1
Q
What does the OSI model stand for?
A
Open Systems Interconnection
2
Q
How many layers are there in the OSI model?
A
7 layers
3
Q
Name all seven layers
A
- Physical
- Data-Link
- Network
- Transport
- Session
- Presentation
- Application
4
Q
What is the primary purpose of the OSI model?
A
Standardizes functions, not implementations
5
Q
Main Comms-Reliability Principle for the Application Layer (Layer 7)
A
Make failure recoverable and observable
nothing you do at the app layer should require a perfect link; when failure happens, it’s cheap to retry and easy to diagnose.
6
Q
Main Comms-Reliability Principle for the Presentation Layer (Layer 6)
A
Protect content without fragility surprises
confidentiality & integrity that behave predictably over noisy links.
7
Q
Main Comms-Reliability Principle for the Session Layer (Layer 5)
A
Keep long-lived conversations alive across disruptions
sessions survive AOS/LOS, handovers, and key rotation with minimal human intervention
8
Q
Main Comms-Reliability Principle for the Transport Layer (Layer 4)
A
Match windows and timers to the space path
keep the pipe full without thrash; don’t mistake RTT/jitter for congestion
9
Q
Main Comms-Reliability Principle for the Network Layer (Layer 3)
A
Deliver even when paths change or disappear
the network layer routes around LOS, site outages, and policy quirks without losing data.
10
Q
Main Comms-Reliability Principle for the Data-Link Layer (Layer 2)
A
- Frame well
- Correct smartly
- Pace fairly
make frames robust to burst errors and play nicely with RTT and ARQ
11
Q
Main Comms-Reliability Principle for the Assumption Layer (Layer 1)
A
Win the margin and keep the loops locked
hold lock, stay linear, meet masks, and deliver enough margin where it matters (low elevation)
12
Q
What does the Application layer (Layer 7) encompass?
A
User-facing network services
Examples include HTTP, FTP, SMTP, DNS, CCSDS mission apps. PDU is Data.
13
Q
What is the role of the Presentation layer (Layer 6)?
A
Data representation/translation
Examples include TLS/SSL encryption, codecs, serialization (ASN.1, CBOR), compression. PDU is Data.
14
Q
What is the main function of the Session layer (Layer 5)?
A
Dialog control, sessions, checkpoints, resumes
Examples include RPC sessioning, NetBIOS sessions, gRPC streams. PDU is Data.
15
Q
What does the Transport layer (Layer 4) provide?
A
End-to-end reliability, flow/control, multiplexing
Examples include TCP (reliable), UDP (best-effort), SCTP; ARQ windows, ACK/NAK. PDU is Segment (TCP) / Datagram (UDP).
16
Q
What is the function of the Network layer (Layer 3)?
A
Logical addressing and routing between nodes
Examples include IP, ICMP, CCSDS Space Packets w/ routing, DTN (BPv7). PDU is Packet.
17
Q
What does the Data Link layer (Layer 2) handle?
A
Framing on a single link; MAC addressing; error detection/correction
Examples include Ethernet (MAC/PHY split), AX.25, CCSDS TM/TC channel coding, HDLC. PDU is Frame.
18
Q
What is the focus of the Physical layer (Layer 1)?
A
Bits over a medium (voltages, light, RF)
Examples include 100BASE-TX, 802.11 OFDM, UHF QPSK with RRC shaping, symbol timing, carrier/AFC. PDU is Bit(s)/symbols.
19
Q
What is the process of encapsulation in the OSI model?
A
App data is wrapped as it goes down (add headers/trailers each layer)
20
Q
What is one benefit of using the OSI model for troubleshooting?
A
Helps identify which layer is failing
21
Q
What does the OSI model help with in terms of design partitioning?
A
Separates waveform/PHY (Layer 1) from framing/FEC (Layer 2), routing (Layer 3), and reliability (Layer 4)
22
Q
How does the OSI model enhance interoperability?
A
Provides clear contract surfaces for standards and vendors
23
Q
What is the relationship between the OSI model and the TCP/IP model?
A
Application + Presentation + Session (OSI 7–5) ≈ Application (TCP/IP), Transport (OSI 4) ≈ Transport (TCP/IP), Network (OSI 3) ≈ Internet (IP), Data Link + Physical (OSI 2–1) ≈ Link
24
Q
What is a typical symptom at Layer 1 (Physical)?
