TDM PWs

1
TDM PWs

• Yaakov (J) Stein
• Chief Scientist
• RAD Data Communications

2
Outline

• 1) Pseudowires
• 2) Emulating TDM
• 3) TDMoIP encapsulation formats
• 4) TDM signaling transport
• 5) Timing recovery
• 6) Packet loss and mis-ordering

3
Pseudowires
Pseudowire (PW) A mechanism that emulates the
essential attributes of a native service while
transporting over a packet switched network (PSN)
4
The old model (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)
• doesnt help to design transport networks

5
Simple telephony counter-example
OSI application layer
?
OSI physical layer

• 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

6
The new model (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 ATM
• G.806 CO networks G.781 timing G.8010
  Ethernet
• G.809 CL networks G.783 SDH G.8110 MPLS

7
Layer networks

• Layering
• Network may be decomposed (vertically) into layer
  networks
• Client-server relationship between adjacent layer
  networks
• Layer network
• Basic topological component for information
  transfer
• Link in layer network supported by network below
• Layer network provides link connection to layer
  above
• Layers are completely independent
• Trail
• Transport entity in layer network
• Contains client payload and OAM
• Partitioning
• Network may be decomposed (horizontally) into
  subnetworks connected by links
• Recursively, each subnetwork is similarly
  decomposed
• Peer-peer relationship between adjacent
  subnetworks

8
Network Modes
Circuit Switched (CS)
Packet Switched (PSN)
Connection Oriented (CO)
Connectionless (CL)

• 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

9
Network interworking (tunneling)

• Network interworking may be provided by tunneling
  (edge to edge)
• Service Interworking requires more complex
  mechanisms

10
PseudoWire Emulation Edge to Edge PWE3
Pseudowire (PW) mechanism that emulates
essential attributes of a native service while
transporting over a PSN
Customer Edge (CE)
Customer Edge (CE)
Providers PSN
Customer Edge (CE)
Provider Edge (PE)
Provider Edge (PE)
Customer Edge (CE)
Customer Edge (CE)
native service
PseudoWires (PWs)
native service
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Emulating TDMFrom PSTN to PSN
12
Classic Telephony
Access Network
Core (Backbone) Network
analog lines
SONET/SDH NETWORK
T1/E1
extensions
Synchronous Non-packet network
T1/E1 or AAL1/2

• 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)

13
A few G.XXX sayings

• G.114 (One-way transmission time)
• delay lt 150 ms acceptable
• 150 ms lt delay lt 400 ms conditionally
  acceptable
• delay gt 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

14
TDM PWs
Access Network
analog lines
Packet Switched Network
T1/E1/T3/E3
extensions
Asynchronous network No timing information
transfer
T1/E1 or AAL1/2

• TDMoIP replaces CS core with a PSN
• The access networks and their protocols remain !
• TDM Pseudowire
• Can G.xxx compliance be maintained?

15
Network 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)

16
TDMoIP 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

17
TDMoIP 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

18
TDMoIP encapsulation formats
19
TDMoIP layering structure
PSN / multiplexing
Optional RTP header
TDMoIP Control Word
Adapted TDM payload
higher layers
20
PW Multiplexing

• to reduce resources in core network
• PWs 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

21
Packet 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

22
TDM over IP and MPLS
PSN label
control word
PW label
TDM payload
23
TDMoIP 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

24
TDM 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

timeslots
25
Why not N bytes?

• Why dont we simply encapsulate N bytes frame?

N TDM octets
UDP
IP
RTP?

• because a single lost packet would cause service
  interruption
• need constant N (else dont 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)

26
Why not one frame?

• Why dont we simply encapsulate the T1/E1 frame?

• 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

27
TDM Structure
handling of TDM depends on its structure unstructu
red TDM (TDM arbitrary stream of
bits) structured TDM
28
TDM 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)

29
Structure aware encapsulations

• Structure-locked encapsulation (CESoPSN)
• Structure-indicated encapsulation (TDMoIP AAL1
  mode)
• Structure-reassembled encapsulation (TDMoIP
  AAL2 mode)

headers
AAL1 subframe
AAL1 subframe
AAL1 subframe
AAL1 subframe
30
Structure 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

ptr
seqnum (with CRC)
T1/E1 frames (only timeslots in use)
UDP/IP
for example
_at_
7
TS1 TS2 TS5 TS7 TS1 TS2 TS5 TS7
31
Structure 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

32
TDM Signaling
33
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 (EM, ground-start/loop-start,
  ring-voltage, etc)
• In-band (dial-tone, ring-back, DTMF,etc)
• CAS Channel Associated Signaling
• CCS Common Channel Signaling
• we neednt discuss the analog techniques

34
In-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

35
CAS

• 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

36
CCS

• 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 network
• or signaling gateway employed
• encapsulate (relay) the native signaling
• forward as additional traffic through the PSN

37
HDLCoIP

• HDLCoIP 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

38
TDM Timing Recovery
39
TDM Jitter and Wander

• Jitter short term timing variation
• (i.e. fast jumps - frequency gt 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 lt 10 Hz)
• Measure in MTIE(t) or TDEV(t)
• MTIE - max pk-pk error
• TDEV expected deviation
• Mask as function of t

compared to reference clock Note requirements
for E1 given in G.823
for T1 given in G.824
40
PSN - 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

41
Jitter 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 rate
• How do we know the right rate?
• How do we guard against buffer overflow/underflow?
  

42
Clock 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
• lttn gt Tn d n / R d
• and the packet rate R has been recovered

43
What timing information ?

• We have three clues to the source clock rate
• TOA
• Jitter buffer level
• TOD (if in protocol)

44
FLL

• We can estimate the rate R
• by counting the number of arrivals N per unit
  time T
• the longer the averaging the better the estimate
• R N / T
• Open loop frequency setting
• Better method is closed loop
• Measure reception rate Fn 1 / (tn -
  tn-1)
• Correct present rate F according to filtered Fn
• F F a lt Fn - F gt

tn
F
45
PLL

• Phase 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
counter
46
Packet Loss and Misordering
47
Reasons 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)

48
Handling of packet loss

• In order to maintain timing SOMETHING must be
  output
• towards the TDM interface when a packet is lost
• Packet Loss Concealment methods
• fixed
• replay
• interpolation

49
Voice Quality Comparison

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

50
Mis-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