De
DOCSIS 3.0 MAC and Upper Layer Protocols Interface Specification zegt er het volgende over:
5.2.2.2.2.1 Traffic Segmentation Overview
The upstream channels within the bonding group may have very different physical-layer characteristics. One channel may be 1280 ksps with QPSK data regions and TDMA framing while another may be 5.12 Msps with 64-QAM data regions and S-CDMA framing. The CMTS decides how to segment the bandwidth based on the bandwidth requested by the CM and the other traffic on the upstream channels. Figure 5–4 shows an example of four upstream TDMA channels with varying mini-slot sizes. Each row in the figure represents bandwidth across a single upstream channel. The vertical lines demarcate the mini-slot boundaries.
The letters and shadings in the figure represent the service flow to which the block of bandwidth has been allocated by the CMTS. Blocks E and D represent small grants to different flows supporting voice service. In this example, the CMTS chooses to grant A's request by using bandwidth on only Channels #1 and #2. Similarly the CMTS chooses to grant B's request by using only Channels #3 and #4. The CMTS chooses to grant C's request spread across all four upstream channels.
Figure 5–4 - Segmentation Example
Each contiguous group of mini-slots assigned to the same service flow on the same channel in the figure becomes a segment. Thus the grant to service flow B consists of 2 segments and the grant to service flow C consists of 4 segments. Since the grant to service flow A on Channel #1 consists of two portions separated by the grant to service flow E, the overall grant to service flow A consists of 3 segments: two on Channel #1 and one on Channel #2. Each of these segments is treated like a legacy grant from the standpoint of physical layer overhead. Each segment will need a preamble at the beginning and, if TDMA transmission is used, guard time at the end. The physical layer properties of each segment are specified by the channel's physical parameters and the segment's burst parameters. The set of channels over which the CMTS may segment bandwidth for a given service flow is called the service flow's Upstream Bonding Group. The Upstream Bonding Group is used by the CMTS to know on which channels it may allocate grants to a service flow. The Upstream Bonding Group is also used by the CM to know on which channels it may send requests and on which channels it must look for grants for a given service flow.
De modems sturen Request Frame (REQ) berichten naar de CMTS met het verzoek om te mogen zenden en de CMTS bepaalt wie er in welke upstream en op welk moment mag zenden. Om dat kenbaar te maken aan de modems in het coaxsegment worden er continue Upstream Bandwidth Allocation MAP DOCSIS management berichten via de downstreams verzonden samen met informatie over de samenstelling van de upstreams via de Upstream Channel Descriptor berichten en de MAC Domain Descriptors voor de configuratie van o.a. de downstreams. Onderstaand een telling van het aantal DOCSIS management berichten (PDU Count) dat een CMTS in een tijdsbestek van ongeveer 45 seconden via een downstream verzendt (meting op 274 MHz in mijn coaxsegment):
code:
1
2
3
4
5
6
7
| ==============================================================================================================================================================================================================================================================================
Destination Address Manufacturer Frequency FC Type Header/Frame Type MAC Management Message Type Source Address PDU Count Byte Count Comment
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
01-E0-2F-00-00-01 274 MAC Specific Header MAC Management Header Upstream Bandwidth Allocation C8-4C-75-XX-XX-4C 93684 5999484
01-E0-2F-00-00-01 274 MAC Specific Header MAC Management Header Upstream Channel Descriptor (DOCSIS 2.0/3.0 Only) C8-4C-75-XX-XX-4C 96 45408
01-E0-2F-00-00-01 274 MAC Specific Header MAC Management Header MAC Domain Descriptor C8-4C-75-XX-XX-4C 24 15168
01-E0-2F-00-00-01 274 MAC Specific Header MAC Management Header MAC Domain Descriptor C8-4C-75-XX-XX-4D 24 816 |
De CMTS wijst dus meer dan 2000 keer per seconde de zendtijd van de modems toe. Een deel van die zendtijd wordt gereserveerd voor administratief verkeer tussen modem en CMTS. Modems moeten immers ook de kans krijgen zich te registreren bij een CMTS en een modem moet ook met een bepaalde regelmaat Ranging Request berichten naar de CMTS versturen die door de CMTS dan weer met Ranging Responses beantwoord worden. Als dat proces niet goed verloopt krijg je van die T3/T4 errors in het status overzicht van de modem.
