tailieunhanh - Practical TCP/IP and Ethernet Networking- P42

Practical TCP/IP and Ethernet Networking- P42: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | LAN system components 187 Figure Implementing a WAN with 2-port routers gateways Access routers Access routers are 2-port routers that use dial-up access rather than a permanent . connection to connect a LAN to an ISP and hence to the communications cloud of the Internet. Typical options are ISDN or dial-up over telephone lines using either the ITU kbps or ITU 56 kbps standard. Some models allow multiple phone lines to be used using multilink PPP and will automatically dial up a line when needed or redial when a line is dropped thereby creating a virtual leased line . Border routers Routers within an autonomous system normally communicate with each other using an interior gateway protocol such as RIP. However routers within an autonomous system that also communicate with remote autonomous systems need to do that via an exterior gateway protocol such as BGP-4. Whilst doing this they still have to communicate with other routers within their own autonomous system . via RIP. These routers are referred to as border routers. Routing vs bridging It sometimes happens that a router is confronted with a layer 3 network layer address it does not understand. In the case of an IP router this may be a Novell IPX address. A similar situation will arise in the case of NetBIOS NetBEUI which is non-routable. A brouter bridging router will revert to a bridge if it cannot understand the layer 3 protocol and in this way forward the packet towards its destination. Most modern routers have this function built in. Gateways Gateways are network interconnection devices not to be confused with default gateways which are the IP addresses to which packets are forwarded for subsequent routing indirect delivery . A gateway is designed to connect dissimilar networks and could operate anywhere from layer 4 to layer 7 of the OSI model. In a worst case scenario a gateway may be required to decode and re-encode all seven layers of two .