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Ethernet Over SDH (EoS or EoSDH) or Ethernet over SONET refers to a set of protocols which allow Ethernet traffic to be carried over synchronous digital hierarchy networks in an efficient and flexible way. The same functions are available using SONET.
Ethernet frames which are to be sent on the SDH link are sent through an "encapsulation" block (typically Generic Framing Procedure or GFP) to create a synchronous stream of data from the asynchronous Ethernet packets. The synchronous stream of encapsulated data is then passed through a mapping block which typically uses virtual concatenation (VCAT) to route the stream of bits over one or more SDH paths. As this is byte interleaved, it provides a better level of security compared to other mechanisms for Ethernet transport.[ citation needed ]
After traversing SDH paths, the traffic is processed in the reverse fashion: virtual concatenation path processing to recreate the original synchronous byte stream, followed by decapsulation to converting the synchronous data stream to an asynchronous stream of Ethernet frames.
The SDH paths may be VC-4, VC-3, VC-12 or VC-11 paths. Up to 64 VC-11 or VC-12 paths can be concatenated together to form a single larger virtually concatenated group. Up to 256 VC-3 or VC-4 paths can be concatenated together to form a single larger virtually concatenated group. The paths within a group are referred to as "members". A virtually concatenated group is typically referred to by the notation pathType-Xv, where pathType is VC-4, VC-3, VC-12 or VC-11 and X is the number of members in the group.
| Container (SDH) | Container (SONET) | Type | Payload Capacity (Mbit/s) |
|---|---|---|---|
| VC-11-Xv | VT-1.5-Xv SPE | Low Order | X x 1.600 (X = 1 to 64) |
| VC-12-Xv | VT-2-Xv SPE | Low Order | X x 2.176 (X = 1 to 64) |
| VC-2-Xv | - | Low Order | X x 6.848 (X = 1 to 64) |
| VC-3-Xv | STS-1-Xv SPE | High Order | X x 48.384 (X = 1 to 256) |
| VC-4-Xv | STS-3c-Xv SPE | High Order | X x 149.76 (X = 1 to 256) |
EoS also drops the "idle" packets of the Ethernet frame before encapsulating the Ethernet frame to GFP, which is recreated at the other end during decapsulation process. Hence this provide a better throughput compared to native Ethernet transport.
An additional protocol, called link capacity adjustment scheme (LCAS), allows the two endpoints of the SDH paths to negotiate which paths are working and can carry traffic versus which paths should not be used to carry traffic.
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Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by the American National Standards Institute and ITU-T for digital transmission of multiple types of traffic. ATM was developed to meet the needs of the Broadband Integrated Services Digital Network as defined in the late 1980s, and designed to integrate telecommunication networks. It can handle both traditional high-throughput data traffic and real-time, low-latency content such as telephony (voice) and video. ATM provides functionality that uses features of circuit switching and packet switching networks by using asynchronous time-division multiplexing. ATM was seen in the 1990s as a competitor to Ethernet and networks carrying IP traffic as it was faster and was designed with quality-of-service in mind, but it fell out of favor once Ethernet reached speeds of 1 gigabits per second.
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Packet over SONET/SDH, abbreviated POS, is a communications protocol for transmitting packets in the form of the Point to Point Protocol (PPP) over SDH or SONET, which are both standard protocols for communicating digital information using lasers or light emitting diodes (LEDs) over optical fibre at high line rates. POS is defined by RFC 2615 as PPP over SONET/SDH. PPP is the Point to Point Protocol that was designed as a standard method of communicating over point-to-point links. Since SONET/SDH uses point-to-point circuits, PPP is well suited for use over these links. Scrambling is performed during insertion of the PPP packets into the SONET/SDH frame to solve various security attacks including denial-of-service attacks and the imitation of SONET/SDH alarms. This modification was justified as cost-effective because the scrambling algorithm was already used by the standard used to transport ATM cells over SONET/SDH. However, scrambling can optionally be disabled to allow a node to be compatible with another node that uses the now obsoleted RFC 1619 version of Packet over SONET/SDH which lacks the scrambler.
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Generic Framing Procedure (GFP) is a multiplexing technique defined by ITU-T G.7041. This allows mapping of variable length, higher-layer client signals over a circuit switched transport network like OTN, SDH/SONET or PDH. The client signals can be protocol data unit (PDU) oriented or can be block-code oriented.
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