N frame. The first field is a 2-octet type value indicating that the frame is a VLAN frame. The second field, also 2 octets long, contains a priority value from 0 to 7, with 7 being the highest priority. It also has a VLAN ID to identify the particular VLAN over which the frame is to be sent. Three advantages accrue not previously available to Ethernet users. 1. It provides a means to expedite time-critical network traffic by setting priorities for outgoing frames. 2. It simplifies network management by making adds, moves, and changes easier to administer. 3. It allows stations to be assigned to logical groups, to communicate across multiple LANs as though they were a single LAN. Here we find bridges and switches that filter destination addresses and forward VLAN frames only to ports of the VLAN to which the traffic is destined. There are three versions of the 100-Mbps Ethernet and all use UTP (unshielded twisted pair) cable: 100Base-TX, 100Base-T4, and 100Base-T2. These designations (e.g., 100Base-TX) are made up of three parts: 100 indicates the transmission rate or 100 Mbps. "Base" means the transmission is baseband, where the raw electrical signal is applied directly to the medium. In fact, all Ethernet transmission today is baseband. Broad- band RF transmission is now obsolete in this application. TX, T2, T4 indicates twisted pair or fiber-optic cable. T2, two twisted pair, T4, four twisted pair. Each of the three types of Ethernet uses different encoding and a different set of media- dependent sublayers. Table 11.1 gives an overview of the physical layer interface for the three versions compared to their 10-Mbps counterpart. 5VLAN stands for virtual LAN. ENTERPRISE NETWORKS I: LOCAL AREA NETWORKS Table 11.1 Summary of 100BASE-T Physical Layer Characteristics Ethernet Transmit Full-Duplex Version Symbol Rate Encoding Cabling Operation 10Base-T 10 MBd Manchester Two pairs of UTP Category 3 or Supported better 100Base-TX 125 MBd 4B/5B Two pairs of UTP Category 5 or Supported Type 1 STP 100Base-T4 33 MBd 8B/6T 