Overigens wordt er een upstream kanaal niet met één vast modulatie type gebruikt, het door de modem te gebruiken modulatie type hangt af van het soort toepassingen waarvoor de CMTS zend tijd toewijst. Die toepassingen worden gedefinieerd in de Upstream Channel Descriptor management berichten die de CMTS verstuurt. Onderstaand een voorbeeld van zo'n upstream kanaal definitie zoals verstuurd in mijn coaxsegment:
├─■ Upstream Channel Descriptor (DOCSIS 2.0/3.0 Only)
│ ├─■ Upstream Channel ID = 1
│ ├─■ Configuration Change Count = 11
│ ├─■ Mini-Slot Size = 2
│ ├─■ Downstream Channel ID = 73
│ └─■ TLVs:
│ ├─■ Modulation Rate = 5120 { Ksym/sec }
│ ├─■ Frequency = 52000000 { Hz }
│ ├─■ Maintain Power Spectral Density = off
│ ├─■ Ranging Required = no ranging required
│ ├─■ Preamble Pattern
│ │ └─■ data = 0000 03 F0 28 33 EB F0 28 33 EB F0 28 33 EB F0 28 33 ..(3..(3..(3..(3
│ │ 0010 EB F1 64 28 92 A9 97 47 67 DA 04 17 BB C2 75 8F ..d(...Gg.....u.
│ │ 0020 36 FF 57 39 35 0D C1 87 19 88 D3 D2 2B 60 3F 29 6.W95.......+`?)
│ │ 0030 6B 0D F3 DE C0 ED F3 DE C0 ED F3 DE C0 ED F3 DE k...............
│ │ 0040 C0 ED F1 64 28 92 A9 97 47 67 DA 04 17 BB C2 75 ...d(...Gg.....u
│ │ 0050 8F 36 FF 57 39 35 0D C1 87 19 88 D3 D2 2B 60 3F .6.W95.......+`?
│ │ 0060 29 6B 00 00 00 00 00 00 00 00 00 00 00 00 00 00 )k..............
│ │ 0070 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
│ ├─■ Burst Descriptor (DOCSIS 2.0/3.0)
│ │ ├─■ Interval Usage Code = Request
│ │ └─■ Upstream Physical-Layer Burst Attributes:
│ │ ├─■ Modulation Type = 16-QAM
│ │ ├─■ Differential Encoding = off
│ │ ├─■ Preamble Length = 36
│ │ ├─■ Preamble Value Offset = 396
│ │ ├─■ FEC Error Correction (T) = 0
│ │ ├─■ FEC Codeword Information Bytes (k) = 16
│ │ ├─■ Scrambler Seed = 338
│ │ ├─■ Maximum Burst Size = 0
│ │ ├─■ Guard Time Size = 22
│ │ ├─■ Last Codeword Length = fixed
│ │ ├─■ Scrambler on/off = on
│ │ ├─■ R-S Interleaver Depth (Ir) = 1
│ │ ├─■ R-S Interleaver Block Size (Br) = 2048
│ │ └─■ Preamble Type = QPSK1
│ ├─■ Burst Descriptor (DOCSIS 2.0/3.0)
│ │ ├─■ Interval Usage Code = Initial Maintenance
│ │ └─■ Upstream Physical-Layer Burst Attributes:
│ │ ├─■ Modulation Type = 16-QAM
│ │ ├─■ Differential Encoding = off
│ │ ├─■ Preamble Length = 384
│ │ ├─■ Preamble Value Offset = 6
│ │ ├─■ FEC Error Correction (T) = 5
│ │ ├─■ FEC Codeword Information Bytes (k) = 34
│ │ ├─■ Scrambler Seed = 338
│ │ ├─■ Maximum Burst Size = 0
│ │ ├─■ Guard Time Size = 48
│ │ ├─■ Last Codeword Length = fixed
│ │ ├─■ Scrambler on/off = on
│ │ ├─■ R-S Interleaver Depth (Ir) = 1
│ │ ├─■ R-S Interleaver Block Size (Br) = 2048
│ │ └─■ Preamble Type = QPSK1
│ ├─■ Burst Descriptor (DOCSIS 2.0/3.0)
│ │ ├─■ Interval Usage Code = Station Maintenance
│ │ └─■ Upstream Physical-Layer Burst Attributes:
│ │ ├─■ Modulation Type = 16-QAM
│ │ ├─■ Differential Encoding = off
│ │ ├─■ Preamble Length = 384
│ │ ├─■ Preamble Value Offset = 6
│ │ ├─■ FEC Error Correction (T) = 5
│ │ ├─■ FEC Codeword Information Bytes (k) = 34
│ │ ├─■ Scrambler Seed = 338
│ │ ├─■ Maximum Burst Size = 0
│ │ ├─■ Guard Time Size = 48
│ │ ├─■ Last Codeword Length = fixed
│ │ ├─■ Scrambler on/off = on
│ │ ├─■ R-S Interleaver Depth (Ir) = 1
