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PWE3 Introduction for OptiX RTN 910/950 P-0

Content

Confidential Information of Huawei. No Spreading Without Permission

1. PWE3 Overview...............................................................................Page 4

2. TDM Service to PWE3....................................................................Page

14

3. ATM Service to PWE3................................................................ ...Page 23

4. Ethernet Service to PWE3..............................................................Page

27

5. PWE3 Application Scenarios..........................................................Page

29

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Reference


RTN 950(V100R001C00_01) Feature Description

RFC 4553

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Chapter one is the PWE3 principles overview, it is focus on the content which related to


OptiX RTN 910/950 product application.

Chapter 2,3 and 4 will describe the encapsulation for the typical services, they are the

emphasis for this course.

And the last chapter three introduces the application of the PWE3 in OptiX RTN910/950.

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Understanding the PWE3 position in the OptiX RTN 910/950 network and its basic


concepts is the emphasis of this chapter.

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In the OptiX RTN 910/950, the different services are all be transmitted in the MPLS


tunnel by packets. To encapsulate the different service frames such as the ATM, IMA E1,

TDM E1, Channel STM-1 or Ethernet into MPLS tunnel, we use the PWE3 protocol.

Encapsulation

The packets transmitted on PWs use the standard PW encapsulation type andtechnology. The RFC 4446 defines multiple encapsulation types of PWE3

packets on PWs.

Tunnel

A mechanism to transparently bear the information on the network. It is used to

bear PWs. A tunnel can bear multiple PWs. In most cases, a tunnel is an MPLS

tunnel. The tunnel is a channel that directly connects a local PE to a peer PE. In

the tunnel, the data is transmitted transparently between PEs.

Tunnel: Carries one or multiple pseudo wires (PW)

PWs are transparently transmitted in the core network

PWs are created through signaling (LDP)

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AC: Attachment Circuit


It is the physical or virtual circuit that connects a CE to a PE. For example, anAC can be an FR DLCI, an ATM VPI/VCI, an Ethernet port, a VLAN, a PPPconnection on a physical interface, a PPP session from an L2TP tunnel, or anMPLS LSP.

Forwarder

A PE sub-system that selects a PW to transmit the payload received on the AC.

PW signaling

The basis on which PWE3 is implemented. It is used for creating and maintainingPWs. Currently, the primary PW signaling is LDP.

PW: Pseudo Wire.

It is a mechanism that carries the essential elements of an emulated circuitbetween PEs over a PSN.

CE: Customer Edge.

It is a device that originates or terminates a service. The CE cannot be awarewhether an emulated service or a local service is in use.

PE: Provider Edge. It is a device that provides PWE3 to a CE. It is usually the edge router that is

connected to a CE on a backbone network. A PE is responsible for processingthe VPN service. A PE performs the mapping and forwarding of the packets fromthe private network to the public-network tunnels and that in the reverse order.

CW: Control Word.

A control word is a 4-byte encapsulated packet header. It is used to transmitpackets in an MPLS PSN.

VCCV: Virtual Circuit Connectivity Verification.

The VCCV is a tool used to manually test the virtual circuit connectivity. It isimplemented through the extended LSP ping.

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Concept of PW


The Mechanism that bears the simulated layer 2 services between clients to the

packet switch network (PSN).

The following takes the flow direction of Virtual Private Network 2 (VPN2) packets from

CE2 to CE4 as an example to show how PWE3 works:

1. CE2 sends the service flow to be Emulated to PE1 through an AC.

2. After PE1 receives the packets, the forwarder selects a PW for forwarding the

packets.

3. PE1 generates double MPLS labels according to the forwarding entry of the PW

(the private network label is used to identify the PW; the public network label is

used to forward packets to PE2 through the tunnel).

4. After emulated service packets arrive at PE2 through the public network tunnel,

the system removes the label.

5. The forwarder of PE2 selects an AC for forwarding the original service flow to

CE4.

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PW control plane: Create and remove PWs and monitor the status


PW forwarding plane: Convert the L1/2 header information and transmit PDUs in

sequence

MIB: Management Information Base, storage configuration information.

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The CW is negotiated at the control plane, and is used for packet sequence detection,


packet fragmentation, and packet reassembly at the forwarding plane. In the PWE3

protocols, ATM Adaptation Layer Type 5 (AAL5) and FR require the support for the CW.

The CW has the following functions:

Carries the sequence number for forwarding packets

If the control plane supports the CW, a 32-bit CW is added before the data

packet to indicate the packet sequence. When the load balancing is

supported, the packets may be out of sequence. The CW can be used to

number the packets so that the peer can reassemble the packets.

Fills the packet to prevent the packet from being too short.

For example, if Ethernet is between PEs and PPP is between PEs and

CEs, the size of the PPP control packet is smaller than the smallest MTU

supported by the Ethernet. Then the PPP negotiation fails. You can avoid

this by adding the CW, that is, by adding the fill bit.

Carries the control information of the Layer 2 frame header.

