11.1 Hubs and switches 431 such as RS232, RS422, and RS485. Selecting network equipment requires an analysis of parameters such as area network type, form factor, memory, performance, and features. There are three types of enterprise network: local area network (LAN), metro area network (MAN), and wide area network (WAN). Form factor choices include chip, board and module. Memory specifications include random access memory (RAM), synchronous dynamic random access memories (SDRAMs), and Flash memory. Data rate, operating temperature, and number of concurrent connections are additional specifications to consider. In terms of features, some products are stackable, rack-mounted, hardened, or suitable for virtual LAN (VLAN). Others have an alarm or indicator, provide IP addressing, and have an integral firewall for security. Network equipment should meet the Restriction of Hazardous Substances (RoHS) directive from the European Union (EU). Full-duplex products can transmit data in both directions. 11.1 HUBS AND SWITCHES 11.1.1 Overview A hub (sometimes referred to as a concentrator) is to a network device which receives a data packet from one network node and retransmits to all other connected nodes. In its simplest form, a hub works by duplicating the data packets received at its entry port and making them available to all other ports,