Transcript TDMoIP : IPR - DSPCSP Pages

TDM PWs
Yaakov (J) Stein
Chief Scientist
RAD Data Communications
Outline
1) Pseudowires
2) Emulating TDM
3) TDMoIP encapsulation formats
4) TDM signaling transport
5) Timing recovery
6) Packet loss and mis-ordering
TDMoIP
Slide 2
Pseudowires
Pseudowire (PW): A mechanism that emulates the
essential attributes of a native service while transporting
over a packet switched network (PSN)
TDMoIP
Slide 3
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)

doesn’t help to design transport networks
APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORK
LINK
PHYSICAL
TDMoIP
Slide 4
Simple telephony counter-example
voice channel
OSI application layer
E1 (TDM)
E3 (PDH)
E1 (TDM)
?
STM1 (SDH)
OC3 (OTN)
voice channel
E3 (PDH)
STM1 (SDH)
OSI physical layer
OC3 (OTN)

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
TDMoIP
Slide 5
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.806 CO networks
G.809 CL networks
G.705 PDH
G.781 timing
G.783 SDH
G.732 ATM
G.8010 Ethernet
G.8110 MPLS
TDMoIP
Slide 6
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
TDMoIP
Slide 7
Network Modes
Circuit Switched
Packet Switched
(CS)
(PSN)
Connection Oriented
Connectionless
(CO)
(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
TDMoIP
Slide 8
Network interworking (tunneling)
Network interworking may be provided by tunneling (edge to edge)
network
Native
Service
Native
Service
edge
edge
Service Interworking requires more complex mechanisms
Native
Service
A
network
Native
Service
B
TDMoIP
Slide 9
PseudoWire Emulation Edge to Edge
PWE3
Pseudowire (PW): mechanism that emulates essential
Customer
Edge
attributes of a native service while transporting over a PSN
(CE)
Customer
Edge
(CE)
Customer
Edge
(CE)
Provider’s PSN
Customer
Edge
Provider
Edge
Provider
Edge
(CE)
(PE)
(PE)
Customer
Edge
native
service
PseudoWires (PWs)
native
service
(CE)
TDMoIP
Slide 10
Emulating TDM
From PSTN to PSN
TDMoIP
Slide 11
Classic Telephony
Access Network
Core (Backbone) Network
analog lines
CO
SWITCH
T1/E1
extensions
P
B
X
T1/E1
or
AAL1/2

