Concepts and Principles of Operation
Service type | Number of channels |
DA64 | One B-channel |
DA128 | Two B channels |
Data Communication Services
Two widely used inter-networking mechanisms are bridging and routing. Bridging offers a straightforward method of interconnecting network segments. Bridges are simple to use. However, if you are bridging across a WAN using ISDN, you can incur unnecessary costs from the ISDN bill. A more controllable way of moving data cross networks is by routing. For Internet access, TCP/IP is the routing protocol. For Intranet access, IPX, and bridging for other protocols (e.g. NETBIOS) may be needed occasionally.
Bridging
A bridge automatically learns addresses of all active stations on its Local Area Network (LAN). It examines all the packets on the LAN, reading their source and destination addresses, and does not forward those packets which are intended for a local destination, while forwarding all others across the WAN to the next LAN. This means that traffic that is intended only for the local segment does not cross the bridge. However, there are some packets which a bridge must forward across the WAN to all parts of the network, such as broadcasts, multicasts and packets with unknown destinations to the bridge. The broadcast and multicast transmission or transmission to an unknown destination may initiate an ISDN call. Every ISDN call costs money. Bridging is not recommended unless absolutely necessary.
Routing
Routing is moving user data traffic across the inter-network from source to destination based on the network layer address. Along the way, at least one intermediate router is typically encountered. Routing and bridging is designed to accomplish precisely the same purpose. The primary difference between the two is that bridging occurs at Layer 2 (the link layer) of the OSI reference model, while routing occurs at Layer 3 (the network layer). This distinction provides routing and bridging with different information to use in the process of moving user data from source to destination. There are several different kinds of routing in use, most widely used are IP, and IPX routing.