Twisted Pair: Ten Questions and Answers
1. How do you read the RJ45 interface pin numbering on twisted pair cables?
Answer: With the metal contacts of the connector facing up and the plastic latch facing down, and the end that plugs into the RJ-45 jack facing outward, the contacts are numbered 1-8 in sequence.
2. What is an 8P8C connector?
Answer: The RJ-45 connector required for making network cables has eight grooves on the front, referred to as "8P" (positions); the metal contacts inside the grooves total eight, referred to as "8C" (contacts). The industry commonly calls this an "8P8C" connector.
3. Why are two mutually insulated copper conductors twisted together in twisted pair cable instead of being laid parallel? Wouldn't parallel wires be easier to manufacture?
Answer: This is because network cables carry high-frequency signals above 10MHz. In this case, the insulation between the conductors acts as a dielectric plate, and the capacitance formed between it and the conductors causes a bypass attenuation effect on the high-frequency signal (the signal phase is delayed). Due to the high signal frequency, this effect becomes non-negligible, so parallel wires cannot effectively transmit high-frequency network signals.
However, if we twist the parallel wire pair, a series inductance is formed along with the capacitance between the pair. From electronic circuit theory, we know that inductance has the opposite effect to capacitance—it causes the signal phase to advance. By adjusting the twist rate so that the inductance and capacitance formed by the pair exactly cancel each other out, and since both effects increase equally with the length of the pair, the signal can ideally be transmitted over the twisted pair without attenuation.
4. Does twisted pair transmit analog or digital signals? Can it also carry power?
Answer: Twisted pair can transmit both analog and digital signals. This depends on the technology used for transmission and reception.
Twisted pair can also carry power, but you must ensure that the power delivered meets the requirements of the equipment. For example, if the equipment operating power is 10-12VDC with a current of 50mA, and the twisted pair has an internal resistance of 10Ω/100 meters (measure in actual use), then if the power delivered is 12VDC, to ensure normal equipment operation, you can only transmit power over a distance within 200 meters. If the distance is relatively short (200-300 meters) and low-voltage DC power is being transmitted, it is feasible. For longer distances (around 1500 meters) where power is not readily available at the remote end, one pair of wires can be used to transmit 220V 50Hz AC power. However, it is not recommended to transmit both signals and power in the same twisted pair cable. If power must be run, use dedicated power cables and maintain a certain distance from the twisted pair.
5. What types of shielded twisted pair cable are available?
Answer: According to the 568B standard, shielded twisted pair cable is only defined for Category 7, though some manufacturers have produced shielded twisted pair cables in Category 3 and Category 5 through their own technological innovations.
6. Are all twisted pair cables 4-pair 8-wire?
Answer: Not all twisted pair cables are 4-pair 8-wire. Early Category 2 cables were 2-pair 4-wire. In the American Wire Gauge (AWG) standard, Category 3, Category 4, Category 5, and Enhanced Category 5 twisted pair cables are all defined as 4-pair. When Ethernet uses twisted pair as the transmission medium, only 2 pairs (4 wires) are needed to complete signal transmission and reception. In Fast Ethernet using twisted pair as the transmission medium, there are three standards: 100Base-TX, 100Base-T2, and 100Base-T4. Among these, the 100Base-T4 standard requires all 4 pairs for signal transmission, while the other two standards only require 2 pairs. The most widely adopted standard in Fast Ethernet is 100Base-TX, so when purchasing twisted pair for 100M networks, some network vendors or companies may provide twisted pair with only 2 pairs, reasoning that Fast Ethernet only needs 2 pairs to transmit information.
7. When making a straight-through cable, is it necessary to follow the 568B standard?
Answer: No. A straight-through cable is a twisted pair used to connect computers to hubs (or switches) in a network. It can be divided into a one-to-one connection method and a 100M connection method. The one-to-one connection method means the wires at both ends of the twisted pair are connected in a one-to-one correspondence—pin 1 at one end must connect to pin 1 at the other end. Although there is no sequence requirement, the correspondence must be consistent. The 100M connection method refers to a connection that supports 100Mbps communication rates. Although its connection is also one-to-one, the color of each pin is fixed and must follow the 568B standard, with the specific arrangement: White/Orange, Orange, White/Green, Blue, White/Blue, Green, White/Brown, Brown.
