Archive 27 Mar 2007 #58
Ethernet's New Frontier:
"Self-Insured" Ethernet - Part 2
By Vladimir Kaminsky, Contributing Editor and Sr. Consultant
This is Part II from a two-part paper prepared by Flanagan Consulting on
an emerging Ethernet protocol whose goal is to compete with MPLS and
Ethernet-over-SONET transport techniques for Carrier-Grade transmission.
To date, the combination of MPLS with Ethernet over SONET (EoS) have
been the prevailing technology option that supports Carrier-Grade
applications requirements, specifically:
-- Hard QoS / traffic management,
-- Service management, and
-- Support for TDM services.
PBT FOR A CONNECTION-ORIENTED CORE
This ViewsLetter introduces a new technology called Provider Backbone
Transport (PBT), which can be employed in the service provider domain of
a Provider Backbone Bridge Network (802.1ah) to allow configuration of
engineered, resilient, SLA driven point-to-point Ethernet trunks, also
called Ethernet Virtual Circuits (EVCs) by the MetroEthernet Forum. PBT
trunks allow carriers to engineer traffic-managed circuits which may be
monitored, along with the rest of the 802.1ah network, using the 802.1ag
protocols. PBT focuses on applying Ethernet technology directly to the
transport of Ethernet services.
Introduced in October, 2005, PBT is a connection-oriented Ethernet
technology combining telecom network and Ethernet technology
characteristics. It is based on, but is not equal to, 802.1 MAC-in-MAC,
the encapsulation of one Layer 2 frame in another using the Ethernet
Media Access Control (MAC) header.
PBT maintains the advantages of Ethernet, such as simplicity,
scalability, and ubiquity, while addressing carrier grade quality
concerns without the need for MPLS. The idea is to subtract certain
things from Ethernet properties to make it more carrier-friendly. For
example: rather than allow Ethernet to learn the surrounding network,
as it was created to do, PBT lets carriers dictate network knowledge to
The core concepts of PBT are to:
-- configure Ethernet services in network switches via network
management and the network control plane, thereby creating
connection-oriented Ethernet services,
-- disable self-learning functions, such as the Spanning Tree protocol,
and VLAN features in order to avoid broadcast packet flooding,
-- reuse forwarding tables in the switches, and
-- discard all packets not found in the PBT forwarding table.
Carriers tell a node where to forward traffic, giving it multiple
options and the ability to define protection paths which provide for
<50-ms service recovery. With the introduction of Ethernet Virtual
Circuits (EVCs) a carrier realizes such functions as protection
switching, OAM, QoS, and traffic engineering. Each EVC may be assigned
performance parameters such as a committed information rate (CIR) and
committed burst size (CBS) that assure throughput, and similar limits
for "excess" traffic to be carried if capacity is available. Discard
thresholds are also available to limit throughput to what the subscriber
The reason to do all this is to create determinism, the lack of which is
the main issue Ethernet has always had against it in a carrier environment.
PBT offers a mechanism to permit scalable point-to-point trunks to be
configured or signaled in an Ethernet subnetwork. Such trunks perform
the functions of a packet switched network (PSN) and supports
pseudowires to replace TDM circuits. PBT builds upon the concepts of
Provider Bridging in 802.1ad (Q-in-Q), which is limited by the VLAN
address space of 4096.
Provider Backbone Bridging, in 802.1ah, doubles the length of the MAC
address space (which squares an already large number) to provide an
extremely scalable solution. That is, both the customer's original
frame and the carrier's encapsulation header contain source and
destination addresses in the form of Ethernet MAC addresses.
PBT forwarding decisions are made based on a Backbone MAC destination
(B-MAC) address (associated with an EVC) and Backbone VLAN ID (B-VID),
the encapsulated header. With these enhancements, PBT provides a “pure”
Ethernet backbone solution, which meets carrier class requirements
without the need for additional protocol layers such as MPLS or new
generation SONET (NGS).
By basing the solution on existing standards and technologies, PBT also
ensures that existing Ethernet network deployments can be re-used and
enhanced to support this new concept. PBT leverages the existing IEEE
802.1ah frame format, without modification.
PBT may be deployed in place of PBB (Provider Backbone Bridging), or may
run in parallel with PBB. In both cases, PBT eliminates the need for
backbone core devices to perform learning and ﬂooding. Instead,
point-to-point tunnels to transport L2 VPNs are provisioned using a
sophisticated management platform. Rather than using RSTP/MSTP to
prevent loops, the management platform traffic engineers the provider
backbone network utilizing significantly more capacity.
BACKUP TUNNEL FOR H.A.
Primary and backup PBT tunnels may be pre-configured by a management
system. This enables the operator to traffic engineer according to
path, bandwidth, and service requirements. Customers and services are
associated with tunnels taking into account the aggregate committed
information rate (CIR) and excess information rate (EIR) bandwidth
requirements. Tunnels are monitored through the use of IEEE 802.1ag
Connectivity Fault Management (CFM) Continuity Check Messages (CCM).
CCM control frames are sent and received every few milliseconds across
PBT tunnels. If the primary tunnel should experience a fault, the
tunnel endpoints automatically begin using the backup tunnel. The
forwarding database entries are pre-configured along the backup path to
minimize the failover and restoration times.
PBT has been proposed to the IEEE, IETF, and ITU-T. It is expected that
IEEE 802.1 will standardize this innovative and increasingly popular
BT has selected Nortel (www.nortel.com) and Siemens (www.siemens.com) as
key suppliers of carrier-scale Ethernet equipment for its 21CN network.
BT plans to use PBB-TE (Provider Backbone Bridging with Traffic
Engineering), an emerging IEEE standard, for PBT)deployment. The
network will be used to transport high-bandwidth services, from mission
critical business applications to full-streaming video. BT has selected
Nortel's Metro Ethernet Routing Switch 8600 and Metro Ethernet Services
Unit 1850 for the Ethernet component of its 21CN. Meriton Networks
(www.meriton.com) is combining PBT, what they call Carrier Ethernet
Transport, with their wave division multiplexing optical switch.
Provider Backbone Transport transforms Ethernet technology,
traditionally restricted to small-scale, local networks, into a more
reliable, scalable and deterministic technology making it suitable as
the basis for fixed and mobile carrier networks to deliver live video
and broadcast, multimedia, broadband data, and voice services.
Traditional connectionless, best-effort Ethernet poses challenges for
service providers needing to guarantee service delivery and quality of
service for real-time applications. PBT is a simple point-to-point
tunneling technology that adds determinism to Ethernet, enabling service
providers to specify the path that an Ethernet service should take
across the network. PBT allows for QoS guarantees by reserving
bandwidth for real-time services.
Perspective on carrier Ethernet compared to other transport methods.
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