Generic Stream Encapsulation

File:GSE diagram.png

The protocol specification has been published as ETSI TS

102 606.ETSI TS 102 606: "Digital Video

Broadcasting (DVB); Generic Stream Encapsulation (GSE)

Protocol" An accompanying implementation guidelines

document has been published as ETSI TS 102 771.ETSI TS 102 771: "Digital Video Broadcasting (DVB); Generic Stream Encapsulation (GSE) implementation guidelines"

IP datagrams, Ethernet Frames, or other network layer packets are encapsulated in one or more GSE Packets. The encapsulation process adds control information such as the network protocol type and address label, and provides an overall integrity check when needed.

The payload frame may be encapsulated in a single GSE Packet or sliced into fragments and encapsulated in several GSE Packets. GSE Packets have in general variable length, in order to match the input IP traffic with minimum overhead.

GSE Packets may be sent in different Base Band frames, not necessarily consecutive or with the same transmission parameters (modulation format, coding rate). No constraint on the GSE Packet position within the Base Band frame is assumed. However, GSE Packets may not be reordered between the encapsulator and the de-encapsulator. In general, a Base Band frame can contain more than a single GSE Packet. Base Band frames may have fixed, or variable length.

GSE does not provide a mechanism for integrity check of single GSE Packet. A CRC-32 is only appended to the last fragment of a fragmented payload to verify the correctness of the reassembly operation. GSE relies on the physical layer being able to ensure the required error detection and/or correction probability.IETF {{IETF RFC|3819}}: "Advice for Internet Subnetwork Designers"

The GSE Packet header is highly dynamic and provides for many options. The minimum header is two bytes, comprising three flags fields, and a 12-bit payload length field. The diagram below shows all possible fields.

class="wikitable" style="text-align:center" |+ Unrolled GSE Header
width="4%"|bit offset ! colspan="1" width="3%"| 0 ! colspan="1" width="3%"| 1 ! colspan="2" width="6%"| 2-3 ! colspan="4" width="12%"| 4–7 ! colspan="8" width="24%"| 8-15 ! colspan="8" width="24%"| 16-23 ! colspan="8" width="24%"| 24-31
0 | colspan="1" | Start | colspan="1" | End | colspan="2" | Label Type | colspan="12"| GSE Length | colspan="8" bgcolor="#CCFFCC"| Fragment ID | colspan="8" bgcolor="#CCFFCC"| Total Length
32 | colspan="8" bgcolor="#CCFFCC"| Total Length | colspan="16" bgcolor="#CCFFCC"| Protocol Type | colspan="8" bgcolor="#CCFFCC"| Label (3 bytes)
64 | colspan="16" bgcolor="#CCFFCC"| Label (continuation, length 3 bytes) | colspan="16" bgcolor="#AADDBB"| Label (continuation, length 6 bytes)
96 | colspan="8" bgcolor="#AADDBB"| Label (continuation, length 6 bytes) | colspan="24" bgcolor="#FFBBBB"| ULE Extension Headers (Optional)
... | colspan="24" bgcolor="#FFBBBB"| ... | colspan="8" | Data
... | colspan="32"|   Data

The basic mechanism of GSE payload fragmentation uses the Start and End Flags, where the Start flag indicates the beginning of a payload frame, and the End flag indicates its end. This is shown in the diagram below.

class="wikitable" style="text-align:center" |+ Fragmentation Principle
width="10%" | Start ! width="10%" | End ! width="80%" | GSE Packet Content
1 | 0 | style="text-align:left" | Total payload size / Protocol type / Payload start
0 | 0 | style="text-align:left" | Payload continuation
0 | 1 | style="text-align:left" | Payload end / CRC-32

On DVB-S2, DVB-T2, and DVB-C2 the ACM/VCM modes may cause the Base Band frames to vary in size depending on the transmission conditions. Hence there may be situations where the first fragments of a payload frame have been sent, but the encapsulator is forced to set aside the current payload frame, and start working on a new one. This may e.g. occur when large fragments have been prepared while transmission conditions were fine, but suddenly the conditions deteriorate, and only small Base Band frames are available.

This is when the Fragment ID field becomes important. It is a short-term identification of the payload frame. Whenever the encapsulator needs to move on to the next payload frame, without having finished transmitting the previous one, it uses the next available Fragment ID. That way, up to 256 payload frames can be "kept open" at any time. The decapsulator uses the Fragment ID to pick the reassembly buffer in which to store the fragment.

The "Label Type" (LT) bits determine how the GSE packet address is encoded according to the following table:

class="wikitable" style="text-align:center" |+ Addressing Mode
width="10%" | LT bits ! width="80%" | Addressing mode
00 | style="text-align:left" | Indicates that a 6 bytes label is present and shall be used for filtering.
01 | style="text-align:left" | Indicates that a 3 bytes label is present and shall be used for filtering.
10 | style="text-align:left" | No label present. All receivers shall process this packet.
11 | style="text-align:left" | Label re-use: no label is present; the label is the same as the previous GSE packet in the same base band frame. LT=11 is also used for intermediate and end packets (i.e. Start bit 0). LT=11 shall not be used for the first GSE packet in a base band frame with Start bit 1.