A
No lock, constellation smeared, high EVM, Doppler not tracked
25
What could indicate an issue at Layer 2 (Data Link)?
CRC fails, burst errors, framing loss, PER spikes despite good EVM
26
What might happen at Layer 3 (Network) if packets are routed incorrectly?
Packets routed wrong or dropped across hops
27
What is a possible Layer 4 (Transport) issue if there are good frames but poor throughput?
Window/timeout mismatched to RTT; flow control issues
28
What are some typical issues that could arise at Layers 5–7?
App/encoding/crypto mismatches, schema errors
29
What is a one-liner summary of the OSI model?
The OSI model is a 7-layer blueprint that separates bits & RF (L1), frames & error control (L2), routing (L3), end-to-end reliability (L4), and application semantics (L5–7)
30
What does Layer 1 of the OSI model focus on?
Physical (SDR DSP & RF)
## Footnote
This layer includes antenna/polarization, front-end NF/selectivity, TX PA headroom, and clocks.
31
What are common symptoms indicating issues at Layer 1?
Bursty losses correlated with elevation, body rate, polarization changes, PA temperature, or Doppler residual
## Footnote
These symptoms often show up as constellation spreads, increased EVM, and dips in Eb/N0.
32
What actions can be taken to mitigate Layer 1 issues?
* Lower symbol rate
* Strengthen FEC/interleaving
* Widen AFC capture
* Tune loop bandwidths
* Ensure RRC roll-off match
* Implement dual-pol or diversity combining
* Fix IF bandwidth / decimation / AGC law
* Lock LO with GPSDO
33
What key performance indicator (KPI) is used to measure Layer 2 performance?
Packet Error Rate (PER)
## Footnote
This layer includes CCSDS TM/TC framing, LDPC/RS/convolutional FEC, interleaver, and CRC.
34
What are the failure fingerprints for Layer 2?
CRC failures or lost frames in bursts despite clean EVM/SNR
## Footnote
Site-to-site PER differences with identical passes can indicate GS profile drift.
35
What actions can be taken to address Layer 2 issues?
* Confirm exact framing/scrambler and code ID
* Correct LLR scaling and decoder iterations
* Increase interleaver depth
* Add dual-pol diversity
* Freeze GS profiles with hashes
36
What is the main function of Layer 3 in the OSI model?
Routing/addressing
## Footnote
This includes IP over CCSDS and CCSDS Space Packets.
37
What symptoms indicate issues at Layer 3?
Frames arrive (L2 OK) but telemetry disappears later
## Footnote
Mismatched VCs/VCIDs or route tables can also indicate problems.
38
What actions should be taken to verify Layer 3 functionality?
Packet capture at L3; verify VC mapping and routes during handovers
39
What is the role of Layer 4 in the OSI model?
Transport (end-to-end reliability/flow)
## Footnote
This layer includes TCP/UDP, CFDP, and link-local ARQ.
40
What symptoms suggest Layer 4 issues?
Only some packets delivered despite good L1/L2
## Footnote
Issues may arise from window/timeout mismatches or periodic stalls.
41
What actions can be taken to resolve Layer 4 issues?
* Size window to W ≈ 2·R·RTT
* Set ACK/timeout accordingly
* Drive ACM from SDR Eb/N0/EVM
42
What are the primary functions of Layers 5-7 in the OSI model?
Session/Presentation/Application
## Footnote
These layers handle encryption/compression, CFDP sessions, and mission applications.
43
What symptoms indicate issues at Layers 5-7?
Bit-level errors magnified by crypto/auth
## Footnote
Schema/codec mistakes can be misinterpreted as RF loss.
44
What is the first step in cross-layer triage for a CubeSat fleet?
Prove L1 first: log Eb/N0, EVM, residual CFO, timing error, AGC level, PA temp/VSWR, body rates, GS profile hash
45
What is the minimal acceptance matrix for Layer 1?
Constellation clean, lock stable
## Footnote
EVM below spec at high SNR; residual CFO within loop linear; Eb/N0 vs elevation matches budget.
46
What actions fall under 'SDR-only' fixes?
* RRC match
* AFC/TED tuning
* FEC/interleaver selection
* ACM hooks
* Profile control
* I/Q capture
* Diversity combining
47
What actions require broader system changes?
