From small beginnings MultiProtocol Label Switching (MPLS) has come a long way in ten years. Although there are a considerable number of detractors who believe it costly and challenging to manage, it has now been deployed in just about all carriers around the world in one guise or another (MPLS-TE) as discussed in The rise and maturity of MPLS. Moreover, it is now extending its reach down the stack into the optical transmission world through activities such as T-MPLS covered in PBB-TE / PBT or will it be T-MPLS? (Picture: GMPLS: Architecture and Applications by Adrian Farrel and Igor Bryskin).In the same way that early SDH standards did not encompass appropriate support for packet based services as discussed in Making SDH, DWDM and packet friendly, initial MPLS standards were firmly focussed on IP networks not for use with optical wavelength or TDM switching.
The promise of MPLS was to bring the benefits of a connection-oriented regime to the inherently connectionless word of IP networks and be able to send traffic along pre-determined paths thus improving performance. This was key for the transmission of real time or isochronous services such as VoIP over IP networks. Labels attached to packets enabled the creation of Label Switched paths (LSPs) which packets would follow through the network. Just as importantly, it was possible to specify the quality of service (QoS) of an LSP thus enabling the prioritisation of traffic based on importance.
It was inevitable that MPLS would be extended to enable it to be applied to the optical world and this is where the IETF’s Generalised MPLS (GMPLS) standards comes in. Several early packet and data transmission standards bundled together signalling and data planes in vertical ‘stove-pipes’ creating services that needed to be managed from top to bottom completely separately from each other.
The main vision of GMPLS was to create a common control plane that could be used across multiple services and layers thus considerably simplifying network management by automating end-to-end provisioning of connections and centrally managing network resources. In essence GMPLS extends MPLS to cover packet, time, wavelength and fibre domains. A GMPLS control plane also lies at the heart of T-MPLS replacing older proprietary optical Operational Support Systems (OSS) supplied by optical equipment manufacturers. GMPLS provides all the capabilities of those older systems and more.
GMPLS is also often referred to as Automatic Switched Transport Network (ASTN) although GMPLS is really the control plane of an ASTN.
GMPLS extends MPLS functionality by creating and provisioning:
- Time Division Multiplex (TDM) paths, where time slots are the labels (SONET / SDH).
- Frequency Division Multiplex (FDM) paths, where optical frequency such as seen in WDM systems is the label.
- Space Division Multiplexed (SDM) paths, where the label indicates the physical position of data – photonic Cross-connects
|Switching Domain||Traffic Type||Forwarding Scheme||Example of Device|
|Packet, cell||IP, ATM||Label||IP router, ATM switch|
|Time||TDM SONET/SDH||Time slot||Digital cross-connects|
|Physical space||Transparent||Fiber, line||OXC|
GMPLS has extended and enhanced the following aspects of MPLS:
- Signalling RSVP-TE and CR–LDP
- Routing protocols – OSPF–TE and IS-IS-TE
GMPLS has also added:
- Extensions to accommodate the needs of SONET / SDH and optical networks.
- A new protocol, link-management protocol (LMP), to manage and maintain the health of the control and data planes between two neighbouring nodes. LMP is an IP-based protocol that includes extensions to RSVP–TE and CR–LDP.
As GMPLS is used to control highly dissimilar networks operating at different levels in the stack, there are a number of issues it needs to handle in a transparent manner:
- It does not just forward packets in routers, but needs to switch in time, wavelength or physical ports (space) as well.
- It should work with all applicable switched networks OTN, SONET / SDH, ATM and IP etc.
- There are still many switches that are not able to inspect traffic and thus not able to extract labels – this is especially true for TDM and optical networks.
- It should facilitate dissimilar network interoperation and integration.
- Packet networks work at a finer granularity than optical networks – it would not make sense to allocate a 622Mbit/s SDH link to a 1Mbit/s video IP stream by mistake.
- There is a significant difference in scale between IP and optical networks from a control perspective – optical networks being much larger with thousands of wavelengths to manage.
- There is often a much bigger latency in setting up an LSP on an optical switch than there is on an IP router.
- SDH and SONET systems can undertake a fast switch restoration in less than 50mS in case of failure – a GMPLS control plane needs to handle this effectively.
GMPLS / ASTN is now well entrenched in the optical telecommunications industry with many, if not most, of the principle optical equipment manufacturers demonstrating compatible systems.
It’s easy to see the motivation to create a common control plane (GMPLS was defined under the auspices of the IETF’s Common Control and Measurement Plane (ccamp) working group) as it would would considerably reduce the complexity and cost of managing fully converged Next Generation Networks (NGNs). Indeed, it is hard to see how any carrier could implement a real converged network without it.
As discussed in Path Computation Element (PCE): IETF’s hidden jewel converged NGNs will need to compute service paths across multiple networks, across multiple domains and automatically pass service provision at the IP layer down to optical networks such as SDH and ASTN. Again, it is hard to see how this vision can be implemented without a common control plane and GMPLS.
To quote the concluding comment in GMPLS: The Promise of the Next-Generation Optical: Control Plane (IEEE Communiction Magazine July 2005 Vol.43 No.7):
“we note that far from being abandoned in a theoretical back alley, GMPLS is very much alive and well. Furthermore, GMPLS is experiencing massive interest from vendors and service providers where it is seen as the tool that will bring together disparate functions and networks to facilitate the construction of a unified high-function multilayer network operators will use as the foundation of their next-generation networks. Thus, while the emphasis has shifted away from the control of transparent optical networks over the last few years, the very generality of GMPLS and its applicability across a wide range of switching technologies has meant that GMPLS remains at the forefront of innovation within the Internet. “