BDS DOCSIS Standards and Protocols 2 — Questions and Answers
Question 1: What does BPI+ stand for and what does it protect in DOCSIS?
- Basic Protocol Interface — protects headend hardware
- Baseline Privacy Interface Plus — encrypts downstream data between CMTS and modem (Correct answer)
- Broadband Protection Interface — encrypts upstream QAM channels
- Burst Protection Interface — prevents upstream collisions
Correct answer: Baseline Privacy Interface Plus — encrypts downstream data between CMTS and modem
BPI+ (Baseline Privacy Interface Plus) provides authentication and AES encryption of downstream traffic between the CMTS and each cable modem, preventing unauthorized access.
Question 2: What is the difference between a primary service flow and a secondary service flow in DOCSIS?
- Primary flows are downstream; secondary are upstream
- Primary flows are always active; secondary flows are created on demand for specific traffic types (Correct answer)
- Primary flows use QAM; secondary flows use OFDM
- Primary flows are encrypted; secondary flows are unencrypted
Correct answer: Primary flows are always active; secondary flows are created on demand for specific traffic types
A primary service flow exists for the life of the modem session and carries default traffic, while secondary service flows are provisioned dynamically to handle traffic with specific QoS requirements.
Question 3: In DOCSIS, what is a Service ID (SID)?
- A unique identifier assigned to each downstream channel
- A numerical label that maps upstream bandwidth reservations to a specific cable modem service flow (Correct answer)
- The modem's serial number used for authentication
- An IP address prefix assigned to a node segment
Correct answer: A numerical label that maps upstream bandwidth reservations to a specific cable modem service flow
A Service ID (SID) is assigned by the CMTS to each upstream service flow and is used to grant bandwidth allocation in MAP messages.
Question 4: What upstream channel access method does DOCSIS use to coordinate multiple modems sharing the same channel?
- CSMA/CD
- TDMA (Time Division Multiple Access) (Correct answer)
- FDMA
- Token Ring
Correct answer: TDMA (Time Division Multiple Access)
DOCSIS uses TDMA (and A-TDMA/S-CDMA in later versions), where the CMTS schedules transmission slots via MAP messages so modems take turns transmitting without collision.
Question 5: What is the significance of the DOCSIS 3.1 upstream OFDMA technology?
- It eliminates the need for ranging
- It allows multiple modems to share subcarriers simultaneously within the same upstream channel (Correct answer)
- It replaces TDMA with a purely frequency-based access method
- It doubles upstream power output
Correct answer: It allows multiple modems to share subcarriers simultaneously within the same upstream channel
OFDMA (Orthogonal Frequency Division Multiple Access) allows the CMTS to assign specific subcarrier groups to individual modems simultaneously, dramatically increasing upstream efficiency and throughput.
Question 6: What does the term 'node split' mean in the context of DOCSIS capacity planning?
- Replacing a single fiber node with two smaller nodes to reduce the number of homes per node segment (Correct answer)
- Splitting a downstream OFDM channel into two SC-QAM channels
- Dividing a modem's bonded channels between two CMTSes
- Separating upstream and downstream fiber strands
Correct answer: Replacing a single fiber node with two smaller nodes to reduce the number of homes per node segment
A node split replaces one large fiber node with two smaller nodes, each serving fewer homes, reducing contention and increasing the bandwidth available per subscriber.
BDS DOCSIS Standards and Protocols 2
What does BPI+ stand for and what does it protect in DOCSIS?