* Antenna/polarization hardware
* PA headroom/PDN
* On-board FDIR/task priorities
* ARQ window/RTT policies
* Multi-GS scheduling
* Encryption overhead choices
48
Fill in the blank: The __________ is the first layer to check for intermittent packet loss.
Layer 1
49
True or False: Layer 2 actions can include increasing interleaver depth to exceed observed burst length.
True
50
What should the acceptance check for Layer 2 ensure?
PER meets target at 5th-percentile elevation
51
What does 'exact' mean in the context of modem/PHY parameters?
Produce a single, versioned table with tolerances for carrier & clocks, waveform, sync, framing, FEC/interleaver, link-layer, and ops metadata
## Footnote
Includes parameters like center frequency, modulation type, and error correction metrics.
52
What calibration signals should be emitted in a cooperative path?
1. CW tone burst
2. PRBS beacons
3. Mode toggles
## Footnote
These help measure parameters like residual CFO and confirm RRC match.
53
What should be logged in the radio configuration and housekeeping?
1. Center frequency
2. Symbol rate
3. Preamble
4. Scrambler enable
5. FEC mode ID
6. TX power
7. PA temperature
8. VSWR
## Footnote
Also includes logging AFC/PLL states and decoder metrics.
54
What is the purpose of golden lab parity?
Capture golden I/Q at multiple SNRs to serve as references for ground SDR to reproduce
## Footnote
Ensures that BER/PER waterfalls are within ~0.2 dB.
55
How is the symbol rate (R_s) estimated in a black-box path?
1. Cyclostationary/Pilotless baud estimation
2. Gardner TED variance sweep
## Footnote
Both methods help identify peaks in the cyclic spectrum.
56
What methods are used to estimate pulse shape/roll-off (α)?
1. Spectral fit
2. Matched-filter SNR scan
## Footnote
These methods determine the roll-off that maximizes post-filter SNR.
57
What techniques help identify modulation type?
1. Higher-order cumulants
2. EVM vs SNR behavior
## Footnote
These methods differentiate between modulation schemes like BPSK and QPSK.
58
How can preamble/sync and framing be analyzed?
1. Cross-correlate for known ASMs
2. Measure preamble length
3. Bit/byte alignment and scrambling
## Footnote
Descramble trials can help maximize sync hit-rate.
59
What is the purpose of polynomial search for scrambler & CRC?
Identify the correct polynomial that yields flat symbol statistics and valid CRCs across frames
## Footnote
Wrong scrambler leaves spectral lines; the correct one whitens payload.
60
What are the identifiers for FEC family and rate?
1. Convolutional/RS identification
2. LDPC codeword vs payload length
3. Iteration & LLR scaling
## Footnote
These methods confirm FEC characteristics and performance.
61
How is interleaver type/depth determined?
1. Burst-length CDF
2. Depth sweep
## Footnote
Determines minimal interleaver depth for optimal performance.
62
What techniques are used for ARQ/window/RTT analysis?
1. Timestamp frames/ACKs
2. Measure RTT
3. Timeout histogram
## Footnote
These techniques help derive window size and detect retransmits.
63
How are AFC/PLL loop parameters estimated?
1. Residual frequency trajectory
2. Slip statistics
## Footnote
Fitting to a loop model helps identify capture and loop bandwidth.
64
What methods are used to estimate clocks & ppm?
Average residual CFO over high-elevation windows to estimate XO ppm
## Footnote
This can be done using CW-like segments or long payloads.
65
What is the goal of multi-station parity & profile control?
Prevent false attribution by locking GS profiles and ensuring golden I/Q replay across stations match
## Footnote
Ensures consistency in decoding across different ground stations.
66
What is included in the minimal pass plan for satellite contacts?
1. Pass 1: Record full-rate I/Q
2. Pass 2: Diversity via two GS
3. Pass 3: Rate/FEC inference
## Footnote
Each pass serves a specific purpose in data collection and analysis.
67
What acceptance tests confirm modem accuracy?
1. BER/PER waterfall matches spec
2. Sync hit-rate and acquisition time
3. Interleaver depth meets fade requirements
4. RTT/window sizing approaches BDP ceiling
5. Reproducibility of golden I/Q
## Footnote
These tests validate the modem's performance and configuration.
68
What are common pitfalls in modem analysis?
