Four pairs of UTP Category 3 or Not supported better 100Base-T2 25 MBd PAM5 X 5 Two pairs of UTP Category 3 or Supported better Source: Table 7-2 in Cisco, WWW.cisco.com/univercd/cc/td/doc(Ref. 12). 1000-Mbps Ethernet. There are two primary specifications for 1000-Mbps Ethernet: 1000Base-T for UTP (unshielded twisted pair) copper cable and 1000Bbase-X for STP (shielded twisted pair) copper cable, as well as single-mode and multimode optical fiber. 1000Base-T Ethernet provides full-duplex transmission over four-pair Category 5 or better UTP cable. It scrambles each octet in the MAC frame to randomize the bit sequence before it is encoded using a 4-D, 8-state trellis forward error correction (FEC) coding in which four PAM5 symbols are sent at the same time over four wire pairs. Four of the five levels of each PAM5 symbols represent two bits in the data octet. The fifth level is used for FEC coding, which enables symbol recovery in the presence of noise and crosstalk. Separate scramblers for the master and slave PHYs create essentially uncorrelated data streams between the two opposite-traveling symbol streams on each wire pair. Clock recovery and master/slave loop timing procedures are essentially the same as those used in 100Base-T2. Which NIC (network interface card) will be master (typically the NIC in a multiport intermediate network node) and which will be slave is determined during autonegotiation. Each transmitted frame is encapsulated with start-of-stream and end-of-stream delimiters, and loop timing is maintained by continuous streams of IDLE symbols sent on each wire pair during interframe gaps. 1000Base-T supports both half- duplex and full-duplex operation. There are three versions of 1000Base-X all of which support full-duplex binary trans- mission at 1250 Mbps over two strands of optical fiber or two STP copper wire pairs. Transmission coding is based on ANSI Fibre Channel 8B/10B encoding scheme. Each 8-bit data octet is mapped into a 10-bit code group for serial bit transmission. Similar to earlier E