Generic Stream Encapsulation
Encyclopedia
Generic Stream Encapsulation, or GSE for short, is a Data link layer
protocol defined by DVB. GSE provides means to carry packet oriented protocols, like for example IP
, on top of uni-directional physical layer
s like e.g. DVB-S2
, DVB-T2
and DVB-C2
.
GSE provides additional features beyond the pure carriage of IP datagrams that increase the protocol flexibility and applicability. Some key GSE functions/characteristics are:
specification has been published as ETSI TS
102 606 . An accompanying implementation
guidelines
document has been published as ETSI TS 102 771.
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 .
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.
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:
If the last fragment of a paylod 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.
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:
instance ASTRA2Connect
from SES Astra
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.
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
protocol defined by DVB. GSE provides means to carry packet oriented protocols, like for example IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
, on top of uni-directional physical layer
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
s like e.g. DVB-S2
DVB-S2
Digital Video Broadcasting - Satellite - Second Generation is a digital television broadcast standard that has been designed as a successor for the popular DVB-S system. It was developed in 2003 by the , an international industry consortium, and ratified by ETSI in March 2005...
, DVB-T2
DVB-T2
DVB-T2 is an abbreviation for Digital Video Broadcasting – Second Generation Terrestrial; it is the extension of the television standard DVB-T, issued by the consortium DVB, devised for the broadcast transmission of digital terrestrial television....
and DVB-C2
DVB-C2
DVB-C2 is a digital cable transmission system developed by the DVB Project. It uses the latest modulation and coding techniques to enable highly efficient use of cable networks where, up to now, in many cases downstream transmission capacity is already being used to its limit...
.
GSE provides additional features beyond the pure carriage of IP datagrams that increase the protocol flexibility and applicability. Some key GSE functions/characteristics are:
- Support for multi-protocol encapsulation (IPv4IPv4Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
, IPv6IPv6Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
, MPEG, ATMAsynchronous Transfer ModeAsynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
, EthernetEthernetEthernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, 802.1pQIEEE 802.1QIEEE 802.1Q is the networking standard that supports Virtual LANs on an Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames...
VLANs, etc.) - Transparency to network layer functions, including IPInternet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
encryptionEncryptionIn cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
and IP header compressionVan Jacobson TCP/IP Header CompressionVan Jacobson TCP/IP Header Compression is a data compression protocol described in RFC 1144, specifically designed by Van Jacobson to improve TCP/IP performance over slow serial links. Van Jacobson compression reduces the normal 40 byte TCP/IP packet headers down to 3-4 bytes for the average case...
. - Support of several addressing modes. In addition to the 6-byte MAC addressMAC addressA Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
(including multicastMulticastIn computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
and unicastUnicastright|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
), it supports a MAC address-less mode, and an optional 3-byte address mode. - A mechanism for fragmenting IPInternet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
datagrams or other network layerNetwork layerThe network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
packets over Base BandBasebandIn telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
frames to support ACMLink adaptationLink adaptation, or adaptive coding and modulation , is a term used in wireless communications to denote the matching of the modulation, coding and other signal and protocol parameters to the conditions on the radio link Link adaptation, or adaptive coding and modulation (ACM), is a term used in...
/VCM. - Support for hardwareHardwareHardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....
filteringFirewall (computing)A firewall is a device or set of devices designed to permit or deny network transmissions based upon a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass....
. - Extensibility: additional link protocolLink protocolIn communications, a link protocol is a method and specification for transmission of data from one node on a local network or network link to another node on the same link...
s can be included through specific protocol type values (e.g. Layer 2 security, IP Header Compression, etc.).
Protocol Outline
The protocolCommunications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
specification has been published as ETSI TS
102 606 . An accompanying implementation
Implementation
Implementation is the realization of an application, or execution of a plan, idea, model, design, specification, standard, algorithm, or policy.-Computer Science:...
guidelines
document has been published as ETSI TS 102 771.
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 .