8. Can Enhanced Category 5 support Gigabit? Can it support 10-Gigabit?
Answer: a. Not all Enhanced Category 5 cables can run Gigabit Ethernet. In fact, according to the EIA/TIA 568B standard, Category 5 cable is applicable to 100Mb/s 100Base Ethernet. Category 6 cable is suitable for Gigabit Ethernet. High-quality Enhanced Category 5 UTP can also achieve 1000M—1000BASE-T. The quality is mainly reflected in whether the electrical performance meets the 4-pair full-duplex transmission requirements of Gigabit Ethernet, such as Enhanced Category 5 UTP that has passed 3Com/Mohawk/CDT's 1000BASE-T Cabling verification.
In fact, there is a standard for Gigabit Ethernet over Enhanced Category 5 UTP, namely the 802.3ab standard. 802.3ab defines the 1000BASE-T specification based on Category 5 UTP, with the ultimate goal of achieving effective 1000Mbps Ethernet transmission over 100 meters on Category 5 UTP. The main significance of 802.3ab is to protect the investment of a large number of users who have already deployed Category 5 cabling systems. Category 5 cabling systems have been widely used in various buildings constructed in recent years, and as the information industry advances rapidly, new technologies that do not consider protecting users' previous investments will inevitably lose their vitality.
In this case, Enhanced Category 5 uses four pairs, with each pair transmitting bidirectionally, i.e., full-duplex. (Although Category 6 also uses four pairs for Gigabit, only two pairs transmit data while the other two pairs receive data, i.e., simplex.) Note: Enhanced Category 5 and Category 6 operate at different frequencies and use different encoding methods, so the equipment used for Gigabit also differs. The encoding method for Gigabit over Enhanced Category 5 is relatively complex, and the equipment is relatively expensive. (Running 1000Mbps over Enhanced Category 5 distributes the traffic across 8 copper wires, with each wire carrying 125Mbps, but its frequency range is only up to 100MHz, meaning 1Hz must produce 1.25 bits, making the encoding and modulation relatively complex. In contrast, Category 6 uses one pair to achieve 500Mbps, with each wire carrying 250Mbps, and its frequency range extends to 250MHz, so 1 bit per 1Hz is sufficient, making the encoding method simpler.
Enhanced Category 5 in Gigabit Ethernet operates in full-duplex mode and is very sensitive to return loss. For example, when a switch receives a data packet, it may be difficult to distinguish whether it was sent normally by the remote end or reflected back from its own transmission. Therefore, network equipment supporting 1000Base-T must include active digital signal processors to compensate for return loss.)