In certain cases, the frame does not need to be transmitted completely in

the L2VPN packets on the network. The frame header is stripped at the

ingress and added at the egress. This method, however, cannot be used

if the information in the frame header needs to be carried. You can use

the CW to solve this problem. The CW can carry the negotiated

information between the ingress PE and the egress PE.

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Unframed E1


Using all the time slots as a whole to transmit user data.

So, the total bandwidth for one unframed E1 connection is 2.048Mbps, just like

the bandwidth provided by a serial interface.

Framed E1

Time slot 0 used for signaling or other purpose.

Time slot 1-31 can be used for transmit service data for different users.

For example: Time slot 1 can be used for user1 to provide 64 Kbps bandwidth,

and time slot 11 to 12 can be used for user 2 to provide 128 Kbps bandwidth.

To use a PW to emulate the transmission of TDM service over a PSN, the following

elements must be carried to the other end of the PW.

TDM data

Frame format of TDM data

TDM alarm and signaling at the AC side

Synchronous timing information of TDM

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In the SAToP mode:


The equipment regards TDM signals as constant rate bit flows, instead of

sensing structures in the TDM signals. The entire bandwidth of TDM signals is

emulated.

The overhead and payload in the TDM signal are transparently transmitted.

The Ethernet frame carries the CES service. Usually the loading time is 1 ms.

In the CESoPSN mode:

The RTN 910/950 equipment senses frame structures, frame alignment modes

and timeslots in the TDM circuit.

The RTN 910/950 equipment processes the overhead and extracts the payload

in TDM frames. Then, the PTN equipment delivers the timeslot of each channel

to the packet payload according to certain sequence. As a result, the service in

each channel in the packet is fixed and visible.

Each Ethernet frame that carries the CES service loads TDM frames of a fixed

number. Usually the loading time is 1 to 5 ms.

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Principle


The SAToP mode does not detect any structure in the TDM circuit. It treats the

TDM circuit as a bit stream of a constant rate. One unit contains 8 bits. Cut a

certain length (reflected as loading time for the customer) from the TDM bit

stream and then load it to the emulation packet. The framing information and

channel information are not considered. Hence, the TDM service is transparentlytransmitted.

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Tunnel label: MPLS label, generally distributed by RSVP-TE or manually configured in


PTN equipments.

PW label: distributed by LDP or manually configured.

Control Word: optional, can choose use CW or not, if not, CW header is not required.

RTP (Real Time Protocol) Header/ Time Stamp/ SSRC Identifier: if RTP used, these

encapsulations are required; if not, not required.

We can encapsulate one E1 frame into one PW encapsulation, or we can encapsulate

more E1 frames into one PW header.

If one E1 frame encapsulated into one PW header, more bandwidth will be used

but the encapsulation time is short.

If more E1 frames encapsulated into one PW header, can improve the bandwidth

efficiency but need more time to encapsulate all the frames.

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Principle


The CESoPSN mode detects the frame structure, framing mode, timeslot

information in the TDM circuit. The payload of the TDM frame is extracted first

and the timeslot of each channel is added into the packet payload according to a

certain sequence.

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Tunnel label: MPLS label, generally distributed by RSVP-TE or manually configured in


PTN equipments.

PW label: distributed by LDP or manually configured.

Control Word: optional, can choose use CW or not, if not, CW header is not required.

RTP Header/ Time Stamp/ SSRC Identifier: if RTP used, these encapsulations are

required; if not, not required.

The CESoPSN different from SAToP is:

The CESoPSN protocol can identify frame structure of TDM service. It may not

transmit idle timeslot channels, but only extracts useful timeslots of CE devices

from the E1 traffic stream and then encapsulates them into PW packets for

transmission.

For example: only time slot 1-5 have data, all the other time slots are idle,

CESoPSN can choose only transmit time slot 1-5s data to another PE, the

opposite PE can reconstruct the original E1 frame, and then send it to

appropriate CE.

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Between BTS and BSC, the CES service is transported through the PTN equipment. A


CES service is available between BTS and BSC that are connected to NE1. Two CES

services are available between BTS and BSC that are connected to NE2.

BTS1 use one E1 connection connected to PE1, BTS2 use two E1 connections

connected to PE2. PE3 use one channelized STM-1 connection connected to BSC.

So, we need create one PW between PE1 and PE3, and two PWs between PE2 andPE3.

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In the SAToP mode:


The equipment regards TDM signals as constant rate bit flows, instead of

sensing structures in the TDM signals. The entire bandwidth of TDM signals is

emulated.

The overhead and payload in the TDM signal are transparently transmitted.

The Ethernet frame carries the CES service. Usually the loading time is 1 ms.

In the CESoPSN mode:

The PTN equipment senses frame structures, frame alignment modes and

timeslots in the TDM circuit.

The PTN equipment processes the overhead and extracts the payload in TDM

frames. Then, the PTN equipment delivers the timeslot of each channel to the

packet payload according to certain sequence. As a result, the service in each

channel in the packet is fixed and visible.

