Understand PBB-TE/PBT and T-MPLS.

Connection Oriented Packet Transport is a general term to describe the concept underlying both Provider Backbone Bridge - Traffic Engineering (PBB-TE) and Transport MPLS (T-MPLS). Common for both technologies are the following:

• End-to-end bidirectional, point-to-point connections or tunnels, which can support a number of packet based services
• Protection of these tunnels on an end-to-end basis with the ability to switchover in less than 50 milliseconds
• Assurance of OAM integrity on an end-to-end basis to support protection switching, fault detection and monitoring of tunnels
• Use of network management systems to control how tunnels are configured or provisioned

PBB-TE/PBT

In PBB-TE, or Provider Backbone Transport (PBT) as it was originally known, the solution is based on the IEEE 802.1ah Provider Backbone Bridging (PBB) standard. However, many of the features in PBB are turned off with new features added to support transport requirements.

PBB-TE reuses the MAC-in-MAC frame format, which encapsulates a standard IEEE 802.1ad Q-in-Q Ethernet packet with stacked VLANs in a frame with a unique Backbone MAC (B-MAC) address and Backbone VLAN ID (B-VID), as well as a Service Identifier (I-TAG). This allows highly scalable, transparent transport of a large number of packet services.

The B-MAC indicates the destination and source addresses for the ends of a tunnel. The B-VID indicates the path taken by the tunnel through the network. It is thus possible to establish a backup path to the same destination that can be used for protection.

PBB uses Spanning Tree Protocol to establish routes through the network. In PBB-TE, this protocol is removed and a managment system is used to traffic engineer connections similar to current practices for SONET/SDH networks. This ensures full control over transport connection routes.

PBB-TE also uses a subset of the IEEE 802.1ag Operation, Administration and Maintenance (OAM) standard, specifically Continuity Fault Management (CFM). This allows Continuity Check (CC) OAM messages to be monitored and sub-50 msec protection switching to a pre-defined backup path to be performed.

For more information on PBB-TE see:

 TPACK PBB-TE White Paper

http://en.wikipedia.org/wiki/Provider_Backbone_Transport

T-MPLS

Transport MPLS (T-MPLS) is the equivalent Connection Oriented Packet Transport mechanims to PBB-TE based on MPLS rather than Ethernet. Taking IETF defined MPLS standards as a starting point, T-MPLS attempts to align MPLS with ITU-T G.805 transport network architecture principles in order to create a transport focused version of T-MPLS.

T-MPLS has been standardized by ITU-T in recommendations:

• G.8110.1: Architecture of Transport MPLS (T-MPLS) Layer Network
• G.8112: Interfaces for the Transport MPLS (T-MPLS) Hierarchy (TMH)
• G.8121: Characteristics of multi-protocol label switched (MPLS) equipment functional blocks

One of the main charcteristics of T-MPLS is that it does not use routing and signalling protocols to establish Label Switched Paths (LSP), but relies on traffic engineering using a central network management solution, similar to SONET/SDH networks today. This allows maximum control of how network resources are used. The LSPs are also bi-directional rather than uni-directional as is the case in IP/MPLS.

One of the major differences between T-MPLS and MPLS is that a number of IP related functions have been removed in order to ensure the end-to-end integrity of Operation, Administration and Maintenance (OAM) packets used for monitoring and protection switching. The following functions have been removed:

• Penultimate Hop Popping (PHP) - where the last link in the LSP path is based on IP and not MPLS in order to minimize router processing
• LSP merging - where all traffic sent to the same destination can use the same label
• Equal Cost Multiple Path (ECMP) - Routing of a single LSP over multiple paths

T-MPLS uses a special OAM methodology based on ITU-T Y.1711 but with enhancements inspired by ITU-T Y.1731 and IEEE 802.1ag. The ITU-T standards associated with this are G.8113/Y.1372 (ex-Y.17tor) and G.8114/Y.1373 (ex-Y.17tom)

With these enhancements, performance monitoring can be performed to indicate transmission problems and initiate protection switching to a backup end-to-end LSP. This protection mechanism is defined in ITU-T G.8131/Y.1382 (T-MPLS linear protection switching with 1+1, 1:1 and 1:N options) and G.8132/Y.1383 (T-MPLS ring protection switching).

The protection mechanism differs from existing MPLS Fast-Re-Route (FRR) as it is based on end-to-end bidirectional LSPs rather than link-by-link protection. MPLS FRR also requires LSP merge functionality, which has been removed from T-MPLS in order to preserve OAM integrity.

For more information on T-MPLS see:

 TPACK T-MPLS White Paper

http://en.wikipedia.org/wiki/T-MPLS