b. Globally, 50% of cabling is Category 5, and 30% is Category 6. For 10-Gigabit, to be deployed as quickly as possible, support for the above two cabling systems must be considered. However, some 10-Gigabit research initiatives hold different views, arguing that 10-Gigabit running over fiber is sufficient and that Category 5 and Category 6 cabling systems need not be considered. The international standard for 10-Gigabit, 802.3ae, was released as early as the end of 2003. However, the development of 10-Gigabit has not been as smooth as originally anticipated. The biggest challenge is how 10-Gigabit technology can seamlessly connect with existing networks. Users want to smoothly upgrade to 10-Gigabit without changing the existing network structure and cabling systems. In practical applications, access-layer cabling may not need to be changed, and existing cabling systems may suffice, but such a structure would limit access-layer equipment to the existing 100Mbps transmission rate, making it impossible to run 10-Gigabit speeds on it. If 10-Gigabit only plays a role at the core layer while the access layer continues to use existing technology, the era of large-scale 10-Gigabit deployment will not arrive soon. This is not the original intention of 10-Gigabit development. Whether running 10G over Category 5 and Enhanced Category 5 cables should be included in the emerging 10GBase-T standard is a debate raised by some companies when defining next-generation physical layer chips. Many electronics and cable companies doubt whether Enhanced Category 5 cables, used by more than half of enterprises today, can complete 10Gbit/s data transmission tasks. Currently, the IEEE 802.3an task group drafting the standard has excluded Enhanced Category 5 cables, though this is not a final decision. SolarFlare has transmitted 10Gbit/s signals over 50 meters of Enhanced Category 5 cable using a custom transceiver and a 10G-sample 8-bit A/D converter. The company stated that when its custom 9-bit A/D converter is released, it will be able to transmit over 100 meters. However, SolarFlare cannot release products until the 10GBase-T standard is completed in early 2006. Its products may include a new version of the A/D converter and a transceiver integrating 4 million transistors on a 90nm process. Force10, an Ethernet router and switch manufacturer, will also support 10G fiber connections, believing that from a technical perspective, Enhanced Category 5 will bring 10G into enterprise and low-cost domains, as fiber is too expensive. Cisco, Intel, and an end-user alliance at U.S. national laboratories support including Enhanced Category 5 cables in the 10GBase-T standard. However, cable manufacturers oppose this plan, arguing that existing cables cannot meet the performance requirements for 10G transmission. 100MHz is the specified bandwidth for Category 5 and Enhanced Category 5 cables, though some companies have tested enhanced versions at 350MHz. The 802.3an task group has now specified 10GBase-T bandwidth at 650MHz, though this figure may be reduced to 500MHz. SolarFlare and other companies believe Enhanced Category 5 can achieve 350 to 400MHz bandwidth and state that the idea that 10G requires CAT7 is incorrect. The 802.3an task group has set targets specifying 10G over 200MHz CAT6 cables at 55 to 100 meters, or 600MHz CAT7 cables at 100 meters. Some believe the final version of the specification may include Enhanced Category 5 cables. Currently, the task group has discussed two signaling methods. SolarFlare suggests modulation based on 10-lane 833 Msample/s PAM (Pulse-Amplitude Modulation) format coding, while Intel suggests low-density parity check with 8-lane 1000 Msample/s PAM. Most laboratories currently use Enhanced Category 5, and by the time the standard is released in 2006, half of all cables will also be Enhanced Category 5. If it can run 10G at a low cost, people will certainly not reject it. Some users have cable lengths not exceeding 50 meters and may connect to 10G in the future, but redesigning Enhanced Category 5 to implement this technology is not ideal, as some types of Enhanced Category 5 perform worse at 450MHz than at 250MHz.
9. What is the maximum transmission distance for twisted pair?
Answer: Whether under the 10Base-T and 100Base-TX standards or the 1000Base-T standard, the maximum transmission distance is clearly specified as 100 meters. In structured cabling specifications, it is also explicitly required that horizontal cabling not exceed 90 meters and that the total link length not exceed 100 meters. In other words, 100 meters is a limit for wired Ethernet. This limit applies to the link length from the network card to the hub device. In fact, we can break through the 100-meter limit through certain methods.
High-quality cabling products, high-grade network equipment, and reduced transmission rates. To extend the network beyond 150 meters, dedicated network equipment such as network extenders is required.
10. How can the network range be expanded?
Answer:
a. In a 10Base-T Ethernet network, to expand the network range, repeaters can be installed between two segments of twisted pair cable (typically implemented through hub or switch cascading), but a maximum of 4 repeaters can be installed, extending the maximum network range to 500m. This connection method is also called cascading.
b. In a 100Base-T network, there are two scenarios.
1. When the connected device is a 100Mbit/s hub: a maximum of two hubs can be connected simultaneously, and the maximum distance between hubs is only 5m, resulting in a maximum network connection distance of 205m;
2. When the connected device is a 100Mbit/s switch: the connection situation is the same as in a 10Base-T network, i.e., a connection distance of 500m. This is because switches operate in switching mode.