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Category 5 also known as "CAT5"

Category 5 cable is a twisted pair high signal integrity cable type often referred to as Cat5e or Cat-5. Most cables are unshielded ("aka" UTP- unshielded twisted pair), relying on the twisted pair design for noise rejection, and for long distances it may required using shielded twisted pair. Category 5 has been superseded by the Category 5e specification structured cabling for computer networks such as Ethernet, and is also used to carry many other signals such as basic voice services, token ring, and ATM (at up to 155 Mbit/s, over short distances).

The specification for Category 5 cable was defined in ANSI/TIA/EIA-568-A, with clarification in TSB-95. These documents specified performance characteristics and test requirements for frequencies of up to 100 MHz. Category 5 cable includes twisted pairs in a single cable jacket. This use of balanced lines helps preserve a high signal-to-noise ratio despite interference from both external sources and other pairs (this latter form of interference is called crosstalk). It is most commonly used for 100 Mbit/s networks, such as 100BASE-TX Ethernet, although IEEE 802.3ab defines standards for 1000BASE-T - Gigabit Ethernet over category 5 cable. Cat5e cable typically has four twists pairs in it and each of the conductors is 24 gauge (AWG) copper wire.

The Cat 5e "350 MHz" debacle

The 350 MHz term started a couple of years before the arrival of Category 6 cable by the Telecommunication Manufacturers and promised better performance. Although the performance of this new 350 MHz cable was slightly better it was an easy way to sell the consumer on future proofing their needs while charging around 15% more and leading to a higher margin on the 350 MHz cable than the standard 5e cable. Some low-end cable manufacturers have the term "tested to 350 MHz" printed on the jacket as a way to appear to the consumer that they are receiving a better quality of 5e cable, but the cable was only "tested to 350 MHz" and no promise of a performance guarantee is ever mentioned.

As the 350 cables and term gained momentum, many manufacturers began offering a 400 MHz 5e cable, a 550 MHz 5e cable, and so on. This led to the consumer and communications contractor assuming and leaving them confused that the higher a MHz rating meant for a better performing cable. The arrival of the Category 6 cable standard which specified a delivered performance at 250 MHz left many people confused.

To make thinks simpler the standard only recognizes the Category 5e as guaranteeing performance at 100 MHz. No standard is issued for 350 MHz cable so far. But the most important thing in this standard is the speed rates.CAT5e is capable of transmitting data at speeds of up to 1000 Mbps (1 Gigabit per second). This speed rate supports many applications, higher speeds (CAT6 and higher) are for very limited market. The CAT6

Category 6 also known as "CAT6"

Category 6 cable, commonly referred to as Cat-6, is a cable standard for Gigabit Ethernet and other network protocols that are backward compatible with the Category 5/5e and Category 3 cable standards. Compared with Cat-5 and Cat-5e, Cat-6 features more stringent specifications for crosstalk and system noise. The cable standard provides performance of up to 250 MHz and is suitable for 10BASE-T, 100BASE-TX (Fast Ethernet), 1000BASE-T / 1000BASE-TX (Gigabit Ethernet) and 10GBASE-T (10-Gigabit Ethernet). Category 6 cable has a reduced maximum length when used for 10GBASE-T; Category 6a cable, or Augmented Category 6, is characterized to 500 MHz and has improved alien crosstalk characteristics, allowing 10GBASE-T to be run for the same distance as previous protocols. Category 6 cable can be identified by the printing on the side of the cable sheath.

Like most earlier cables, Category 6 cable contains four twisted wire pairs. Although it is sometimes made with 23 AWG wire, the increase in performance with Cat-6 comes mainly from better insulation; 22 to 24 AWG copper is allowed so long as the ANSI/TIA-568-B.2-1 performance specifications are met.

Category 6 cable was designed to offer higher performance for better transmission of data at speeds up to 10Gbps.
Maximum Length
The maximum allowed length of a Cat-6 cable is 100 meters (330 ft) when used for 10/100/1000baseT. When used for 10GbaseT, Cat-6 cable's maximum length is 55 meters (180 ft).

Just to give you some references, most of routers and switches today are rated 10/100Mbps which means maximum data transfer rates up to 100Mbps (10% from CAT5e max capabilities). Another example, most internet accounts today rated 2-10Mbps which means maximum data transfer rates up to 10Mbps (1% from CAT5e max capabilities).

I hope things are more clear now.