Using GFP, Virtual Concatenation, LCAS and GMPLS for Ethernet Transport over SONET/SDH Networks. By James D. Jones Student ID

Transcription

1 Using GFP, Virtual Concatenation, LCAS and GMPLS for Transport over SONET/SDH Networks By James D. Jones Student ID EETS-8391 Summer 2002

2 Abstract As the bandwidth demand of continues to grow, the influence of is migrating into the transport world. While is the dominant technology in data networking environments, SONET and SDH dominate the MAN/WAN transport world. Pressure is increasing to offer low cost, long reach connectivity between remote nodes or networks using SONET/SDH transport. Several key technologies are enabling this trend, including: - Generic Framing Procedure (GFP): a method to encapsulate any data type into a format suitable for optical transport. - Virtual Concatenation: a technique to assign payload areas of arbitrary size within a SONET/SDH signal. It can be used to transport payloads that do not fit efficiently in the standard STS-Nc SPE sizes that are typically supported by existing SONET/SDH network elements (NEs). - Link Capacity Adjustment Scheme (LCAS): a signaling mechanism to dynamically adjust the size of a container transported in a SONET/SDH Network with Virtual Concatenation. - Multi-Protocol Label Switching (MPLS) and Generalized MPLS (GMPLS): efficient data forwarding techniques, originally designed for packet-oriented networks, later extended by GMPLS to support any transport type. GFP, VC and LCAS have been developed in parallel and their main benefits are realized when they are used in combination. These elements combined with the discovery, routing, signaling capabilities of GMPLS enable powerful dynamic bandwidth control applications. This report will explore the role played by each of these technologies in enabling efficient transport of over SONET and SDH. (For simplicity, this article from here on will refer to SONET, although it is meant to include both SONET and SDH). Motivation for GMPLS-Controlled over SONET The case for GMPLS-Controlled over SONET (EoS) is primarily one of economics. The low cost and wide range of equipment and applications supported by continues to expand. As the bit rate and number of users grow, so does the need to provide higher bandwidth and longer reach transport systems to interconnect nodes and networks. The broad base of SONET equipment is the mainstay of carriers MAN and WAN transport, but was originally architected for voice/tdm traffic. Given these factors, there is significant interest in providing a - 1

3 more cost-effective, flexible method of transporting over SONET. At the same time, carriers need to reduce the operating costs of provisioning these services, while offering them in a reduced set up time. GMPLS has been developed to enable automated rapid provisioning of high bandwidth connections over SONET networks. Existing methods of providing long-distance connectivity include Packet-over-SONET (PoS) and X.86 using LAPS framing. These techniques utilize flag word delineation in combination with byte stuffing to map packets or MAC frames into SONET SPEs. The primary limitation of such techniques is the complex termination and adaptation processes that take place at the ingress and egress of the networks. The byte stuffing process required for transparency also causes a variation in the bit rate required of the transport network. The aim of EoS using GFP, VC and LCAS is to deliver the MAC layer simply and in a native format using standards-based techniques between the DTEs or networks. In essence, the end-users would view the transport network as a simple switch providing local connectivity. Further, the elimination of byte stuffing by GFP removes variations in bandwidth needed to transport. The delivery of over SONET networks requires co-existence of data and TDM signals within the SONET payload. The key to this migration is a hybrid network element at the edge of the SONET network that could multiplex either data or TDM traffic into the same SPE. Sample applications enabled by such hybrid nodes include [1]: - private lines (EPLs), providing simple point-to-point connectivity between remote nodes by using SONET transport networks to emulate transport. EPLs could either use dedicated TDM channels for each user or statistically multiplex frames from multiple users within a TDM channel. - Transparent LAN services, providing multipoint connectivity to extend learning and bridging functions over the SONET network. Over the last 3 years, considerable effort has gone into definition and development of GFP, VC and LCAS to deliver this capability. The following sections will discuss each of these, along with GMPLS, then show how they may be used in combination to provide effective EoS. - 2

