Serial Communications

Summary

This video explains serial communication, focusing on its role in wide area networks (WANs) and leased line connections. It covers how serial links use time-division multiplexing (TDM) to transmit multiple channels, such as phone calls or data, over a single circuit like T1. The video also discusses various protocols like HDLC (High-Level Data Link Control) and PPP (Point-to-Point Protocol), their differences, and their usage in Cisco devices. It touches on synchronous and asynchronous circuits, and the evolution of packet switching technologies like X.25, Frame Relay, and MPLS.

Takeaways

• 😀 Serial communication is a method of data transmission where bits are sent sequentially, one after another, as opposed to parallel communication, which sends multiple bits at a time.
• 😀 Leased lines, also known as serial links or connections, are permanent, dedicated connections between two sites that provide a continuous, always-on connection paid for by the subscriber.
• 😀 Serial communication is often used for longer distances, while parallel communication is better for short-distance data transfer.
• 😀 The T1 circuit, also known as DS1, is a digital service hierarchy used in serial communication, capable of transmitting 24 separate channels of 64,000 bits per second each.
• 😀 The North American digital services hierarchy uses T1 circuits, while Europe uses the SONET (Synchronous Optical Network) and other optical carrier standards.
• 😀 HDLC (High-Level Data Link Control) is the default encapsulation method for point-to-point links on Cisco devices, and it is commonly used for serial communication.
• 😀 PPP (Point-to-Point Protocol) is similar to HDLC but includes additional features such as security mechanisms (PAP and CHAP) and is used in various types of serial connections.
• 😀 Synchronous circuits use a common timing signal between devices, whereas asynchronous circuits do not require synchronization and are common in public switched telephone networks (PSTN).
• 😀 Legacy protocols like X.25, Frame Relay, and ATM were once widely used for packet switching, but MPLS (Multiprotocol Label Switching) is now more common in modern networks.
• 😀 When troubleshooting serial communication issues, commands like 'show interface serial' and 'show controllers' can be used to verify encapsulation settings and physical connections on Cisco devices.

Q & A

What is serial communication, and how is it used in the context of leased line connections?

Serial communication refers to the transmission of data bits one after another over a single communication channel. In the context of leased line connections, it is used to establish a continuous, reserved, and always-on connection between two remote sites, often involving various equipment like multiplexers and repeaters.

How do serial communication and parallel communication differ?

Serial communication transmits data bits one at a time, while parallel communication sends multiple bits simultaneously. Although parallel communication can be faster over short distances, serial communication is used over longer distances due to synchronization issues and cost-effectiveness.

What is the significance of T1 circuits in serial communication?

T1 circuits are a type of digital service that can carry 24 channels, each transmitting 64,000 bits per second, over a single physical connection. This allows multiple phone calls or data streams to be transmitted simultaneously, often used in leased line and point-to-point connections.

What does 'HDLC' stand for, and why is it important in serial communications?

HDLC stands for High-Level Data Link Control, and it is the default encapsulation protocol for serial connections on Cisco devices. It defines the frame structure used for communication over point-to-point links, offering a standard method for data transmission.

What is the difference between synchronous and asynchronous circuits?

Synchronous circuits use a common timing signal between two devices to ensure synchronized data transfer, often provided by the service provider. Asynchronous circuits, on the other hand, do not require a common clock signal, and data is transmitted without a fixed time reference.

What role does the DCE cable play in serial communication?

The DCE (Data Communication Equipment) cable provides the clocking signal for synchronous circuits in serial communications. It is typically connected to the provider's equipment or a lab setup, ensuring that data transmission occurs with correct timing.

How does PPP compare to HDLC in serial communication?

PPP (Point-to-Point Protocol) is another type of encapsulation for serial links, similar to HDLC, but it includes additional features such as built-in security mechanisms like PAP (Password Authentication Protocol) and CHAP (Challenge Handshake Authentication Protocol). PPP is more flexible and commonly used in packet-switched networks.

What is Time Division Multiplexing (TDM), and how does it relate to leased lines?

Time Division Multiplexing (TDM) is a technique used to send multiple data channels over a single communication link by dividing the transmission time into discrete intervals. In leased line connections, TDM allows for the transmission of several 64kbps channels over a T1 circuit, enabling efficient use of bandwidth.

What is the significance of the term 'fragmented T1' in serial communication?

A fragmented T1 refers to purchasing only a portion of the full 1.544 Mbps capacity of a T1 circuit for data transmission. This allows for more flexible and cost-effective use of T1 circuits when full bandwidth is not necessary.

What does 'encapsulation' mean in the context of serial communication, and how do you configure it on Cisco devices?

Encapsulation in serial communication refers to the method of packaging data for transmission over the link. On Cisco devices, the default encapsulation for serial interfaces is HDLC. To configure this, you can use the 'encapsulation hdlc' command in the interface configuration mode.