TDM PWsTDM PWs

Yaakov (J) Stein Chief ScientistRAD Data Communications

TDMoIP Slide 2

OutlineOutline

1) Pseudowires

2) Emulating TDM

3) TDMoIP encapsulation formats

4) TDM signaling transport

5) Timing recovery

6) Packet loss and mis-ordering

TDMoIP Slide 3

PseudowiresPseudowires

Pseudowire (PW): Pseudowire (PW): A mechanism that emulates the A mechanism that emulates the essential attributes of a native service while transporting essential attributes of a native service while transporting over a packet switched network (PSN)over a packet switched network (PSN)

TDMoIP Slide 4

The The oldold model model (X.200, OSI)(X.200, OSI)

Once upon a time networks were exclusively described by the OSI

model

However

few networks actually work only that way

highly inflexible (always need more layers!)

some features only in one place (security, mux)

missing features (OAM)

doesn’t help to design transport networks PHYSICAL

LINK

NETWORK

TRANSPORT

SESSION

PRESENTATION

APPLICATION

TDMoIP Slide 5

Simple telephony counter-exampleSimple telephony counter-example

this type of scenario important to carriers, and thus to ITU-T

not captured by ISO layering model

there can be an arbitrary large number of intervening layers

all intermediate layers fulfill the same function -- transport

voice channel

E1 (TDM)

E3 (PDH)

STM1 (SDH)

OC3 (OTN)

voice channel

E1 (TDM)

E3 (PDH)

STM1 (SDH)

OC3 (OTN)OSI physical layer

OSI application layer

?

TDMoIP Slide 6

The The newnew model model (G.805)(G.805)

A more general and applicable model for transport networks

Layer network and trail

Layering and partition

Basic network modes

Interworking

Diagrammatic technique

References:G.805 generic G.705 PDH G.732 ATMG.806 CO networks G.781 timing G.8010 Ethernet G.809 CL networks G.783 SDH G.8110 MPLS

TDMoIP Slide 7

Layer networksLayer networks

LayeringNetwork may be decomposed (vertically) into layer networksClient-server relationship between adjacent layer networks

Layer networkBasic topological component for information transferLink in layer network supported by network belowLayer network provides link connection to layer aboveLayers are completely independent

Trail Transport entity in layer network Contains client payload and OAM

PartitioningNetwork may be decomposed (horizontally) into subnetworks connected by linksRecursively, each subnetwork is similarly decomposedPeer-peer relationship between adjacent subnetworks

TDMoIP Slide 8

Network ModesNetwork Modes

Many native network types (technologies) for each mode– CS: TDM, PDH, SDH, OTN– CO: ATM, FR, MPLS, TCP/IP, SCTP/IP– CL: UDP/IP, IPX, Ethernet, CLNP

Can layer any mode over any mode – BUT some layerings may involve performance loss– CL over CO over CS is EASY– CO over CL, or CS over CO is harder– CS over CL is very hard

Circuit Switched

(CS)

Packet Switched

(PSN)

Connection Oriented

(CO)

Connectionless

(CL)

TDMoIP Slide 9

Network interworking (tunneling)Network interworking (tunneling)Network interworking may be provided by tunneling (edge to edge)

Service Interworking requires more complex mechanisms

networkNativeService

NativeService

edge edge

NativeService

A

NativeService

B

network

TDMoIP Slide 10

PPseudoseudoWWire ire EEmulation mulation EEdge to dge to EEdge dge PWEPWE33

ProviderEdge

(PE)Customer

Edge

(CE)

CustomerEdge

(CE)

CustomerEdge

(CE)

nativeservice

Provider’s PSN

PseudoWires (PWs)

CustomerEdge

(CE)

CustomerEdge

(CE)

ProviderEdge

(PE)nativeservice

Pseudowire (PW): Pseudowire (PW): mechanism that emulates essential mechanism that emulates essential attributes of a native service while transporting over a PSNattributes of a native service while transporting over a PSN

TDMoIP Slide 11

Emulating TDMEmulating TDM

From PSTN to PSNFrom PSTN to PSN

TDMoIP Slide 12

Classic TelephonyClassic Telephony

Circuit switched ensures signal integrity Very High Reliability (“five nines”) Low Delay and no noticeable echo   Timing information transported over the network Mature Signaling Protocols (over 3000 features)

T1/E1

COSWITCH

analog lines

SONET/SDHNETWORK

PBX

extensions

Access Network

T1/E1 or AAL1/2

PBX

Core (Backbone) Network

SynchronousNon-packet network

COSWITCH

TDMoIP Slide 13

A few G.XXX sayings …A few G.XXX sayings …

G.114 (One-way transmission time)– delay < 150 ms acceptable– 150 ms < delay < 400 ms conditionally acceptable– delay > 400 ms unacceptable– G.126/G.131 echo control may be needed

G.823/G.824 (timing)– primary vs. secondary clocks– jitter masks– wander masks

G.826 (error performance)– BER better than 2 * 10-4

– strict limitation on errored seconds

TDMoIP Slide 14

TDM PWsTDM PWs

TDMoIP replaces CS core with a PSNThe access networks and their protocols remain !TDM PseudowireCan G.xxx compliance be maintained?