GSE Header
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.| bit offset | 0 | 1 | 2-3 | 4–7 | 8-15 | 16-23 | 24-31 | |||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | Start | End | Label Type | GSE Length | Fragment ID | Total Length | ||||||||||||||||||||||||||
| 32 | Total Length | Protocol Type | Label (3 Byte) | |||||||||||||||||||||||||||||
| 64 | Label (3 Byte) | Label (6 Byte) | ||||||||||||||||||||||||||||||
| 96 | Label (6 Byte) | ULE ULE ULE may refer to:* Unidirectional Lightweight Encapsulation, Protocol for network layer packets over MPEG transport streams.* ULE, a scheduler called SCHED_ULE in the FreeBSD OS... Extension Headers (Optional) |
||||||||||||||||||||||||||||||
| ... | ... | Data | ||||||||||||||||||||||||||||||
| ... | Data |
|||||||||||||||||||||||||||||||
Fragmentation and Reassembly
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.| Start | End | GSE Packet Content |
|---|---|---|
| 1 | 0 | Total payload size / Protocol type / Payload start |
| 0 | 0 | Payload continuation |
| 0 | 1 | Payload end / CRC-32 |
On DVB-S2
DVB-S2
Digital Video Broadcasting - Satellite - Second Generation is a digital television broadcast standard that has been designed as a successor for the popular DVB-S system. It was developed in 2003 by the , an international industry consortium, and ratified by ETSI in March 2005...
, DVB-T2
DVB-T2
DVB-T2 is an abbreviation for Digital Video Broadcasting – Second Generation Terrestrial; it is the extension of the television standard DVB-T, issued by the consortium DVB, devised for the broadcast transmission of digital terrestrial television....
, and DVB-C2
DVB-C2
DVB-C2 is a digital cable transmission system developed by the DVB Project. It uses the latest modulation and coding techniques to enable highly efficient use of cable networks where, up to now, in many cases downstream transmission capacity is already being used to its limit...
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.
GSE addresses
The "Label Type" (LT) bits determine how the GSE packet address is encoded according to the following table:| LT bits | Addressing mode |
|---|---|
| 00 | Indicates that a 6 bytes label is present and shall be used for filtering. |
| 01 | Indicates that a 3 bytes label is present and shall be used for filtering. |
| 10 | No label present. All receivers shall process this packet. |
| 11 | 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 shall not be used for the first GSE packet in a base band frame. |
CRC-32 Trailer
Each GSE Packet containing the last fragment for a payload frame, carries a CRC-32 checksumCyclic redundancy check
A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
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:
If the last fragment of a paylod 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.
Products Supporting GSE
Since GSE packets are directly inserted into base-band frames of themodulation 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
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
or a similar LAN
Län
Län and lääni refer to the administrative divisions used in Sweden and previously in Finland. The provinces of Finland were abolished on January 1, 2010....
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-S2
Digital Video Broadcasting - Satellite - Second Generation is a digital television broadcast standard that has been designed as a successor for the popular DVB-S system. It was developed in 2003 by the , an international industry consortium, and ratified by ETSI in March 2005...
, DVB-T2
DVB-T2
DVB-T2 is an abbreviation for Digital Video Broadcasting – Second Generation Terrestrial; it is the extension of the television standard DVB-T, issued by the consortium DVB, devised for the broadcast transmission of digital terrestrial television....
or DVB-C2
DVB-C2
DVB-C2 is a digital cable transmission system developed by the DVB Project. It uses the latest modulation and coding techniques to enable highly efficient use of cable networks where, up to now, in many cases downstream transmission capacity is already being used to its limit...
modulator through a second
interface.
Here is a (very likely incomplete) list of GSE en- and decapsulators:
- Newtec
- WORK Microwave GmbH
- gcs Global Communication & Services GmbH
- Advantech Wireless Inc.
- Comtech EF Data Corporation
- K.S.Transplaneta Ltd.
- Computer Modules, Inc.
GSE-based IP Service Offerings
There are many IP-over-satellite service offerings, including forinstance ASTRA2Connect
ASTRA2Connect
ASTRA2Connect is a two-way satellite broadband Internet service available across Europe, which launched in March 2007, and uses the ASTRA series of geostationary satellites...
from SES Astra
SES Astra
Astra is the name for the geostationary communication satellites, both individually and as a group, which are owned and operated by SES S.A., a global satellite operator based in Betzdorf, in eastern Luxembourg. The name is sometimes also used to describe the channels broadcasting from these...
or Tooway
Tooway
Tooway is a bi-directional satellite broadband Internet service available across Europe. The service was launched in 2007 via two Eutelsat geostationary satellites, Hot Bird 6 and Eurobird 3, respectively at the 13° and 33° East orbital positions....
from
Eutelsat
Eutelsat
Eutelsat S.A. is a French-based satellite provider. Providing coverage over the entire European continent, as well as the Middle East, Africa, India and significant parts of Asia and the Americas, it is one of the world's three leading satellite operators in terms of revenues.Eutelsat’s satellites...
. 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.