1. Wrong roll-off or IF bandwidth
2. LLR scaling/decoder iterations off
3. Scrambler mismatch
4. AFC capture too tight
5. GS config drift
## Footnote
Identifying and avoiding these pitfalls is crucial for accurate analysis.
69
What is the TL;DR workflow for modem parameter analysis?
1. Cooperative? Emit CW + PRBS + toggle modes
2. Otherwise: Use analysis techniques on I/Q
3. Prove with waterfalls and golden replays
4. Freeze in a versioned modem profile
## Footnote
This summarizes the steps for effective modem analysis.
70
List all layers in order
1. Physical (PHY)
2. Data Link
3. Network
4. Transport
5. Session
6. Presentation
7. Application
71
What is the highest layer of the OSI model?
Application layer
72
What interfaces does the Application layer expose downward?
Presentation APIs or directly to Transport sockets
73
List common protocols used in the Application layer.
* HTTP(S)/REST
* gRPC
* MQTT
* AMQP
* SNMP
* CCSDS Space Packet Protocol
74
What are the primary responsibilities of the Application layer?
* Business/mission logic
* Command/telemetry semantics
* File/asset transfer
* Observability endpoints
* AuthN/authZ policy hooks
75
Identify typical vulnerabilities of the Application layer.
* Input validation
* Auth/session bugs
* Command authorization
* Replay/CSRF on consoles
* Misconfigured TLS
* Privacy leakage in logs/telemetry
76
What hardware is commonly associated with the Application layer?
* Servers
* Operator consoles
* Flight computers
* Payload controllers
77
What is the primary function of the Presentation layer?
Serialization/deserialization and data representation
78
List common protocols and encodings used in the Presentation layer.
* TLS 1.2/1.3
* DTLS
* SSH
* CBOR
* JSON
* ASN.1
* Protobuf
79
What are the responsibilities of the Presentation layer?
* Data representation
* Encryption
* Integrity/authentication
* Compression
* Session presentation
80
Identify vulnerabilities associated with the Presentation layer.
* Weak cipher suites
* Nonce/IV misuse
* Padding or mode misuse
* Downgrade attacks
* Decompression bombs
81
What hardware is relevant to the Presentation layer?
* Crypto accelerators
* HSM/TPM
* Secure enclaves
82
What is the primary function of the Session layer?
Session establishment and teardown
83
List typical protocols used in the Session layer.
* TLS sessions
* SSH channels
* HTTP/2 streams
* QUIC connections
* CCSDS SDLS
84
What responsibilities does the Session layer have?
* Checkpointing & resumes
* Dialogue control
* Multiplexing logical streams
* Key rollover
85
Identify vulnerabilities associated with the Session layer.
* Session fixation/hijack
* Ticket/key reuse
* Poor timeout/idle policies
86
What hardware is commonly associated with the Session layer?
Primarily software; Ground key management appliances
87
What is the primary function of the Transport layer?
Reliable and ordered byte streams or messages
88
List common protocols used in the Transport layer.
* TCP
* UDP
* QUIC
* SCTP
* CCSDS Proximity-1 transport
89
What are the responsibilities of the Transport layer?
* Reliability
* Flow control
* Congestion control
* Segmentation/reassembly
90
Identify vulnerabilities associated with the Transport layer.
* RST/DoS attacks
* SYN floods
* Head-of-line blocking
91
What hardware is relevant to the Transport layer?
* Host NIC offloads
* User-space stacks
* Flight OBC network stack
92
What is the primary function of the Network layer?
Logical addressing and routing
93
List common protocols used in the Network layer.
* IPv4/IPv6
* ICMP/ND
* IPsec
* MPLS
94
What are the responsibilities of the Network layer?
* Routing
* Fragmentation
* QoS marking
* Tunneling/VPN
95
Identify vulnerabilities associated with the Network layer.
* BGP/route hijack
* IP spoofing
* Header exposure
96
What hardware is commonly associated with the Network layer?
* Routers
* Firewalls
* DTN nodes
97
What is the primary function of the Data Link layer?
Framing and addressing
98
List common protocols used in the Data Link layer.
* Ethernet
* Wi-Fi
* PPP/HDLC
* CCSDS TM/TC frames
99
What are the responsibilities of the Data Link layer?
* Error detection
* Flow control
* Multiplexing
* Synchronization
100
Identify vulnerabilities associated with the Data Link layer.
