bstan- times. As Network Layer connectors, routers are protocol specific; indeed the earliest routers could only connect LANs with identical APPLICATION APPLICATION Network Layer protocols. LAYER LAYER 56.5.4 Gateways PRESENTATION PRESENTATION LAYER LAYER Network users who want to connect two totally dissimilar LANs together use a 'gateway'. There is still some confusion regarding the SESSION SESSION use of this term. Many people still refer to routers as gateways. The LAYER LAYER following definitions are now more widely accepted: TRANSPORT TRANSPORT 1. Router: a device that operates at Layer 3 of the OSI LAYER LAYER Model. 2. Gateway: a device that operates at Layers 4, 5, 6 and 7, (i.e. the NETWORK NETWORK Transport, Session, Presentation and Application Layers) of the OSI Model. LAYER LAYER DATA LINK DATA LINK Gateways effectively provide a protocol conversion service, at LAYER LAYER each of the higher layers of the OSI Model for the interconnection of LANs. Therefore, LANs with separate protocol formats such as NetBios and SNA can communicate through a gateway. PHYSICAL PHYSICAL Similarly, gateways permit different machine types to communi- LAYER LAYER cate. For example, IBM PCs and asynchronous terminals can com- municate with IBM hosts through gateways via protocol emulation. MEDIUM Gateways operating at the Session, Presentation and Application Figure 56.22 OSI model of a gateway Layers allow diverse architectures such as SNA or DECnet to Internetworking devices 56/15 Gateway Gateway Gateway Gateway Figure 56.23 Example of an extended network using gateways tially more data processing than the other internetworking devices. the paths between the bridges. The bridge deemed to be the root They are, therefore, slower, less transparent, less flexible and per- bridge is allocated the highest priority of all the bridges in the haps the most network specific of all the LAN interconnectivity network and the network is then configured and routes determined. devices. (See Figure 56.23.) Effectively, the bridges aut