T1/E1/T3/E3

analog lines

PBX

extensions

Access Network

PBX

T1/E1 or AAL1/2

Packet Switched Network

Asynchronous networkNo timing information transfer

TDMoIP Slide 15

Network ComparisonNetwork Comparison

TDM

Circuit switched

Guaranteed BW

Low overhead

Minimal delay

Constant arrival rate

Timing transport

No information loss

PSN

Connection oriented / connectionless

Shared BW

High overhead

Delay (introduced by forwarding)

Packet delay variation (and bursts)

No physical layer clock

Packet loss (congestion, errors)

TDMoIP Slide 16

TDMoIP Protocol ProcessingTDMoIP Protocol Processing

Steps in TDMoIP The synchronous bit stream is segmented The TDM segments are adapted TDMoIP control word is prepended PSN (IP/MPLS) headers are prepended (encapsulation) Packets are transported over PSN to destination PSN headers are utilized and stripped Control word is checked, utilized and stripped TDM stream is reconstituted (using adaptation) and played out

TDMframes

TDMframes

IP Packets

PSN

IP Packets

TDMoIP Slide 17

TDMoIP vs. VoIPTDMoIP vs. VoIP

Two ways to integrate TDM services into PSNs

VoIP Revolution - complete (forklift) CPE replacement New signaling protocols (translation needed) New functionality (e.g. video-phone, presence)

TDMoIP Evolution - CPE unchanged, IWF added at edge No change to signaling protocols (network IW) No new functionality Migration path

TDMoIP Slide 18

TDMoIP encapsulation formatsTDMoIP encapsulation formats

TDMoIP Slide 19

TDMoIP layering structureTDMoIP layering structure

PSN / multiplexing

Optional RTP header

TDMoIP Control Word higher layers

Adapted TDM payload

TDMoIP Slide 20

PW MultiplexingPW Multiplexing

to reduce resources in core networkPWs are sent inside PSN tunnels

we often wish to send several PWs in same tunnel

to demux we use a PW label

for application muxing, IANA has assigned to TDMoIP

UDP port number 0x085E (2142)

in IP networks we use UDP source port number as bundle ID

in MPLS networks we use an inner label

for L2TPv3 we could use L2TP multiplexing

TDMoIP Slide 21

Packet ComponentsPacket Components

PSN headers• ensure packet transported to destination

RTP header• contains timestamp that may help in timing recovery

Control Word • enables detection of out-of-order and lost packets• indicates critical alarm conditions

TDM payload may be adapted• to assist in timing recovery and recovery from packet loss• to ensure proper transfer of TDM signaling• to provide an efficiency vs. latency trade-off

TDMoIP Slide 22

TDM over IP and MPLSTDM over IP and MPLS

IP header (20 bytes)

UDP header (PW label) (8 bytes)

Optional RTP header (12 bytes)

TDMoIP Control Word (4bytes)

TDM payload

PSN label

PW label

controlword

TDM payload

TDMoIP Slide 23

TDMoIP Control WordTDMoIP Control Word

PID (4b) special uses

flags (4 b)– L bit (Local failure)– R bit (Remote failure)

FRG (2 bits) indicates fragmentation (only for special uses)

Length (6 b) used when packet may be padded

Sequence Number (16 b) used to detect packet loss / misordering

PID flags FRG  Length            Sequence Number  

TDMoIP Slide 24

TDM PayloadTDM Payload

What needs to be transported from end to end? Voice (telephony quality, low delay, echo-less) Tones (for dialing, PIN, etc.) Fax and modem transmissions Signaling (there are 1000s of PSTN features!) Timing

T1/E1frame

SYNC TS1 TS2 TS3signaling

bits… … TSn

(1 byte)

“timeslots”

TDMoIP Slide 25

Why not N bytes?Why not N bytes?

Why don’t we simply encapsulate N bytes frame?