* CRC bugs
* MAC spoofing
* VLAN bleed
101
What hardware is relevant to the Data Link layer?
* NICs
* Switches
* Modems
102
What is the primary function of the Physical layer?
Bit-to-waveform mapping
103
List common protocols/specs used in the Physical layer.
* Modulations (BPSK/QPSK/QAM)
* Pulse shaping
* Coding on the wire
104
What are the responsibilities of the Physical layer?
* TX power
* Frequency accuracy
* Spectral compliance
105
Identify vulnerabilities associated with the Physical layer.
* Low SNR
* Interference
* Oscillator drift
106
What hardware is commonly associated with the Physical layer?
* Antennas
* Filters
* ADC/DAC
107
True or False: The Application layer is responsible for error detection.
False
108
Fill in the blank: The _____ layer provides logical addressing and routing.
Network
109
What is a cross-layer interface that matters in SDR/space systems?
* App ↔ Transport
* Presentation ↔ Link/PHY
* Transport ↔ Link
* Link ↔ PHY
* Network ↔ Link
110
List some mitigation strategies for vulnerabilities in the Application and Presentation layers.
* Strict authZ
* Rotate keys
* Sanitize telemetry & logs
111
What is the purpose of idempotent commands in the application layer?
Make failure recoverable and observable by ensuring that repeating commands is safe
## Footnote
Idempotent commands allow operations to be retried without unintended side effects.
112
What are application-level checksums used for?
Catch silent corruption and config drift
## Footnote
Examples include schema hash/profile_id in every message.
113
What is CFDP Class-2 used for?
Resumable transfers of big files with acknowledged delivery.
114
Define 'backoff + budgets' in the context of application reliability.
Expose SLA (deadlines, priorities) to lower layers; degrade gracefully.
115
What does telemetry hygiene refer to?
Emit precise, machine-parsable health/KPIs for fast operational decisions.
116
What is AEAD?
Authenticated Encryption with Associated Data, used to protect content without fragility surprises.
117
What is the recommended nonce discipline for AEAD?
Use correct nonce discipline; 16-byte tags unless proven acceptable.
118
Why is it important to choose the right packet sizes in the presentation layer?
Crypto makes any bit error a drop; MTUs must support post-FEC BER.
119
When should compression be used?
When CPU headroom and latency allow; never compress already-compressed payloads.
120
What should be included in a strict TLS/DTLS configuration?
Modern ciphers only; pin minimum versions; session resumption.
121
What are the benefits of short sessions in the session layer?
Prefer protocols with stream resumption for easy resumes.
122
How should keys be managed during long-lived sessions?
Rotate keys on time/bytes; never stall sessions on KMS hiccups.
123
What is meant by bounded replay windows?
Practical windowing to avoid false drops during reorders.
124
What transport protocol is recommended for RTT/blackouts?
CFDP (Class-2) or LTP/DTN.
125
What is the rule for keeping the send window size?
Window ≥ 2×BDP to match RTT & target throughput.
126
What is the formula for RTO?
RTO = μRTT + 4σ.
127
True or False: Double-ARQ traps should be avoided in strong link-layer ARQ scenarios.
True.
128
What does DTN (BPv7) overlay provide?
Custody transfer across stations; survives line-of-sight and handovers.
129
What is the importance of MTU discipline?
Fit IP/DTN bundles to link frame sizes to avoid fragmentation.
130
What should be done with traffic classes?
Mark critical TC/ACKs higher than bulk TM; enforce at ground station routers.
131
What is meant by strong FEC?
Forward Error Correction tuned to burst CDF.
132
What should link-layer ARQ be sized to?
RTT; window W ≥ 2×BDP (in frames).
133
What is definitive framing?
Clear ASM, CRC/FECF; include VC/seq in AAD if encrypted.
134
What is ACM with hysteresis?
Adaptive Coding and Modulation; switch modes only when margin is stable.
135
What strategy should be used for polarization?
Dual-linear or circular based on mission needs.
136
What is the goal of telemetry for physics?
Per-frame Eb/N0/EVM/loop states + I/Q snapshots to diagnose issues.
137
What does profile immutability ensure?
Version-locked configs with `profile_id`; SDR refuses 'dirty' runtime.
138
What does golden-replay CI do?
Guarantees receiver determinism; finds regressions early.
139
What is root-cause labeling used for?
