Understanding Remote Access, Modems, and Network Devices
Remote Access
A dial-up network device accepts digital data from a PC and converts it into an analog signal (like a phone signal). These signals are received and converted back to digital format.
Modems
Modems modulate (convert binary to analog) and demodulate (convert analog to binary).
- Internal: Inserted into an expansion slot or integrated into motherboards.
- External: Has its own power supply and connects to a serial port.
Modem Elements
Modems consist of three circuits: a receiving circuit for digital data, a data output circuit, and an analog modem control unit.
Transponders
A network element that regenerates the incoming electrical signal to restore its original level (addressing attenuation). It operates at the physical layer, copying bits.
Attenuation is the deterioration of the signal when the cable length is significant. Transponders restore the signal to its initial state.
Repeaters
Using a repeater:
- Extends the length of a network cable.
- Converts a signal from one cabling system to another (e.g., coaxial to twisted pair).
- Connects different physical media.
- A transceiver changes the nature of the signal.
Advantages of Repeaters:
- Easy operation: simply copies bits from one network segment to another.
- No special configuration required (operates at the physical layer).
- Repeats the network signal at high speed.
Disadvantages of Repeaters:
- Not isolated from traffic problems (collisions spread).
Concentrators or Hubs
A hub divides what comes in one port to all other ports. It’s used in a star topology at the physical layer. When a signal reaches one port, it’s sent to all other ports. Also called a multi-port repeater.
Types of Hubs:
- Passive: Concentrates cables but does not regenerate the signal.
- Active: Concentrates cables and regenerates the signal.
Disadvantages of Hubs:
- If the hub fails, the network fails.
- Failure of one station does not affect the network.
- Does not analyze information, so it doesn’t prevent issues like excessive messages (flooding).
- Consumes resources.
Bridges
A network element with some control capabilities. Accepts and redistributes frames based on their content. Operates at Layer 2 (Data Link) of the OSI model (frames). Controls defective frames.
Bridge Functionality:
- Does not send flawed frames.
- Divides a LAN into two or more segments (improving operation).
- Connects two LANs with different protocols or Layer 2 transmission media.
Bridge Operation:
Initially, the bridge’s memory is empty. It receives all frames.
Switches
Operates at Layer 2 (Data Link) of the OSI model, similar to a bridge. It does not use IP addresses, so it lacks routing capabilities. Works with MAC addresses. Always local. Connects network segments instead of networks.
Switch Function:
Takes the MAC address of a frame and sends the information through the corresponding port. Does not connect networks, but segments. The memory table is populated by the user or auto-learned by the switch.