BASE-T uses all four cable pairs for simultaneous transmission in both directions through the use of echo cancellation and a 5-level pulse amplitude modulation (PAM-5) technique. The symbol rate is identical to that of 100BASE-TX (125 Mbaud) and the noise immunity of the 5-level signaling is also identical to that of the 3-level signaling in 100BASE-TX, since 1000BASE-T uses 4-dimensional trellis coded modulation (TCM) to achieve a 6 dB coding gain across the 4 pairs.

The data are transmitted over four copper pairs, eight bits at a time. First, eight bits of data are expanded into four 3-bit symbols through a non-trivial scrambling procedure based on a linear feedback shift register; this is similar to what is done in 100BASE-T2, but uses different parameters. The 3-bit symbols are then mapped to voltage levels which vary continuously during transmission. One example mapping is as follows:

Symbol	Line signal level
000	0
001	+1
010	+2
011	−1
100	0
101	+1
110	−2
111	−1

Automatic crossover

   Automatic MDI/MDI-X Configuration is specified as an optional feature in the 1000BASE-T standard, meaning that straight-through cables will often work between Gigabit capable interfaces. This feature eliminates the need for crossover cables, making obsolete the uplink/normal ports and manual selector switches found on many older hubs and switches and greatly reducing installation errors.