By: Nick Pidgeon. Ethernet switching gave rise to another advancement, full-duplex Ethernet. Full-duplex is a data communications term that refers to the ability to send and receive data at the same time. Legacy Ethernet is half-duplex, meaning information can move in only one direction at a time. In a totally switched network, nodes only communicate with the switch and never directly with each other.
Switched networks also employ either twisted pair or fiber optic cabling, both of which use separate conductors for sending and receiving data. Providing the interframe gap ensures that the interfaces at each end of the link can keep up with the full frame rate of the link. In full-duplex mode, the stations at each end of the link ignore any collision detect signals that come from the transceiver. If the transceiver has a collision detect light, then placing the transceiver in full-duplex mode means that the significance of the collision detect light is undefined, and the light should be ignored.
A station on a full-duplex link sends whenever it likes, ignoring carrier sense CS. There is no multiple access MA since there is only one station at each end of the link and the Ethernet channel between them is not the subject of access contention by multiple stations. Since there is no access contention, there will be no collisions either, so the station at each end of the link is free to ignore collision detect CD.
This leads to asymmetric data patterns, in which most of the data is sent in one direction, and smaller amounts of data in the form of acknowledgments return in the other direction. On the other hand, full-duplex links between switching hubs in a network backbone system will typically carry multiple conversations between many computers.
Therefore, the aggregated traffic on backbone channels will be more symmetric, with both the transmit and receive channels seeing roughly the same amount of traffic.
For that reason, the largest benefit of a full-duplex bandwidth increase is usually seen in backbone links. It is essential that both ends of a link operating in full-duplex mode are configured correctly, or the link will have serious data errors. To ensure correct configuration, the standard recommends that Ethernet Auto-Negotiation see Chapter 5 be used whenever possible to automatically configure full-duplex mode.
However, using Auto-Negotiation to configure full-duplex operation on a link may not be as simple as it sounds. For one thing, support for Auto-Negotiation is optional for most Ethernet media systems, in which case the vendor is not required to provide Auto-Negotiation capability.
Furthermore, Auto-Negotiation was originally developed for twisted-pair Ethernet devices only, and thus it is not supported on all Ethernet media types. The 10 Mbps and Mbps fiber optic media systems do not support the Auto-Negotiation standard, while Gigabit Ethernet fiber optic systems have their own auto-configuration scheme.
Therefore, you may find that you have to manually configure full-duplex support on the station at each end of the link. On a manually configured link, it is essential that both ends of the link be properly configured for full-duplex operation. If only one end of the link is in full-duplex mode, and the other is in half-duplex mode, then the half-duplex end of the link will lose frames due to errors, such as late collisions.
Because the misconfigured link will still support the flow of data despite the errors , it is possible that this problem may not be detected right away. Therefore, you need to be aware that this condition can occur, and make absolutely sure that both ends of a manually configured link are set for the same mode of operation. At each end of the link, you must also ensure that both the Ethernet interface and the transceiver are configured for full-duplex operation.
This can be difficult in some cases, since the 10 Mbps Attachment Unit Interface AUI used between an Ethernet interface and an external transceiver was designed before full-duplex mode was developed and does not support automatic full-duplex configuration. For this reason, the standard recommends that only 10 Mbps Ethernet interfaces with built-in transceivers be used for full-duplex segments.
As a result, when using an external transceiver with a pin connection, you could end up with a transceiver in half-duplex mode connected to an interface in full-duplex mode. On the other hand, the newer pin Medium Independent Interface MII , which supports both 10 and Mbps Ethernet systems, makes it possible for the interface to detect and set the operational mode of an external transceiver.
If an external transceiver is used, then it is essential that the mode of operation of the Ethernet interface and the transceiver at a given station match. There can be problems if an Ethernet interface in full-duplex mode is connected to a transceiver in half-duplex mode, or if the station interfaces are not in the same mode.
The standard notes that this sort of confusion. Table 4. More modern external transceivers with pin MII interfaces are automatically configured by the station interface. Since there is no round-trip timing limit, the only limit on the length of the cabling is the one imposed by the signal transmission characteristics of the cable.
For that reason, some full-duplex segments can be much longer than the same segments operated in half-duplex mode. Twisted-pair segments are limited in distance due to the signal carrying characteristics of the cable, and cannot be extended in length when operated in full-duplex mode.
Due to the restricted signal carrying capability of twisted-pair cable, the maximum limit for a twisted-pair cable segment is the same whether the segment is operated in full-duplex or half-duplex mode. Fiber optic segments, on the other hand, have very good signal carrying characteristics and are mostly limited in length by the timing constraints of half-duplex operation. For that reason, a full-duplex mode fiber optic segment can be considerably longer than the same segment type operating in half-duplex mode.
As an example, a BASE-FX fiber optic segment using a typical multimode fiber optic cable is limited to segment lengths of meters Data is transferred simultaneously from the switch to the endpoint, and vice versa, to ensure optimal network performance and speed. A switch that can deliver Mbps symmetrical, full duplex can transmit and receive at a rate of Mbps.
Even if it is full duplex, a network switch with asymmetrical bandwidth cannot send AND receive at Mbps.
Asymmetrical switches will use an uneven split to transmit at 70Mbps and receive at 30Mbps, for example. Using the same example of moving two Mb files, a Mbps symmetrical, full duplex switch will deliver both files in 1. Even though both devices can be marketed as a Mbps switch, real-world performance is significantly different. In addition to transmission speed, latency also plays a significant role in network performance and service quality.
Latency is the time it takes a piece of information a packet to reach its destination. Latency may not be as crucial for certain endpoints, such as data terminals. However, for real-time applications like voice calls or live video monitoring, low latency is critical to ensure good user experience. To illustrate latency, we tested our long reach Ethernet over Coax switch against a competing product.
Both switches were tested at Mbps, symmetrical, full duplex over 2,ft of RG6 cable. The average delay of the competing product was 4, microseconds, which is times more latency that the CLEER24 switch. See the full performance comparison between these two products. Even at 2,ft, NVT Phybridge Power over Ethernet switches have extremely low latency, on par with standard reach Ethernet solutions from market leaders like Cisco.
Many long reach Power over Ethernet solutions on the market have higher latency levels, which are not suited to support real-time applications. Finally, there is the issue of noise, also known as crosstalk. Crosstalk occurs when a signal transmission results in undesired electromagnetic waves that interfere with surrounding equipment or wiring.
Noise production makes a big impact on large deployments where there is a lot of equipment and cabling in one physical space.
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