Classifying drops as RF vs SW/Queue vs ARQ for appropriate remediation.
140
What is the minimal reliability checklist for PHY?
+3 dB margin @ mask; AFC cap set; PA back-off set at hot; dual-pol enabled or justified.
141
Fill in the blank: The minimal reliability checklist for LINK includes code/interleaver sized to _______.
[crypto BER target].
142
What should be tuned for NET/TRANSPORT?
DTN/CFDP for disruption; QUIC/TCP params tuned to RTT; MTU aligned with frames.
143
What are the key components of the minimal reliability checklist for SESS/PRES?
AEAD with correct nonce; resumable sessions; sane TLS.
144
What should be included in the APP/OPS reliability checklist?
Idempotent commands; resumable files; clear KPIs; signed profiles; golden CI; on-pass root-cause labeling.
145
What is the primary outcome of good engineering across these layers?
Reliability stops being luck—it becomes the inevitable outcome.
146
What is the main goal of the Application layer in the layer philosophy?
Make failure recoverable and observable
## Footnote
Ensuring that operations at the app layer do not require a perfect link
147
What are idempotent commands?
Commands that have a unique ID + epoch; repeating them is safe
## Footnote
Includes a 'do-once until T' guard on the spacecraft
148
Define resumable data flows.
Chunk artifacts into numbered parts with integrity checks; support resume & sparse completion
## Footnote
Follows CFDP Class-2 semantics
149
What is the purpose of explicit contracts in the Application layer?
Schema version + profile_id stamped in every app message
## Footnote
Rejection of unknown versions with human-readable reasons
150
What is graceful degradation?
Sending decimated telemetry or compressed summaries when bandwidth drops
## Footnote
A design pattern for operational modes
151
What should every request/response in the Application layer carry?
Timestamps, execution status, error code, and a minimal trace ID
## Footnote
This bridges flight and ground operations
152
What are the key performance indicators (KPIs) for the Application layer?
Command success rate; median/95p completion time by command type; file transfer goodput vs PHY rate; retry causes
## Footnote
Examples of retry causes include RF, auth, and replay issues
153
Identify a common pitfall in the Application layer.
Hidden state with non-idempotent commands
## Footnote
Other pitfalls include silent schema drift and unbounded retries
154
What is the primary goal of the Presentation layer?
Protect content without fragility surprises
## Footnote
Ensuring confidentiality and integrity over noisy links
155
What does AEAD stand for?
Authenticated Encryption with Associated Data
## Footnote
Used everywhere in the Presentation layer
156
What is nonce discipline?
Using a per-packet nonce that is never reused
## Footnote
Derived from {sat_id, session_id, seq} and logged in telemetry
157
What should packet sizes be set to in the Presentation layer?
Set so required post-FEC BER is achievable
## Footnote
Example: 1000 B @ 1e-3 drop ⇒ BER ≤1.25e-7
158
What is selective AAD?
Including VC/seq/profile_id in AAD to detect misframing early
## Footnote
A design pattern to enhance reliability
159
What are the KPIs for the Presentation layer?
Overhead per packet; decrypt/verify latency; auth-fail vs replay-drop counts; handshake failure rate
## Footnote
These metrics help assess the performance of the Presentation layer
160
What is a major pitfall in the Presentation layer?
Nonce reuse
## Footnote
Other pitfalls include large packets that exceed BER targets
161
What is the goal of the Session layer?
Keep long-lived conversations alive across disruptions
## Footnote
Minimizing human intervention during AOS/LOS and key rotation
162
Define resumable sessions/streams.
QUIC streams or CFDP transactions with checkpoints
## Footnote
Designed to survive disruptions
163
What role do short-lived, renewable credentials play in the Session layer?
Allow token/ticket rotation without tearing down the data path
## Footnote
Renewing ahead of loss of signal (LOS)
164
What does state minimization entail in the Session layer?
Stashing session state in endpoints instead of the network
## Footnote
Ensures deterministic re-establish semantics
165
What are pause/resume hooks?
Explicit signals to pause before LOS and resume after AOS
## Footnote
These hooks are informed by pass predictors
166
What are the KPIs for the Session layer?
Resume success rate; rekey success rate; median resume time after AOS
## Footnote
Also includes session lifetime without manual touch
167
What is a common pitfall in the Session layer?
