2.4.1 Defining QoS

Quality of Service is the ability of a network to provide different levels of service to different types of traffic.

Study Deep: The MOS Score (Mean Opinion Score)

How do we measure if QoS is "good enough" for humans?

• MOS 5: Excellent (Face-to-face quality).
• MOS 4: Good (Cell phone quality).
• MOS 3: Fair (Noticeable distortion).
• MOS 1: Unsatisfactory (Impossible to communicate).
• The Impact: If Jitter exceeds 30ms or Packet Loss exceeds 1%, the MOS score typically drops below 3.5, which is the "threshold of frustration" for most users.

2.4.2 Key Performance Indicators (KPIs):

1. Bandwidth (Throughput): The data rate (measured in bps).
2. Latency (Delay): The time it takes for a packet to travel from source to destination.

• Propagation Delay: Speed of light in fiber.
• Queuing Delay: Time spent in buffers.
• Processing Delay: Time spent checking headers.

1. Jitter (Delay Variation): The difference in latency between packets. High jitter kills VoIP and Video calls.
2. Packet Loss: The percentage of packets dropped. Real-time apps prefer a drop over a late packet; data apps (TCP) hate drops.

2.4.3 ATM Service Categories:

ATM was the first major standard to define formal QoS classes:

• Fixed Bit Rate (CBR): Emulates a circuit. Perfect for uncompressed voice.
• Real-Time Variable Bit Rate (rt-VBR): For compressed video/voice with strict timing.
• Non-Real-Time Variable Bit Rate (nrt-VBR): For critical data apps.
• Available Bit Rate (ABR): For normal data; the network provides feedback on how much speed to use.
• Unspecified Bit Rate (UBR): Best effort, no guarantees (like standard IP).

2.4.4 Class of Service (CoS) marking:

In Ethernet, QoS is managed using the 802.1p bits (3 bits, allowing 8 priority levels). In IP, we use the DSCP (Differentiated Services Code Point) bits (6 bits, allowing 64 classes).

2.4.5 RSVP (Resource Reservation Protocol):

RSVP is a signaling protocol that allows an application to "book" bandwidth across every router in a path before it starts sending data. It is the "fine-grained" approach to QoS.