Circuit emulation service (CES)

Overview

Circuit Emulation is required to preserve the context and nature of a circuit over a packet network. Backhaul networks are moving from circuit switched to packet-based (Ethernet, IP/MPLS) network transport. However, service providers have to preserve investments in the network equipment. The Alcatel-Lucent 1850 TSS-5 with a VLNC60/VLNC61/VLNC62/VLNC64 circuit pack provides the ability to handle TDM services using the pseudowire technology over an Ethernet network. The Alcatel-Lucent 1850 TSS-5 provides interfaces to connect to the existing network equipment to backhaul the traffic over a packet network.

For specific configuration information and CLI command information see the Alcatel-Lucent 1850 Transport Service Switch (TSS-5) Command Line Interface Guide.

Pseudowire architecture

The VLNC60/61/62 circuit pack supports DS1/E1 access interfaces and Ethernet trunk interfaces. The VLNC64 circuit pack supports a channelized OC-3/STM-1 interface and Ethernet trunk interfaces. The VLNC60/VLNC61/VLNC62/VLNC64 circuit pack can connect directly to the EATN, or can connect through a VLNC40/42/42B Ethernet Aggregator circuit pack. When the VLNC60/VLNC61/VLNC62/VLNC64 circuit pack is connected to a VLNC40/42/42B the trunk interfaces interact as if the VLNC60/VLNC61/VLNC62/VLNC64 circuit pack is connected to a standard Ethernet switch or an IP router. No proprietary protocols/messaging is used on the data path. If the VLNC60/VLNC61/VLNC62/VLNC64 circuit pack and VLNC40/42/42B are in the same shelf, backplane GbE interfaces can be used between the two circuit packs.

The VLNC60/VLNC61/VLNC62/VLNC64 circuit pack supports Structure Agnostic Transport over Packet (SAToP) pseudowire (RFC 4553). Each physical DS1/E1 interface is associated with a single pseudowire. The pseudowire is associated with a pair of unidirectional tunnels, configured between shelves, as a link between the DS1/E1 interfaces on those shelves. In Unstructured Emulation Mode, the VLNC60/VLNC61/VLNC62/VLNC64 circuit pack treats the data on the incoming pseudowire interface as if it were unstructured. All signalling CAS, CCS is carried in-band without knowledge of the signalling protocol itself.

View the figure
Pseudowire operation

TDM frames are sent over the packet network encapsulated in RFC 4553 format. The Real-time Transport Protocol (RTP) header can be included in the encapsulated packet. Beginning in Release 7.2, the user can configure the RTP header to be excluded. When there is no RTP header, differential timestamp based clock recovery is not available. Because the packet network is asynchronous, RTP packets with TDM payloads may not arrive in sequence, or in synchronization with SONET timing. Packets are resequenced, if needed, using a control word in the RTP header, and placed in a de-jitter buffer so that their TDM payloads can be synchronized, and delivered to the DACS. Alcatel-Lucent 1850 TSS-5 shelves at either end of an EATN are synchronized using differential clock recovery.

Differential clock recovery is based on SRTS (Synchronous Residual Time Stamp) requires a common clock between the 2 devices doing Circuit Emulation Function. IEEE 1588v2 (pending ratification) is a protocol used to supply clocks across an Ethernet network. It ensures that the clock on the 2 IWFs are in sync. RTP used to carry time stamps through the network. Clock recovery is done on a per pseudowire basis. Each DS1/E1 can be asynchronous to other DS1/E1 signals on the same shelf. On the VLNC64, the DS1/E1 signals can be grouped in up to 28 different differential timing domains.

Refer to Synchronization features for more information about Differential Timing Domain.

The following illustration shows how timing is accomplished across the packet network using IEEE 1588 protocol.

The activities that take place, from right to left are:

Figure A-9: Example of pseudowire timing
Example of pseudowire timing
QOS

Depending on the header (P bits for VLAN, EXP for MPLS), the VLNC60/VLNC61/VLNC62/VLNC64 circuit pack supports QoS marking on a per-pseudowire basis. Intermediate devices in the EATN must honor this marking in order to ensure end to end QoS. The QoS mapping is can be configured. Each DS1/E1 port can be provisioned to map to a p bit or EXP.

Jitter management

The VLNC60/VLNC61/VLNC62/VLNC64 circuit pack provides de-jitter buffers that can be configured for each pseudowire. The de-jitter buffers absorb the jitter introduced in the packet network and stabilize data towards the TDM interface. The jitter buffer value can be configured.

Protection for pseudowires

Bi-directional forwarding detection (BFD) is a network protocol that provides low-overhead method of detecting faults across media that does not support failure detection, like asynchronous messaging across Ethernet. Alcatel-Lucent 1850 TSS-5 uses BFD to monitor the integrity of MPLS tunnels between shelves. Duplicate pairs of unidirectional MPLS tunnels with associated pseudowires can be configured to provide two bi-directional links using different paths between Alcatel-Lucent 1850 TSS-5 shelves. One pair of tunnels can carry the active pseudowire and its traffic, while the other pair of tunnels serve as a backup. BFD is configured on each tunnel. If the BFD detects a failure or and RFI, the VLNC60/VLNC61/VLNC62/VLNC64 circuit pack directs all pseudowire traffic onto the backup tunnel.

Note:

Prior to Release 6.0, the protection switching time of Circuit Emulation pseudowires was in the range of 600 msec to 5 sec. Beginning in Release 6.0, tunnel failure detection is now in tens of milliseconds.

MEF-8

In order to support a Metro Ethernet Forum 8 (MEF-8) based circuit emulation service, the user can specify an option for the mode of Circuit Emulation Service on the pack, either MPLS or Ethernet. If MPLS is selected, then the circuit emulation service is based on RFC 4553. If Ethernet is selected, then the circuit emulation service is based on MEF-8. The VLNC60/VLNC61/VLNC62/VLNC64 circuit pack cannot support mixed transport modes (RFC 4553 and MEF-8) simultaneously. If the mode of circuit emulation service is changed (from MPLS to Ethernet, or Ethernet to MPLS, all existing configurations are cleared and the pack is reset. Alcatel-Lucent 1850 TSS-5 can connect to either an MPLS network or a Metro Ethernet network; Alcatel-Lucent 1850 TSS-5 cannot connect to both simultaneously.

Ethernet mode (MEF-8)

In the Ethernet mode, PSN Tunnel requires an endpoint which is based on an Ethernet MAC address. The PWEs transported in Ethernet mode will have an Ethertype of 0x88d8. In the Ethernet mode, there is no UPSR/SNCP-like protection solution for the PWEs and end-to-end protection via BFD is not supported. If there is physical failure on Ethernet port that supports the PSN Tunnel, the PWE switches to the backup PSN Tunnel.

Ethernet mode specifies a differential timestamp frequency of 25 MHz. By default, the frequency for differential timestamp on the VLNC60/61/62/64 packs is 77.76 MHz. Beginning in Release 5.1, the packs support 25 MHz frequency as well.

Note:

The VLNC64 must be Series 1:2 (S1:2) or later, to support the 25 MHz frequency.

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