MAC QoS Framework

Why Do We Need QoS?

Not all internet traffic is the same.

Think about it:

• A VoIP call needs instant delivery. Even 200ms delay ruins the conversation.
• A video stream needs steady bandwidth. Buffering is annoying.
• A file download just wants to finish eventually. Speed matters, timing doesn’t.
• An email can arrive whenever. Nobody notices a 5-second delay.

The problem: Without QoS, the network treats everything equally. Your important voice call competes with someone’s bulk download.

What is QoS?

QoS = Quality of Service

It means giving different treatment to different types of traffic based on their needs.

• Voice? High priority, low latency
• Video? Guaranteed bandwidth
• Downloads? Best effort, whatever’s available

QoS lets the network be smart about how it allocates resources.

The 5 Service Classes

WiMAX defines 5 service classes, from most demanding to least:

1. UGS (Unsolicited Grant Service)

For: VoIP, video conferencing, T1/E1 leased lines

What it does:

• Provides constant bit rate (CBR)
• Fixed-size grants at regular intervals
• No need to request bandwidth

Why “unsolicited”?

• The base station automatically sends bandwidth grants
• User doesn’t have to ask for it each time
• Zero delay for getting transmission slots

Example: A VoIP call needs 64 kbps every 20ms. UGS guarantees exactly that.

2. rtPS (Real-time Polling Service)

For: Streaming video, MPEG, live broadcasts

What it does:

• Supports variable bit rate (VBR)
• Base station polls the user periodically
• User can request bandwidth when polled

How polling works:

1. Base station asks: “Do you have data to send?”
2. User responds with how much bandwidth it needs
3. Base station grants the requested amount

Why not UGS?

• Video bit rate changes (action scenes vs still frames)
• UGS would waste bandwidth during quiet moments
• rtPS adapts to actual needs

3. ertPS (Extended Real-time Polling Service)

For: VoIP with silence suppression

The problem it solves:

• VoIP with silence suppression doesn’t send data during quiet periods
• UGS would waste bandwidth during silence
• rtPS would add latency when speech resumes

The solution:

• Works like UGS when you’re talking (automatic grants)
• Works like rtPS during silence (no wasted bandwidth)
• Best of both worlds

ertPS is basically “smart VoIP mode”

4. nrtPS (Non-real-time Polling Service)

For: FTP, file transfers, large downloads

What it does:

• Guaranteed minimum bandwidth
• Polling at regular intervals (but less frequent than rtPS)
• Can tolerate some delay

The difference from rtPS:

• rtPS: “I need bandwidth NOW for real-time content”
• nrtPS: “I need bandwidth eventually, but guarantee me a minimum”

Example: Downloading a large file. You want decent speed, but a few extra seconds doesn’t matter.

5. BE (Best Effort)

For: Web browsing, email, background updates

What it does:

• No guarantees at all
• Gets whatever bandwidth is left over
• Lowest priority

When it’s used:

• Checking email
• Browsing websites
• Any traffic that doesn’t need special treatment

BE is the “economy class” of WiMAX. You’ll get there, but no promises about when.

Quick Comparison

How a Connection Gets QoS

Step 1: User requests a connection

• Specifies what service class it needs
• Provides traffic parameters (bandwidth, latency)

Step 2: Admission control

• Base station checks: “Can I meet these requirements?”
• If network is overloaded, request may be rejected

Step 3: Connection granted

• Base station assigns resources
• Scheduler treats this connection according to its class

Step 4: Ongoing service

• UGS: automatic grants
• rtPS/ertPS: regular polling
• nrtPS: periodic polling
• BE: opportunistic access

Why This Matters

Without QoS:

• Voice calls would stutter
• Video would buffer constantly
• Real-time apps would be unusable

With QoS:

• Critical traffic gets priority
• Network resources used efficiently
• Different apps coexist happily

WiMAX QoS ensures that a VoIP call never loses to a file download.