Each GSE Packet containing the last fragment for a payload frame, carries a CRC-32 checksum over the payload frame. The checksum is used to detect loss of intermediate fragments.

The checksum is a 32 bit value calculated according to the generator polynomial represented by 0x104C11DB7:

$y\; =\; x^\{32\}\; +\; x^\{26\}\; +\; x^\{23\}\; +\; x^\{22\}\; +\; x^\{16\}\; +\; x^\{12\}\; +\; x^\{11\}\; +\; x^\{10\}\; +\; x^\{8\}\; +\; x^\{7\}\; +\; x^\{5\}\; +\; x^\{4\}\; +\; x^\{2\}\; +\; x^\{1\}\; +\; x^\{0\}$

If the last fragment of a payload frame is lost, the decapsulator can not directly detect that fact. It never sees the GSE frame with the End flag set and containing the CRC-32. For this situation, the decapsulator must choose a suitable time-out based on the data-rate and application.

Since GSE packets are directly inserted into base-band frames of the

modulation scheme, GSE products come in the form of "GSE Routers" or

"GSE Modems", which - from the outside - act very much like a DSL

Router or DSL Modem used by consumers. More generically these devices

are also referred to as "GSE Encapsulators". These products have a standard

IP network interface (most often Ethernet or a similar LAN

interface) to collect IP traffic which is to be forwarded over the

uni-directional link on the other end. To optimise the packaging into

base-band frames, these devices typically generate complete base-band

frames with the GSE packets as payload, which are then transferred to

the DVB-S2, DVB-T2 or DVB-C2 modulator through a second

interface.

Here is a (very likely incomplete) list of GSE en- and decapsulators:

• [https://web.archive.org/web/20120517023254/http://www.newtec.eu/ Newtec]
• [http://www.newtec.eu/products/professional-equipment/elevation/modems/ip-satellite-modem-el470/ EL470 IP Satellite Modem]
• [http://www.newtec.eu/products/professional-equipment/elevation/demodulators/ip-satellite-demodulator-el970/ EL970 IP Satellite Demodulator]
• [http://www.work-microwave.de/ WORK Microwave GmbH]
• [http://www.work-microwave.de/dvb-s_s2.html#modems Challenge Series Satellite High Speed DVB-S2 IP Modem SK-IP] {{Webarchive|url=https://web.archive.org/web/20110410220742/http://www.work-microwave.de/dvb-s_s2.html#modems |date=2011-04-10 }}
• [http://www.tebkom.at/ Tebkom GmbH]
• [https://web.archive.org/web/20150707163826/http://www.tebkom.at/product_odg.html ODG200 IP/DVB-S2 Encapsulator/Modulator with ACM support]
• [http://www.advantechwireless.com/ Advantech Wireless Inc.]
• [https://web.archive.org/web/20110903085805/http://www.advantechwireless.com/catalogue/products/amt75e-dvb-ss2-high-speed-broadcast-modem-2/ AMT 75e DVB-S/S2 High Speed Broadcast Modem]
• [http://www.comtechefdata.com/ Comtech EF Data Corporation]
• [http://www.comtechefdata.com/products/Advanced-VSAT-Series/pcdm-840.asp CDM-840 Remote Router]
• [http://www.transplaneta.com/ K.S.Transplaneta Ltd.]
• [https://web.archive.org/web/20120322130601/http://www.transplaneta.com/products/ip-encapsulator dpi4502 DVB2 (S2/T2/C2) compliant IPv4 / IPv6 Encapsulator]
• [http://dveo.com/ Computer Modules, Inc.]
• [http://dveo.com/broadcast-systems/ip-over-dvb-encapsulator.shtml DVB Rocket™/S2]
• [http://www.ayecka.com/ Ayecka Communication systems LTD]
• [http://ayecka.com/products-SR1.php#SR1 SR1 - Advance DVB-S2 demodulator with hardware based, wire speed, GSE Decapsulator]
• [http://ayecka.com/products-ST1.php#ST1 ST1 - Advance DVB-S2 modulator with hardware based, wire speed, GSE encapsulator]
• [http://ayecka.com/products-SM1.php SM1 - Advance DVB-S2 Modem with hardware based, wire speed, GSE EnCapsulator / DeCapsulator]

There are many IP-over-satellite service offerings, including for

instance ASTRA2Connect from SES or Tooway from

Eutelsat. Little detail is however known about the protocols used

since the receivers are provided as part of the service by the

operators and very little technical detail is disclosed.

{{Reflist}}

• [http://www.dvb.org/technology/standards/index.xml#multiplexing Obtain GSE Standard and Guidelines from DVB free of charge]
• [http://www.dvb.org/technology/fact_sheets/DVB-GSE-Fact-Sheet.0709.pdf DVB Fact Sheet on GSE]
• [https://web.archive.org/web/20110722013509/http://telecom.esa.int/telecom/www/object/index.cfm?fobjectid=30265 GSE project home page] at ESA
• [https://github.com/CNES/libgse Opensource implementation of GSE]

Category:Television technology

Category:Link protocols