Digital Signal Multiplexing

Study Notes

4DS-1 channels are multiplexed to form a DS-2 channel with a capacity of 6.312 Mbps, including an overhead of 168 Mbps, and can carry 96 conversations simultaneously.
7DS-2 channels are multiplexed to form a DS-3 channel with a capacity of 44.376 Mbps, including an overhead of 1.368 Mbps, and can carry 672 voice conversations simultaneously.
6DS-3 channels are multiplexed to form a DS-4 channel with a capacity of 274.176 Mbps, including an overhead of 16.128 Mbps, and can carry 4032 voice conversations simultaneously.

WDM is a variation of FDM used in optical fiber transmission.
Multiple transmissions with different wavelengths are combined and sent together via a single, high-capacity optical fiber.
At the receiver side, individual transmissions are retrieved by filtering out based on their wavelengths.
WDM has high accuracy compared to FDM.
Modern research allows up to 200 different channels to share a single high-capacity optical fiber line, known as DWDM (Dense WDM).

Stop and wait is the simplest flow control method.
In this method, the sender sends one frame at a time to the receiver and stops to wait for the acknowledgment.
The intermediate node stores the message in its entirety, checks for transmission errors, inspects the destination address, and then delivers it to the next node.
This method reduces network congestion due to store and forward method.
Each intermediate switching node requires large storage capacity.
Store and forward method introduces delay at each switching node, making it unsuitable for real-time applications.

Transmission media are the physical infrastructure components that carry data from one node to another.
Examples of transmission media include telephone wires, coaxial cables, and optical fibers.
Transmission media can be divided into two main categories: guided media and unguided media.

Guided media are typically based on physical cables.
Twisted pair wires and coaxial cables carry data in the form of electric current signals.
Optical fibers carry data in the form of light.
Twisted pair wires have two classes: Unshielded Twisted Pair (UTP) and Shielded Twisted Pair (STP).

Optical fibers use reflection to guide the light through the fiber.
The densities of the core and cladding of the fiber are adjusted to prevent refraction.
Modes of propagation in optical fibers include multimode and singlemode.
Multimode propagation has two types: step index and graded index.
Singlemode propagation uses a highly focused light beam that travels more or less horizontally.

Bus topology: a single cable connects all devices in the network.
Ring topology: devices are connected in a circular configuration.
Token ring network: a token passing mechanism ensures that every host in the network is given equal chance for data transmission.

Ethernet uses a bus topology.
A limited amount of data is sent when the bus is idle.
If a collision occurs, the transceiver informs the NIC, which stops the transmission and generates a jamming signal.
Ethernet uses a binary exponential back-off policy to resolve collisions.

Token ring network employs a mechanism called token passing.
All hosts on the token ring share the same physical medium and are arranged in a ring.
When a host wants to transmit data, it needs permission to do so.
The circulating mechanism of token frame ensures that every host in the network is given equal chance for data transmission.

FDDI is an alternative to Ethernet and token ring architecture.
It supports a data rate of 100 Mbps.
FDDI uses token passing for media access control.
It has a self-healing mechanism.
FDDI uses two independent wires to connect to every host.
The second ring is used as a backup for network failures, allowing for self-healing.