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The core header is always scrambled to ensure high bit transmission density when transmitting idle GFP frames. Payload scrambling is done using a 1 + x43 self-synchronous scrambler. As in ATM, scrambling is enabled starting at the first transmitted byte after the cHEC field, and disabled after the last transmitted byte of the GFP frame.
Client management provides a generic mechanism to propagate client-specific information, such as performance monitoring and OA&M information.
2.7.3 GFP Client-dependent Functions
The client data can be carried in GFP frames using one of the following two adaptation modes: frame-mapped GFP (GFP-F) or transparent-mapped GFP (GFP-T). Frame-mapped GFP is applicable to most packet data types, and transparent-mapped GFP is applicable to 8B/10B coded signals. In frame-mapped GFP, each received client-frame is mapped in its entirety in a single GFP payload. Examples of such client frames include Ethernet MAC frames, PPP/IP packets, and HDLC-framed PDUs.
The transparent-mapped GFP mode is used to transport continuous bit-rate, 8B/10B block-coded client data and control information carried by networks, such as fiber channel, ESCON, FICON, and Gigabit Ethernet. (Note that in the 8B/10B encoding scheme, each group of eight bits is coded by a 10-bit code. Coding groups of bits is known as block-coding.) Rather than transporting data on a frame-by-frame basis, the GFP transparent-mapped mode transports data as a stream of characters. Specifically, the individual characters are decoded from their client 8B/10B block codes and then mapped into periodic fixed-length GFP frames using 64B/65B block coding. Specifically, the 10-bit codes are first decoded into their original data or control codeword value, and then the decoded characters are mapped into 64B/65B codes. A bit in the 65-bit code is used to indicate whether the 65-bit block contains only data or whether control characters are also included. Eight consecutive 65-bit blocks are grouped together into a single super block, and N super blocks make up a single GFP frame. This procedure reduces the 25% overhead introduced by the 8B/10B block-coding; plus, it reduces latency, which is important for storage-related applications.
2.8 DATA OVER SONET/SDH (DOS)
The data over SONET/SDH (DoS) network architecture provides a mechanism for the efficient transport of integrated data services. The following are some of the features of DoS:
• It provides flexible bandwidth assignment with a 50-Mbps granularity.
• No modifications are required of the intermediate nodes.
SONET/SDH AND THE GENERIC FRAME PROCEDURE (GFP)
• Using GFP, it provides an efficient framing scheme with a small overhead.
• It can accommodate IP packets, Ethernet frames, and constant bit rate data and control information carried by fiber channel, ESCON, and FICON. In particular, it provides an effective mechanism to transport GbE, which has recently been widely deployed in wide area networks (WAN).
• Coexistence of the traditional voice services and the new data services in the same SONET/SDH frame.
• Network management through the SONET/SDH existing and quality-proven network management.
DoS uses three technologies: generic framing procedure (GFP), virtual concatenation, and link capacity adjustment scheme (LCAS). These technologies have been standardized by ITU-T. GFP was described in detail above in Section 2.7. Below, we describe the other two technologies: virtual concatenation and LCAS.
2.8.1 Virtual Concatenation
Virtual concatenation is a SONET/SDH procedure that maps an incoming traffic stream into a number of individual subrate payloads. That is, payloads with a bandwidth less than the bandwidth of a SONET/SDH link. The subrate payloads are switched through the SONET/SDH network independently of each other.
As an example, let us consider the case of transporting a GbE traffic stream over SONET. According to the SONET specifications, an OC-48c (2.488 Gbps) has to be used in order to accommodate the GbE traffic at full speed. However, about 1.488 Gbps of the OC-48c will go unused. Alternatively, we can use an OC-12c (622 Mbps), but this will require appropriately reducing the speed of GbE. The best solution is to use an OC-21c (1.088 Gbps), since this is the SONET payload with a bandwidth that is close to the speed of GbE. However, this payload is not feasible since it has not been implemented in SONET equipment. It will take a major investment to develop this new payload and deploy it into the SONET equipment.
Virtual concatenation provides an efficient and economic solution to this problem. With virtual concatenation, seven independent OC-3c (155 Mbps) subrate payloads can be used to carry the GbE traffic. These seven payloads provide a total payload with 1.088 Gbps bandwidth. The incoming GbE stream is split into seven substreams, and each substream is mapped onto one of the seven OC-3c payloads. These payloads are then switched through the SONET network as individual payloads without the intermediate nodes being aware of their relationship. Virtual concatenation is only required to be implemented at the originating node where the incoming traffic is demultiplexed into the seven subrate payloads and at the terminating node, where the payloads are multiplexed back to the original stream. The seven payloads might not necessarily be contiguous within the same OC-N payload. Also, they do not have to be transmitted within the same SONET fiber. That is, if the SONET/SDH network consists of nodes that are interconnected with f fibers, then each of these seven payloads can be transmitted over any of the f fibers.