Sessions that hard-fail on brief loss
## Footnote
Key rotation blocking the link is another pitfall
168
What is the goal of the Transport layer?
Keep the pipe full without thrash
## Footnote
Avoid confusing RTT/jitter with congestion
169
What transport protocols are recommended for different scenarios?
DTN/LTP/CFDP for bulk file delivery; QUIC for interactive control; UDP for telemetry
## Footnote
Each protocol serves specific needs based on reliability
170
What should the window size be relative to BDP?
Ensure W ≥ 2×BDP
## Footnote
This helps manage data flow effectively
171
What does RTO stand for and how should it be tuned?
Retransmission Timeout; RTO = μRTT + 4σRTT
## Footnote
Measure per-pass to avoid default settings
172
What is ACK economy?
Delayed-ACK/ACK-thinning tuned to uplink scarcity
## Footnote
Ensures congestion control is not starved
173
What is a common pitfall in the Transport layer?
Window size less than BDP
## Footnote
This leads to the pipe never filling
174
What is the main goal of the Network layer?
Deliver even when paths change or disappear
## Footnote
Routing around LOS and outages without losing data
175
What is a DTN overlay?
Custody transfer across ground stations, tolerant of long gaps
## Footnote
Bundle fragmentation sized to link frames
176
What does QoS stand for and its purpose?
Quality of Service; prioritizing traffic classes
## Footnote
Ensures timely delivery of critical messages
177
What is MTU discipline?
Ensuring payloads fit CCSDS frame payload to avoid fragmentation loss
## Footnote
Critical for maintaining data integrity
178
What are the KPIs for the Network layer?
Bundle/packet delivery ratio; path switch loss; queueing delay by class
## Footnote
Fragmentation counts should be approximately zero
179
What is a common pitfall in the Network layer?
IP fragmentation leading to reassembly issues
## Footnote
Mixing traffic classes can also be problematic
180
What is the goal of the Data Link layer?
Frame well, correct smartly, and pace fairly
## Footnote
Making frames robust to burst errors
181
What is the right code + depth in the Data Link layer?
Choosing LDPC/BCH/R-S with a post-FEC BER target
## Footnote
Setting interleaver depth to 1.2–1.5× measured burst-95
182
What is definitive framing?
Using ASM with sane thresholds, CRC/FECF, and monotonic counters
## Footnote
Including seq/VC in AEAD AAD if encrypted
183
What is ACM with hysteresis?
Adaptive Coding and Modulation with thresholds for mode changes
## Footnote
Ensures stability during signal fluctuations
184
What are the KPIs for the Data Link layer?
PER vs elevation; FER vs Eb/N0 waterfalls; ARQ retransmit rate
## Footnote
Burst-length CDF and ACM flapping rate are also important
185
What is a pitfall in the Data Link layer?
Interleaver too shallow for measured bursts
## Footnote
ARQ windows set to incorrect values can also be an issue
186
What is the goal of the Physical layer?
Win the margin and keep the loops locked
## Footnote
Ensuring reliable communication even at low elevation
187
What is link margin at mask?
Ensure ≥ +3 dB margin at minimum elevation
## Footnote
Adjusting mask or mode if margin is insufficient
188
What is the purpose of acquisition & tracking?
AFC capture needs to be ≥ 1.2×(AOS Doppler + XO PPM)
## Footnote
Ensures fast reacquisition after signal slips
189
What is PA linearity?
Setting back-off at hot to meet EVM + mask with margin
## Footnote
Using DPD if available can help reclaim headroom
190
What are the KPIs for the Physical layer?
Eb/N0 & EVM vs elevation; lock/acquire times; residual CFO/timing error
## Footnote
Monitoring PA temp/current/VSWR vs EVM is also critical
191
What is a common pitfall in the Physical layer?
Too-narrow AFC/PLL capture for Doppler
## Footnote
Ignoring polarization dynamics can also lead to issues
192
What is the purpose of cross-layer glue?
Ensures reliability across all layers
## Footnote
Integrates profiles, golden determinism, and root-cause labeling
193
What should be included in a one-page 'ready to ship' checklist?
PHY: +3 dB margin; LINK: FEC & interleaver hit target; NET/TRANSPORT: DTN/CFDP used; SESSION/PRESENTATION: AEAD nonce; APPLICATION: Idempotent commands
## Footnote
This checklist helps ensure system reliability
ELARA/ODIN
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