Each Ethernet frame that carries the CES service loads TDM frames of a fixed

number. Usually the loading time is 1 to 5 ms.

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PSN Transport Header


It depends on the used tunnel technology and performs the packet switching in

the PSN network.

Pseudo Wire Header

It identifies the ATM service in a tunnel and it is the bottom of the label stack in

the MPLS.

ATM Control Word

It can be a length or sequence number and it is the control bit of the transmitted

service.

One-to-one (1-to-1)

A VCC or VPC is mapped to one PW , supports all the AAL types.

The VPI/VCI is not encapsulated.

N-to-one (N-to-1)

Multiple VCCs or VPCs are mapped to one PW.

N >= 1. Supports all the AAL types. The VPI/VCI is encapsulated.

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The encapsulation format of a single cell is required (fast encapsulation), the


encapsulation format of multiple cells is optional. The control character is optional.

The number of cells can be encapsulated is restrained by the management capability of

MTU and Egress PE (router). The number of encapsulated cells can be controlled by

setting the number of concatenated cells and concatenation time on the T2000.

VPI/VCI: The value of Ingress PE can not be changed, but the value of Egress PE canbe changed into a new value.

PTI/CLP: Retain the original cell value.

N-to-One Cell ATM Mode Description

One PW can carry one or more VPCs/VCCs. The format can be the single cell

format, or multiple cell encapsulation, which can enhance the bandwidth

utilization.

The multiple cell encapsulation can enhance the bandwidth utilization, but it

causes network delay variation. In addition, the number of cells to be

encapsulated is determined by the MTU and CTD and CDV requirements of the

ATM connections multiplexed into the PW. Hence, the network environment and

service customization should be considered together.

The multiple VPCs/VCCs encapsulated in the PW may of different service types,

which are of different QoS requirements. Hence, the PSN packets should support

the highest level of the ATM service carried. To ensure the QoS of the PW, it is

recommended that one PW carry one VPC/VCC, or one PW carry the

VPCs/VCCs of the same QoS requirement.

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The CW, which is mandatory, is inserted before the Pseudowire Header. The ATM


pec c ea er s nser e e ore e e ay oa .

For a common ATM VPC, the egress PE cannot change the VCI field in a packet. InVPC cell transport, the egress PE can set a VCI value that is different from the VCI valueon the ingress. The VCI set by the egress PE is determined only by the PW Header.

The following describes the meaning of each field in the ATM Specific Header:

M: It is of one bit. It indicates the transmission mode. That is, a packet containsan ATM cell or a frame payload. The value of 0 indicates that the packet containsan ATM cell; the value of 1 indicates that the packet contains a frame payload.

V: It refers to the VCI flag, which is of one bit. It indicates whether a packetcontains the VCI field or not. The value of 0 indicates that the packet does notcontain the VCI; the value of 1 indicates that the packet contains the VCI. TheVPC is an ATM connection that is switched by the VPI value in the ATM cellheader. Therefore, the VCI is needed for a VPC, and is transmitted in each cell.

VCI: It is of 16 bits. This filed indicates that VCI value after the ATM cell isencapsulated.

One-to-One Cell ATM Mode Description

The number of ATM cells to be encapsulated is negotiated between the ingressdevice and the egress device. Each encapsulated ATM cell has the 1-byte ATMSpecific Header. Therefore, the ATM cell information of 53 bytes can be regardedas being encapsulated in 51 bytes.

The multiple cell encapsulation can enhance the bandwidth utilization, but itcauses network delay variation.

The number of cells to be encapsulated is determined by the MTU and CTD andCDV requirements of the ATM connections multiplexed into the PW.

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Normally, the Ethernet service need not be transmitted according to strict sequence. In


ITU-T G. 802.3, however, it is required that frames from the same session should be

transmitted according to the sequence. It cannot be assumed that the PSN can realize

the frame sorting. If strict sorting is required, the serial number need be used.

The following describes the meaning of each field in the CW:

The first four bits must be 0, which indicates that the data is the PW data. Thepacket must be ignored by the PE that receives the packet.

Reserved: It is of 12 bits. It is the reserved field and is often set to 0.

Sequence Number: It is of 16 bits. It is used to guarantee the packet order. This

field is optional. If the Sequence Number is 0, it means the packet order check is

disabled.

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The PTN V1R1 supports the TDM PWE3 over MPLS. In other words, the PSN network


must be the MPLS network.

The PTN V1R1 only supports PTN V1R1 non-channelized E1, and an E1 link

corresponds to a certain PW service.

TDM PWE3 supports the compression of 64k idle timeslots, and supports the

specification of timeslot restoration value.

The packet loading time can be set from 0.125 ms to 5 ms.

The jitter buffer can be set from 0.375 ms to 16ms.

The CESoPSN and SAToP modes can be set.

The restoration of 16-channel clocks is supported.

The OptiX PTN V1R1 supports the mapping of the TDM service to the MPLS of the

highest priority (exp: 5; scheduling priority: EF).

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OptiX RTN 910950