Understanding Modems, Communication Servers, and Routers

Posted on Mar 27, 2025 in Computers

Networking Devices: Modems, Servers, and Routers

Modems: Facilitate interaction with quality voice services, channel service units (CSU), and digital service units (DSU), which in turn interact with T1/E1 lines using Integrated Services Digital Network (ISDN).

Communication Servers

Communication servers manage user connections, both with and without dialing capabilities.

WAN Physical Layer Standards

WAN physical layer standards include EIA/TIA-232, EIA/TIA-449, V.90, and X.21. The WAN physical layer also defines the interface between data terminal equipment (DTE, i.e., client) and data circuit-terminating equipment (DCE, i.e., service provider). The link layer protocols describe how WAN frames are transported between systems in a single data link, including services and protocols that operate over dedicated point-to-point, multipoint, and switched connections.

Link Layer Standards

  • HDLC (High-Level Data Link Control): While providing high-level data linking, it may not be universally compatible due to manufacturer-specific implementations for point-to-point and multipoint connections.
  • Frame Relay: Utilizes high-quality digital services and a simplified network architecture without error correction mechanisms, enabling faster data transmission compared to other protocols.
  • PPP (Point-to-Point Protocol): Similar to HDLC, operates in a connection-oriented manner, but is considered unreliable.

Routers: Core Components and Functions

Routers contain a CPU, memory, interfaces, and buses, serving as specialized computers for routing purposes. The operating system is often referred to as IOS. Routers make routing decisions using routing tables and routing protocols.

Router Components

  • RAM/DRAM: Stores routing tables, fast-switching cache, and packet queues. Provides memory for the execution of the configuration file. Content is lost upon shutdown or reset.
  • NVRAM: Non-volatile memory that stores the startup configuration file and router backup.
  • FLASH: Erasable and reprogrammable ROM containing the microcode image of the operating system. Allows for content updates and persists through shutdowns.
  • ROM: Contains diagnostic routines for power-on and boot processes, stored on a chip.
  • INTERFACE: Represents the network connection through which incoming and outgoing packets pass.

Router Modes

  • User EXEC: Display mode with limited capabilities; cannot modify configurations.
  • Privileged EXEC: Supports commands for debugging, testing, and router analysis, as well as configuration file management.
  • Setup Mode: An interactive dialog on the console that assists users in creating an initial configuration.
  • RXBOOT Mode: Maintenance mode used to recover lost passwords.

Routing Elements

Routing decisions are made at Layer 3 (Network Layer). The network layer uses the routing table to determine the optimal path for a packet. Routes are represented by numerical addresses. The network layer can grant relay connections or independent interconnections.

Router Functions and Routing Protocols

A router has two basic functions: route determination (choosing the best interface) and switching functions. Routing protocols (e.g., IP) use routing tables, providing the necessary mechanisms for information sharing. Routing protocols enable routers to communicate with each other to update and maintain routing tables. Common routing protocols include:

  • RIP (Routing Information Protocol)
  • IGRP (Interior Gateway Routing Protocol)
  • OSPF (Open Shortest Path First)