4 Generic Framing Procedure In 1999, the ITU-T and ANSI chartered an effort on data over SONET to promote vendor equipment and carrier interworking [2]. This was origin of Generic Framing Procedure (GFP), which is being defined in ITU-T Draft Recommendation G.7041/Y.1303 [3]. GFP is a lightweight adaptation protocol to provide flexible mapping of different bitstream types into a single bytesynchronous channel. It provides efficient encapsulation with fixed, but small overhead per packet. Other techniques can map data into SONET payloads, but as discussed, they exhibit nondeterministic bandwidth expansion due to byte-stuffed transparency processing. GFP has two different mapping techniques, transparent and frame-based. Transparent-mapped GFP provides minimum latency by transporting block-coded signals (presently only 8B/10B) for applications such as SAN. Frame-mapped GFP provides more flexibility and simplicity, and is preferred for most packet data types. Frame-mapped GFP stores and forwards entire client frames in a single GFP frame, thus increasing latency over transparent GFP. When mapping using frame-mapped GFP, the preamble and SFD are discarded and the remainder of the frame is mapped into the GFP payload. If both the transport and bridging capabilities of are integrated into transport NEs, the frame-mapped mode is preferable since the layer 1 aspects of both SONET and interfaces are segregated from layer 2 aspects [1]. Virtual Concatenation Virtual Concatenation is an inverse-multiplexing technique that combines an arbitrary number of SONET channels to create a single byte-synchronous stream. It is defined in ITU-T Recommendation G.707 [4] and ANSI T [5]. Virtual Concatenation is intended to solve the problem of coarse bandwidth granularity of the existing containers in a SONET signal. Two methods for concatenation are defined, contiguous and virtual concatenation. Both methods provide concatenated bandwidth of X times Container-N at the path termination. Contiguous concatenation maintains the contiguous bandwidth through out the whole transport, transported through the same physical resources. Virtual concatenation breaks the contiguous bandwidth into individual virtual containers (VCs), transports the individual VCs and recombines these VCs to a contiguous bandwidth at the end point of the transmission. Virtual concatenation requires concatenation functionality only at the path termination equipment, while contiguous concatenation requires concatenation functionality at each network element [4]. Virtual concatenation does not require a single pipe of the required bandwidth through the network, as continuous concatenation does; it can use smaller increments of available bandwidth to build a larger bandwidth end-to-end pipe. - 3

5 Virtual concatenation can significantly improve the bandwidth utilization of signals mapped into SONET. Table 1 shows examples of bandwidth utilization for both Contiguous and Virtual Concatenation [1, 2]. Table 1: Bandwidth Efficiency Comparison of Contiguous and Virtual Concatenation Data Signal 10 Mbit/s 100 Mbit/s Fast 1 Gbit/s SONET/SDH Payload Mapping and Bandwidth Utilization Contiguous Concatenation STS-1: 20% VC-3: 20% STS-3c: 67% VC-4: 67% STS-48c: 42% VC-4-16c: 42% SONET/SDH Payload Mapping and Bandwidth Utilization Virtual Concatenation VT-1.5-7v: 89% VT-12-5v: 92% STS-1-2v: 100% VC-3-2v: 100% STS-1-21v: 98% VC-3-21v: 98% Link Capacity Adjustment Scheme A companion technology to Virtual Concatenation is the Link Capacity Adjustment Scheme (LCAS), being defined in ITU-T Draft Recommendation G.7042/Y.1305 [6]. LCAS is an extension to Virtual Concatenation that allows dynamic changes to the number of SONET channels in use. LCAS signaling is carried in-band on Path overhead bytes. LCAS bandwidth adjustment is done by the edge NEs to achieve hitless performance of Virtual Concatenation Groups (VCGs). As the bandwidth required by the signals varies, LCAS can be used to signal changes needed to the VCGs, providing elastic bandwidth through the SONET network. LCAS does not signal and set up intermediate resources needed to change the bandwidth from end to end. It is concerned only with coordinating the bandwidth adjustment on the end-points, once the physical resources in between have been provisioned. These intermediate resources can be provisioned through the management plane on an EMS/NMS or dynamically provisioned using a distributed control plane protocol suite such as GMPLS. Generalized MPLS Distributed control plane protocols like GMPLS are intended to provide real-time provisioning, dynamic bandwidth allocation and fast restoration. GMPLS attempts to leverage the Traffic Engineering benefits of MPLS for the control of non-packet switched interfaces such as optical cross-connects (OXCs). Since OXCs generally switch individual wavelengths, this initiative was originally called Multiprotocol Lambda Switching (MPλS). As MPλS progressed, a more general - 4