SONET/SDH
NETWORK
CO
SWITCH
P
B
X
Synchronous
Non-packet network
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)
TDMoIP
Slide 12
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 13
TDM PWs
Access Network
analog lines
T1/E1/T3/E3
extensions
P
B
X
T1/E1
or
AAL1/2
Packet
Switched
Network
P
B
X
Asynchronous network
No timing information transfer
TDMoIP replaces CS core with a PSN
The access networks and their protocols remain !
TDM Pseudowire
Can G.xxx compliance be maintained?
TDMoIP
Slide 14
Network Comparison
TDM
PSN
Circuit switched
Connection oriented / connectionless
Guaranteed BW
Shared BW
Low overhead
High overhead
Minimal delay
Delay (introduced by forwarding)
Constant arrival rate
Packet delay variation (and bursts)
Timing transport
No physical layer clock
No information loss
Packet loss (congestion, errors)
TDMoIP
Slide 15
TDMoIP Protocol Processing
TDM
frames
IP Packets
IP Packets
TDM
frames
PSN
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
TDMoIP
Slide 16
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
TDMoIP
Slide 17
TDMoIP encapsulation formats
TDMoIP
Slide 18
TDMoIP layering structure
PSN / multiplexing
Optional RTP header
TDMoIP Control Word
higher layers
Adapted TDM payload
TDMoIP
Slide 19
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
TDMoIP
Slide 20
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
TDMoIP
Slide 21
TDM over IP and MPLS
IP header
UDP header
(20 bytes)
(PW label)
Optional RTP header
(8 bytes)
(12 bytes)
TDMoIP Control Word
(4bytes)
TDM payload
PSN
label
PW
label
control
word
TDM payload
TDMoIP
Slide 22
TDMoIP Control Word
PID
flags FRG Length
Sequence Number
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
TDMoIP
Slide 23
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”
T1/E1
frame
SYNC
TS1
TS2
TS3
…
signaling
bits
…
TSn
(1 byte)
TDMoIP
Slide 24
Why not N bytes?
Why don’t we simply encapsulate N bytes frame?
IP
UDP
RTP?
N TDM octets
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 25
Why not one frame?
Why don’t we simply encapsulate the T1/E1 frame?
24 or 32 bytes
IP
UDP
RTP?
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
TDMoIP
Slide 26
TDM Structure
handling of TDM depends on its structure
unstructured TDM (TDM = arbitrary stream of bits)
…
structured TDM
framed
S
Y
N
C
(8000 frames per second)
S
Y
N
C
channelized
SYNC
S
Y
N
C
(single byte timeslots)
TS2
TS1
(1 byte)
TS3
…
signaling
bits
…
TSn
multiframed
frame
frame
frame
…
multiframe
frame
TDMoIP
Slide 27
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)
TDMoIP
Slide 28
Structure aware encapsulations
Structure-locked encapsulation (CESoPSN)
headers
TDM structure
TDM structure
TDM structure
TDM structure
Structure-indicated encapsulation (TDMoIP – AAL1 mode)
headers
AAL1 subframe AAL1 subframe AAL1 subframe
AAL1 subframe
Structure-reassembled encapsulation (TDMoIP – AAL2 mode)
headers
AAL2 minicell
AAL2 minicell
AAL2 minicell
AAL2 minicell
TDMoIP
Slide 29
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
UDP/IP seqnum ptr
T1/E1 frames (only timeslots in use)
(with CRC)
for example
7
@
TS1 TS2 TS5 TS7 TS1 TS2 TS5 TS7
TDMoIP
Slide 30
Structure reassembly - AAL2
TDM frame
1
1
1
2
PSN hdrs
TDM frame
TDM frame
3
4
4
4
5
TDM frame
CW
2
2
hdr
1
2
3
4
TS1
3
5
3
hdr
1
2
3
TS2
TDM frame
4
hdr
5
1
2
3
5
4
5
5
TS3
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
TDMoIP
Slide 31
TDM Signaling
TDMoIP
Slide 32
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 33
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
TDMoIP
Slide 34
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
TDMoIP
Slide 35
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
TDMoIP
Slide 36
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
TDMoIP
Slide 37
TDM Timing Recovery
TDMoIP
Slide 38
TDM Jitter and Wander
Jitter = short term timing variation *
Wander = long term timing variation *
(i.e. fast jumps - frequency > 10 Hz)
(i.e. slow moving- frequency < 10 Hz)
Jitter amplitude in UIpp
Measure in MTIE(t) or TDEV(t)
Unit Interval pk-pk
E1 : 1 UIpp = 1/2MHz = 488 ns
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
TDMoIP
Slide 39
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
E1/T1 VOICE
E1/T1 VOICE
TDMoIP
DATA
PSN
GW
TDMoIP
GW
DATA
The arrival
time is not
constant!!!
TDMoIP
Slide 40
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?
E1/T1 VOICE
E1/T1 VOICE
TDMoIP
DATA
GW
PSN
TDMoIP
GW
DATA
Jitter Buffer
TDMoIP
Slide 41
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
<tn > = Tn +
d =n/R+ d
and the packet rate R has been recovered
TDMoIP
Slide 42
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
Fn = 1 / (tn - tn-1)
Measure reception rate
Correct present rate F according to filtered Fn
F = F + a < Fn - F >
tn
TDMoIP
GW
F
TDMoIP
Slide 44
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
360o
write events
read events
270o
counter
TDMoIP
Slide 45
Packet Loss and Misordering
TDMoIP
Slide 46
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)
router
TDMoIP
Slide 47
Handling of packet loss
In order to maintain timing SOMETHING must be output
towards the TDM interface when a packet is lost
PSN
Packet Loss Concealment methods:
 fixed
 replay
 interpolation
TDMoIP
Slide 48
Voice Quality Comparison
See draft-stein-pwe3-tdm-packetloss-00.txt
TDMoIP
Slide 49
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
1
2
3
4
2
1
5
router
3
4
5
router
1
2
4
3
5
Misordering can be handled by
 Reordering (from jitter buffer)
 Handling as packet loss and dropping later
TDMoIP
Slide 50