│ │ ├─■ R-S Interleaver Block Size (Br) = 2048
│ │ └─■ Preamble Type = QPSK1
│ ├─■ Burst Descriptor (DOCSIS 2.0/3.0)
│ │ ├─■ Interval Usage Code = Advanced PHY Short Data Grant
│ │ └─■ Upstream Physical-Layer Burst Attributes:
│ │ ├─■ Modulation Type = 64-QAM
│ │ ├─■ Differential Encoding = off
│ │ ├─■ Preamble Length = 64
│ │ ├─■ Preamble Value Offset = 396
│ │ ├─■ FEC Error Correction (T) = 6
│ │ ├─■ FEC Codeword Information Bytes (k) = 76
│ │ ├─■ Scrambler Seed = 338
│ │ ├─■ Maximum Burst Size = 6
│ │ ├─■ Guard Time Size = 22
│ │ ├─■ Last Codeword Length = shortened
│ │ ├─■ Scrambler on/off = on
│ │ ├─■ R-S Interleaver Depth (Ir) = 1
│ │ ├─■ R-S Interleaver Block Size (Br) = 2048
│ │ └─■ Preamble Type = QPSK1
│ ├─■ Burst Descriptor (DOCSIS 2.0/3.0)
│ │ ├─■ Interval Usage Code = Advanced PHY Long Data Grant
│ │ └─■ Upstream Physical-Layer Burst Attributes:
│ │ ├─■ Modulation Type = 64-QAM
│ │ ├─■ Differential Encoding = off
│ │ ├─■ Preamble Length = 64
│ │ ├─■ Preamble Value Offset = 396
│ │ ├─■ FEC Error Correction (T) = 9
│ │ ├─■ FEC Codeword Information Bytes (k) = 232
│ │ ├─■ Scrambler Seed = 338
│ │ ├─■ Maximum Burst Size = 0
│ │ ├─■ Guard Time Size = 22
│ │ ├─■ Last Codeword Length = shortened
│ │ ├─■ Scrambler on/off = on
│ │ ├─■ R-S Interleaver Depth (Ir) = 0
│ │ ├─■ R-S Interleaver Block Size (Br) = 2048
│ │ └─■ Preamble Type = QPSK1
│ └─■ Burst Descriptor (DOCSIS 2.0/3.0)
│ ├─■ Interval Usage Code = Advanced PHY Unsolicited Grant
│ └─■ Upstream Physical-Layer Burst Attributes:
│ ├─■ Modulation Type = 16-QAM
│ ├─■ Differential Encoding = off
│ ├─■ Preamble Length = 64
│ ├─■ Preamble Value Offset = 396
│ ├─■ FEC Error Correction (T) = 9
│ ├─■ FEC Codeword Information Bytes (k) = 232
│ ├─■ Scrambler Seed = 338
│ ├─■ Maximum Burst Size = 53
│ ├─■ Guard Time Size = 22
│ ├─■ Last Codeword Length = shortened
│ ├─■ Scrambler on/off = on
│ ├─■ R-S Interleaver Depth (Ir) = 1
│ ├─■ R-S Interleaver Block Size (Br) = 2048
│ └─■ Preamble Type = QPSK1
Deze upstream wordt doorgeven op 52 MHz (52000000 Hz) met een symbol rate van 5120 Ksym/sec. Iedere Burst Descriptor tak is een bepaalde toepassing. Er zijn dus 6 verschillende toepassingen voor dit upstream kanaal gedefinieerd met ieder een eigen modulatie type, namelijk:
- Request (16-QAM)
- Initial Maintenance (16-QAM)
- Station Maintenance (16-QAM)
- Advanced PHY Short Data Grant (64-QAM)
- Advanced PHY Long Data Grant (64-QAM)
- Advanced PHY Unsolicited Data Grant (16-QAM)
De Data Grant toepassingen worden gebruikt om de modems data te laten verzenden en zijn tevens de momenten dat het modem het hoogst mogelijke modulatie type mag toepassen. Wat mogelijk is verschilt namelijk per upstream frequentie. Naarmate die frequenties lager worden is er sprake van meer ruis en zal er gekozen worden voor lagere modulatie types zoals bijvoorbeeld QPSK. Dat heeft natuurlijk gevolgen voor de haalbare bitrate binnen een upstream. Een modem laat veelal op de status pagina's het hoogst mogelijke modulatie type zien, maar dat wil dus niet zeggen dat dat modulatietype continu gebruikt wordt voor die upstream.
:strip_icc():strip_exif()/u/2646/shine.jpg?f=community)
/u/12038/crop5a51514a4dec1_cropped.png?f=community)