IP N TDM octetsUDP RTP?

because a single lost packet would cause service interruption need constant N (else don’t know how many TDM bytes were lost) need to conceal lost packet by proper amount of AIS “all ones” TDM synchronization would be lost

SAToP is good for well-engineered networks essentially no packet loss very low PDV (see below)

TDMoIP Slide 26

Why not one frame?Why not one frame?

Why don’t we simply encapsulate the T1/E1 frame?

IP T1/E1 frame

24 or 32 bytes

UDP RTP?

because it is inefficient - however N frames is reasonable (structure-locking)

because a single lost packet could cause service interruption and for CAS, signaling uses a superframe (16/24 frames) so superframe integrity must be respected too

because we want to efficiently handle fractional T1/E1

because we want a latency vs. efficiency trade-off

TDMoIP Slide 27

TDM StructureTDM Structurehandling of TDM depends on its structure

unstructured TDM (TDM = arbitrary stream of bits)

structured TDM

…

SYNC TS1 TS2 TS3signaling

bits… … TSn

(1 byte)

channelized (single byte timeslots)

SYNC

framed (8000 frames per second)SYNC

SYNC

multiframed

frame frame frame … frame

multiframe

TDMoIP Slide 28

TDM transport typesTDM transport types

Structure-agnostic transport (SAToP)• for unstructured TDM• even if there is structure, we ignore it• simplest way of making payload• OK if network is well-engineered

Structure-aware transport (TDMoIP, CESoPSN)• take TDM structure into account• must decide which level of structure (frame, multiframe, …)• can overcome PSN impairments (PDV, packet loss, etc)

TDMoIP Slide 29

Structure aware encapsulationsStructure aware encapsulations

Structure-locked encapsulation (CESoPSN)

Structure-indicated encapsulation (TDMoIP – AAL1 mode)

Structure-reassembled encapsulation (TDMoIP – AAL2 mode)

headers TDM structure TDM structure TDM structureTDM structure

headers AAL1 subframe AAL1 subframe AAL1 subframe AAL1 subframe

headers AAL2 minicell AAL2 minicell AAL2 minicell AAL2 minicell

TDMoIP Slide 30

Structure indication - AAL1Structure indication - AAL1

For robust emulation: adding a packet sequence number adding a pointer to the next superframe boundary only sending timeslots in use allowing multiple frames per packet

for example

UDP/IP T1/E1 frames (only timeslots in use)seqnum(with CRC)

ptr

TS1 TS2 TS5 TS7 TS1 TS2 TS5 TS77 @

TDMoIP Slide 31

Structure reassembly - AAL2Structure reassembly - AAL2

AAL1 is inefficient when timeslots are dynamically allocated each minicell consists of a header and buffered data minicell header contains:

– CID (Channel IDentifier)– LI (Length Indicator) = length-1– UUI (User-User Indication) counter + payload type ID

TDM frame TDM frame TDM frameTDM frameTDM frame

1 2 4 531 1 2 2 3 3 4 4 5 5

1 2 3 4 5hdr 1 2 3 4 5hdr 1 2 3 4 5hdrPSN hdrs CW

TS1 TS2 TS3

TDMoIP Slide 32

TDM SignalingTDM Signaling

TDMoIP Slide 33

Signaling?Signaling?

signaling is used for network control – call setup/tear-down (including routing)– OAM– billing

in TDM networks there may be different types:– Subscriber - CO – CO - CO– CO - CPE (e.g. PBX)

there are four common PSTN signaling techniques:– Analog * (E&M, ground-start/loop-start, ring-voltage, etc)– In-band (dial-tone, ring-back, DTMF,etc)– CAS – Channel Associated Signaling– CCS – Common Channel Signaling

* we needn’t discuss the analog techniques

TDMoIP Slide 34

In-band signalingIn-band signaling

in-band signaling is transferred in the audio (200-3600Hz) band for example:

– call progress tones (dial tone, ring back)– DTMF tones, – FSK for caller identification, – MFR1 in North America or MFCR2 in Europe,

audible tones in TDM time slot automatically forwarded this is not the case for VoIP!