6 need was defined to extend MPLS techniques not just to wavelength switching, but to other entities, such as TDM channels or fibers carrying multiple wavelengths. Consequently, MPλS was redirected into a more universal form called Generalized MPLS (GMPLS). GMPLS is also intended to support a wide variety of NEs with different interface switching types (such as packet-, TDM-, wavelength- and fiber-switched types). Architecture of over SONET with GFP, VC, LCAS and GMPLS This section describes the interworking of the major building blocks of a GMPLS-controlled over SONET network. It begins with the desired behavioral view of such a network, then describe the architectural and protocol aspects. Finally, the architecture of the hybrid edge NEs is discussed to show how SONET networks can accommodate both TDM and data traffic. Figure 1 shows the desired behavioral view of such a network from perspective of the end user systems. The intent is for the SONET transport network to emulate the behavior of a switched network through appropriate adaptation and control. The end users transmit MAC frames to the SONET network, which behaves the same as if it was switched. Figure 1: Behavioral View of over SONET Network (to end users) User A MAC Frames (Virtual) Switched Network MAC Frames User Z Since SONET networks currently handle large volumes of voice/tdm traffic, both data and TDM must gracefully co-exist in the SONET network. The key to this objective is a hybrid edge NE that can aggregate both TDM and data flows to best utilize the available capacity of the SONET network. Figure 2 shows an architectural view of the network with these hybrid NEs originating and terminating both TDM and data flows. The MAC frames, along with other data and TDM traffic are mapped into SONET payloads and transported through the network. By incorporating these features into edge NEs that originate and terminate path layer signals, no other changes are required in intermediate nodes in the SONET network. - 5

7 Figure 2: Architectural View of over SONET Network TDM Traffic MAC Frames SONET Network MAC Frames TDM Traffic User A Other Data Traffic SONET Path Layer Signals (TDM + Data) Hybrid TDM/Data Nodes Other Data Traffic User Z Figure 3 expands on the protocols and functions performed by the edge NEs and the SONET network. To initiate the connection, the end user may signal a connection request to the network over the User-Network Interface (UNI). The UNI, as defined by the Optical Internetworking Forum (OIF) uses GMPLS signaling protocols based on RSVP-TE and CR-LDP to accomplish this. (Alternatively, the connection between end users and the network could be provisioned through an EMS/NMS.) Within the network a routing protocol like OSPF-TE or IS-IS is used to select specific resources that will be used to complete the connection. These resources are set up via signaling protocols. The hybrid NEs on the edge of the SONET network also perform the adaptation and control functions using GFP, Virtual Concatenation and LCAS. The ingress NE encapsulates the MAC frames into GFP frames. It also performs the inverse multiplexing needed to map the GFP frames into VCGs and performs the LCAS signaling to coordinate end-to-end bandwidth changes. LCAS must interwork with the GMPLS protocols on the edge NE. If LCAS needs to increase the bandwidth and an existing Label Switched Path (LSP a GMPLS connection) is available for use, LCAS can signal the bandwidth adjustment. If an existing LSP is not available for LCAS, the GMPLS protocols must establish a new LSP for LCAS to use. - 6

8 UNI Signaling (optional) RSVP-TE CR-LDP Figure 3:Protocol/Functional View GMPLS Signaling and Routing RSVP-TE/CR-LDP OSPF-TE/IS-IS UNI Signaling (optional) RSVP-TE CR-LDP User A User Z Adaptation and Control: GFP Mapping Virtual Concatenation (inverse mux) LCAS Control Adaptation and Control: GFP De-mapping Virtual Concatenation (mux, reassembly) LCAS Control The hybrid NEs at the network edge are critical to this architecture. An expansion of the functionality of a hybrid NE is given in figure 4 (based on [1], [7] and [8]). For simplicity the flow is only shown for signals coming into the NE. (In the figure, the abbreviation Eth represents either 10/100 M or Gb.) Native signals are received and the MAC frames are extracted. The System Packet Interface (SPI) is a standard electrical interface defined by the OIF to separate the synchronous PHY layer from asynchronous packet-based processing. SPI supports transmit and receive data transfers at clock rates independent of the actual line bit rate for the efficient transfer of both variable-sized packet and fixed-sized cell data. In OIF terminology, SPI-3 supports OC-48 rates and SPI-4 supports OC-192 [9]. The MAC Frame is encapsulated in a GFP frame and assigned to a VCG. The LCAS function coordinates the end-end adjustments in bandwidth for each virtual container. - 7

