How Do I Choose the Best Modem?

A router is referred to as a router, which is a more complex port device than a bridge. It has various shapes, but the common point is that the router is an independent device. Unlike a bridge or a network card, it can be parasitic to a certain microcomputer. When making a remote connection, whether it is a router or a bridge, it needs to be used in conjunction with a modem. It has a stronger ability to inspect and guide the carrier circuit. ^[1]

Router Abbreviation

There are two types of routing protocols: internal gateway protocol (IGP) and external gateway protocol (EGP), each of which is used within an autonomous system and between autonomous systems. The IGP is divided into a distance vector and a link vector. The distance vector is to periodically exchange information about the entire routing table with neighboring routers, such as the Routing Information Protocol (RIPvl), IGRP. The link vector communicates link state information to all routers only when the link state changes, such as Open Shortest First (OSPF). EIGRP, for example, has the characteristics of both protocols. ^[3]

Routing is the process of constructing the routing table of network nodes. No matter what kind of packet network, routing is a basic function provided by the network. However, X.25 recommends that routing selection is not clearly defined. Different packet networks are allowed. There are different routing algorithms, how to establish the quality of the routing algorithm? The basic principles of packet routing are as follows: the algorithm is simple and easy to implement to reduce additional overhead; the algorithm is fair to all users; the transmission path with the best performance should be selected so that the end-to-end delay is as small as possible, and the network nodes The workload is balanced to maximize the utilization of network resources. When the network fails, the algorithm can still work normally when the network topology changes, and the circuitous route is automatically selected.

Different packet switched networks may take different routing options. Route selection can be divided into two types: dynamic method and static method.

Router static method

(1) Diffusion routing method, the packet is sent from the original node to its neighboring nodes, and the node receiving the packet checks whether it has received the packet, and if it has received it, it discards it; if it has not received As long as the destination node of the packet is not the node, the packet is broadcast to neighboring nodes, and eventually the packet will reach the destination node. Among them, the process that the packet that reaches the destination node earliest must be an optimal path. With diffuse routing, routing is not related to the network topology, even if the network is severely faulty. As long as there is a path, the packet can reach the end, so the transmission reliability of the packet is very high. However, the disadvantage is that the amount of invalid transmission of packets is large, and the overhead of the network is also large. The increase of traffic in the network will lead to an increase in queuing delay.

(2) Fixed routing table method. A routing table containing the address of the routing destination node and the corresponding output logical channel number is set in each node switch. He specifies the path that should be selected from this node to any destination in the network. The call request packet looks up the routing table according to the destination address of the packet, so that the output logical signals of each transfer node can be obtained, thereby forming an end-to-end virtual circuit. To prevent network failures or path congestion, the routing table can specify primary routes and backup routes.

Router Dynamics

(1) Adaptive routing network. The adaptive routing method refers to that the routing changes according to the change of network conditions. A route is made up of several links in series. The adaptive routing method uses an iterative method to select virtual links one by one to form an end-to-end virtual circuit. However, in this algorithm, all nodes are required to store the entire network topology data, and the change information of each link must be broadcast to all nodes in the network. The adaptive routing algorithm is beneficial to reduce network delay, smooth network load, and prevent network congestion, but frequent replacement of routing tables may cause network instability, generate packet loops, or cause packets to shuttle back and forth between a pair of nodes. Adaptive routing algorithm is the most common routing method used in X.25 packet networks.

(2) Centralized routing exchange. The network management center is responsible for the collection of network status information, route calculation, and download of the routing table. In a packet-switched network, multiple routes are generally selectable between switches. How to obtain a better route, in addition to having a routing algorithm with a short average delay through the network and a strong ability to balance the traffic on the network, it is also necessary to consider the utilization of network resources and the adaptability of the network structure. ^[5]