– speech compression may not pass (need tone relay)– VoIP protocols replace legacy signaling with its own

SIP H.323 Megaco

TDMoIP Slide 35

CASCAS

CAS is carried in the same T1/E1 as payload – but not in the audio – T1 uses robbed bits – E1 uses a dedicated time slot (usually TS16)

Readily handled by TDMoIP (even for fractional T1/E1 links)

VoIP systems need to – detect the CAS bits, – interpret them according to the appropriate protocol – transport them through PSN using a relay protocol – finally regenerate and recombine them at the far end

TDMoIP Slide 36

CCSCCS

Examples: ISDN PRI signaling, SS7

if occupy a TDM timeslot (trunk associated)then forwarded by TDMoIP (see HDLCoIP)

if not trunk associated, then forwarded by signaling networkor signaling gateway employed

encapsulate (relay) the native signaling forward as additional traffic through the PSN

TDMoIP Slide 37

HDLCoIPHDLCoIPHDLCoIP intended to operate in port mode

Data / control messages transparently transported

Assume messages shorter than the MTU (no fragmentation)

Only use when has potential to significantly compress BW

Transmission :– monitor flags until frame detected– test FCS– if incorrect - discarded– if correct -

perform unstuffing flags and FCS removed send frame

TDMoIP Slide 38

TDM Timing RecoveryTDM Timing Recovery

TDMoIP Slide 39

TDM Jitter and WanderTDM Jitter and Wander

Jitter = short term timing variation *(i.e. fast jumps - frequency > 10 Hz)

Jitter amplitude in UIpp

Unit Interval pk-pk

E1 : 1 UIpp = 1/2MHz = 488 ns

Wander = long term timing variation *(i.e. slow moving- frequency < 10 Hz)

Measure in MTIE() or TDEV()MTIE - max pk-pk errorTDEV expected deviation

Mask as function of

* compared to reference clock

Note: requirements for E1 given in G.823 for T1 given in G.824

TDMoIP Slide 40

PSN - Delay and PDVPSN - Delay and PDV

PSNs do not carry timing clock recovery required for TDMoIP

PSNs introduce delay and packet delay variation (PDV) Delay degrades perceived voice quality PDV makes clock recovery difficult

PSN

The arrival time is not constant!!!

E1/T1 VOICE

DATA

E1/T1 VOICE

DATA

TDMoIP

GW

TDMoIP

GW

TDMoIP Slide 41

Jitter BufferJitter Buffer

Arriving TDMoIP packets written into jitter buffer

Once buffer filled 1/2 can start reading from buffer

Packets read from jitter buffer at constant rateHow do we know the right rate?How do we guard against buffer overflow/underflow?

PSN

E1/T1 VOICE

DATA

E1/T1 VOICE

DATA

Jitter Buffer

TDMoIP

GWTDMoIP

GW

TDMoIP Slide 42

Clock RecoveryClock Recovery

The packets are injected into network ingress at times Tn

For TDM the source packet rate R is constant

Tn = n / R

The network delay Dn can be considered to be the sum of

typical delay d and random delay variation Vn

The packets are received at network egress at times tn

tn = Tn + Dn = Tn + d + Vn

By proper averaging/filtering

tn = Tn + d = n / R + d

and the packet rate R has been recovered

TDMoIP Slide 44

FLLFLLWe can estimate the rate R

by counting the number of arrivals N per unit time Tthe longer the averaging the better the estimate

R = N / TOpen loop frequency settingBetter method is closed loop

Measure reception rate Fn = 1 / (tn - tn-1)

Correct present rate F according to filtered Fn

F = F + < Fn - F >

TDMoIP

GW

F

n

TDMoIP Slide 45

PLLPLLPhase difference between

write (arrival) clock and read (present local) clock

Number of packets written into the jitter buffer

minus the number of packets read from the jitter buffer

write events

read events

360o

270o

counter

TDMoIP Slide 46

Packet Loss and MisorderingPacket Loss and Misordering

TDMoIP Slide 47

Reasons for packet lossReasons for packet loss

In a perfect network all packets should reach their destination

In real networks, some packets are lost

Loss is caused by

bit errors invalidating the data (detected by ECC)

intentional dropping by forwarder because of congestion

intentional dropping by forwarder due to policy (e.g. (W)RED)

router

TDMoIP Slide 48

Handling of packet lossHandling of packet loss

In order to maintain timing SOMETHING must be outputtowards the TDM interface when a packet is lost

Packet Loss Concealment methods: fixed replay interpolation

PSN

TDMoIP Slide 49

Voice Quality ComparisonVoice Quality Comparison

See draft-stein-pwe3-tdm-packetloss-00.txt

TDMoIP Slide 50

Mis-orderingMis-ordering

In a perfect network all packets should arrive in proper order

In real networks, some packets are delayed (or even duplicated!)

Misordering is caused by parallel paths– aggravated by load balancing mechanisms

Misordering can be handled by Reordering (from jitter buffer) Handling as packet loss and dropping later

router router1 2 3 4 5 1 2 4 3 5

1 2

3

4

5