9 Figure 4: Signal Flow, Hardware and Simplified Stack Model for Hybrid NE Native Interface Signals (G)MII System Packet Interface GFP Mapped Frames Virtual Concatenation Groups SONET STS-N Signal Hardware PHY layer Termination Switch Fabric GFP Encapsulation L C A S... ADM Switch Fabric Simplified Stack Models Eth MAC Eth MAC SPI-n PHY Eth MAC GFP frame Eth (G)MII Eth (G)MII SPI-n Elec GFP Frame SONET PHY Eth PHY SONET optics Eth optics Conclusion This paper has described some of the key elements for efficient transport of over SONET. Using GFP in this application makes SONET both versatile and flexible, while the combination of LCAS and Virtual Concatenation make it elastic. Applying GMPLS control to such a network adds rapid provisioning and traffic engineering features. All the techniques described GFP, Virtual Concatenation, LCAS and GMPLS are still emerging technologies. Silicon implementations of GFP are becoming available and should be well-supported by multiple vendors. Virtual Concatenation has been standardized, but the LCAS control to fully exploit it is still completing the standardization process and LCAS software developments are not mature. GMPLS signaling is relatively mature across the UNI, but standardized signaling and routing within the networks (NNI Network-Node Interfaces) is still being defined by both ITU-T and OIF. All these technologies should quickly mature and become available in the near future. The critical factor, however is the demand for such products from network operators. Since the main motivation of GFP, LCAS, Virtual Concatenation and GMPLS is to reduce operating costs and maximize network utilization, over SONET networks based on these technologies can be expected to emerge and grow rapidly. - 8

10 References [1] E. Hernandez-Valencia, Hybrid Transport Solutions for TDM/Data Networking Services, IEEE Communications Magazine, May 2002, p [2] P. Bonenfant and A. Rodriguez-Moral, Generic Framing Procedure (GFP): The Catalyst for Efficient Data over Transport, IEEE Communications Magazine, May 2002, p [3] ITU-T Draft Rec. G.7041/Y.1303, Generic Framing Procedure (GFP), October [4] ITU-T Rec. G.707, Network Node Interface for the Synchronous Digital Hierarchy (SDH), October [5] ANSI T , American National Standard for Telecommunications Synchronous Optical Network (SONET): Physical Interfaces Specification, [6] ITU-T Draft Rec. G.7042/Y.1305, Link Capacity Adjustment Scheme (LCAS), [7] D. Cavendish et al, New Transport Services for Next-Generation SONET/SDH Systems, IEEE Communications Magazine, May 2002, p [8] M. Scholten, et al, Data Transport Applications Using GFP, IEEE Communications Magazine, May 2002, p [9] OIF White Paper, OIF Electrical Interfaces,

11 Internet Addresses of Standards Bodies and Forums International Telecommunications Union: Internet Engineering Task Force: Optical Internetworking Forum: Acronyms ANSI American National Standard Institute NMS Network Management System CR-LDP Constraint-based Label Distribution Protocol NNI Network-Network Interface, or Network Node Interface DTE Data Terminating Equipment OC Optical Carrier EoS over SONET OIF Optical Internetworking Forum EMS Element Management System OSPF-TE Open Shortest Path First - Traffic Engineering EPL Private Line OXC Optical Cross-Connect GFP Generic Framing Procedure PHY Physical Layer GMII Gigabit Medium Independent PoS Packet over SONET Interface GMPLS Generalized Multiprotocol Label RSVP-TE Resource Reservation Protocol - Traffic Switching Engineering IETF Internet Engineering Task Force SAN Storage Area Network IS-IS Intermediate System - Intermediate SDH Synchronous Digital Hierarchy System ITU-T SFD Start of Frame Delimiter International Telecommunications Union - Telecommunications Standardization Sector LAN Local Area Network SONET Synchronous Optical NETwork LAPS Link Access Procedure - SONET SPE Synchronous Payload Envelope LCAS Link Capacity Adjustment Scheme SPI System Packet Interface LSP Label Switched Path STS Synchronous Transport Signal MAC Medium Access Control TDM Time Division Multiplexing MAN Metropolitan Area Network UNI User-Network Interface MII Medium Independent Interface VC Virtual Container MPLS Multiprotocol Label Switching VCG Virtual Container Group MPλS Multiprotocol Lambda Switching VLAN Virtual Local Area Network NE Network Element WAN Wide Area Network - 10