1. Chapter 2:
SONET/SDH and GFP
TOPICS
– T1/E1
– SONET/SDH - STS 1, STS -3 frames
– SONET devices
– Self-healing rings
– Generic frame protocol, and Data over SONET
Connection-Oriented Networks - Harry Perros
1
2. T1/E1
• Time division multiplexing allows a link
to be utilized simultaneously by many
users
1
2
M
U
X
link
N
N input
links
Connection-Oriented Networks - Harry Perros
D
E
M
U
X
1
2
N
N output
links
2
3. • The transmission is organized into frames.
• Each frame contains a fixed number of time slots.
• Each time slot is pre-assigned to a specific input
link. The duration of a time slot is either a bit or a
byte.
• If the buffer of an input link has no data, then its
associated time slot is transmitted empty.
• A time slot dedicated to an input link repeats
continuously frame after frame, thus forming a
channel or a trunk.
Connection-Oriented Networks - Harry Perros
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4. Pulse code modulation
• TDM is used in telephony
• Voice analog signals are digitized at the
end office using Pulse Code Modulation.
• A voice signal is sampled 8000 times/sec,
or every 125 µsec.
• A 7-bit or 8-bit number is created every
125 µsec.
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5. The Digital Signal (DS) and
ITU-T standard
• A North American standard that specifies how to
multiplex several voice calls onto a single link.
• The DS standard is a North American standard and
it is not the same as the international hierarchy
standardized by ITU-T.
• Both standards are independent of the transmission.
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6. T carrier / E carrier
• The DS signal is carried over a carrier system
known as the T carrier.
– T1 carries the DS1 signal,
– T2 carries the DS2 signal etc
• The ITU-T signal is carried over a carrier system
known as the E carrier.
• The DS and ITU-T hierarchy is known as the
plesiochronous digital hierarchy (PDH). (Plesion
means “nearly the same”, and chronos means
“time” in Greek).
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7. Digital signal number Voice channels Data Rate (Mbps)
DS0
1
0.064
DS1
24
1.544
DS1C
48
3.152
DS2
96
6.312
DS3
672
44.736
DS3C
1344
91.053
DS4
4032
274.176
Table 2.1: The North American Hierarchy
Level number Voice channels Data Rate (Mbps)
0
1
0.064
1
30
2.048
2
120
8.448
3
480
34.368
4
1920
139.264
5
7680
565.148
Table 2.2: The international (ITU-T) hierarchy
Connection-Oriented Networks - Harry Perros
7
8. The DS1 signal
F
Time
slot 1
Time
slot 2
Time
slot 3
...
Time
slot 24
• 24 8-bit time slots/frame
– Each time slot carries 8 bits/ 125 µsec, or the channel
carries a 64 Kbps voice.
– Every 6th successive time slot (i.e, 6th, 12th, 18th,
24th, etc), the 8 bit is robbed and it is used for
signaling.
• F bit: Used for synchronization. It transmits the
pattern: 10101010…
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9. • T1:
– Total transmission rate: 24x8+1 = 193 bits per 125 µ
sec, or 1.544 Mbps
• E1
– 30 voice time slots plus 2 time slots for
synchronization and control
– Total transmission rate: 32x8 = 256 bits per 125 µsec,
or 2.048 Mbps
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10. Fractional T1/E1
• Fractional T1 or E1 allows the use of only
a fraction of the T1 or E1 capacity.
• For example: if N=2, then only two time
slots are used per frame, which corresponds
to a channel with total bandwidth of 128
Kbps.
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11. Unchannelized frame signal
• The time slot boundaries are ignored by the
sending and receiving equipment.
• All 192 bits are used to transport data followed by
the 193rd framing bit.
• This approach permits more flexibility in
transmitting at different rates.
• This scheme is implemented using proprietary
solutions.
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12. The synchronous optical network
(SONET)
• Proposed by Bellcore (Telecordia).
– It was designed to multiplex DS-n signals and
transmit them optically.
• ITU-T adopted the synchronous digital
hierarchy (SDH), as the international
standard.
– It enables the multiplexing of level 3 signals
(34.368 Mbps)
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13. STS, STM, OC
• The electrical side of the SONET signal is
known as the synchronous transport signal
(STS)
• The electrical side of the SDH is known as
the synchronous transport module (STM).
• The optical side of a SONET/SDH signal is
known as the optical carrier (OC).
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15. • SONET/SDH is channelized.
– STS-3 consists of 3 STS-1 streams, and each STS1 consists of a number of DS-1 and E1signals.
– STS-12 consists of 12 STS-1 streams
• Concatenated structures (OC-3c, OC-12c, etc)
– The frame of the STS-3 payload is filled with
ATM cells or IP packets packed in PPP or HDLC
frames.
– Concatenated SONET/SDH links are commonly
used to interconnect ATM switches and IP routers
(Packets over SONET).
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17. • Main features
– The frame is presented in matrix form and it is
transmitted row by row.
– Each cell in the matrix corresponds to a byte
– The first three columns contain overheads
– The remaining 87 columns carry the
synchronous payload envelope (SPE), which
consists of user data, and additional overheads
referred to as the payload overhead (POH)
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19. The section, line, and path overheads
B1
A1
regenerator
regenerator
STS-1
A
...
STS-1
B
...
STS-12
STS-12
A12
B12
STS-1
STS-1
Section
Line
Section
Section
Line
Section
Section
Line
Path
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20. • Section: a single link with a SONET device
or a regenerator on either side of it.
• Line: A link between two SONET devices,
which may include regenerators
• The section overhead in the SONET frame
is associated with the transport of STS-1
frames over a section, and the line
overhead is associated with the transport of
SPEs over a line.
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23. • The following are some of the bytes in the
section overhead (SOH) :
– A1 and A2: These two bytes are called the
framing bytes and they are used for frame
alignment. They are populated with the value
1111 0110 0010 1000 or 0xF628, which
uniquely identifies the beginning of an STSframe.
– J0: This is called the section trace byte and it
is used for to trace the STS-1 frame back to its
originating equipment.
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24. – B1: This byte is the bit interleaved parity byte
and it is commonly referred to as BIP-8. It is
used to perform an even-parity check on the
previous STS-1 frame after the frame has been
scrambled. The parity is inserted in the BIP-8
field of the current frame before it is scrambled
– E1: This byte provides a 64 Kbps channel can
be used for voice communications by field
engineers.
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25. • The following are some of the bytes in the line
overhead (LOH) that have been defined:
– H1 and H2: These two bytes are known as the pointer
bytes, and they contain a pointer that points to the
beginning of the SPE within the STS-1 frame. The
pointer gives the offset in bytes between the H1 and
H2 bytes and the beginning of the SPE.
– B2: This is similar to the B1 byte in the section
overhead and it is used to carry the BIP-8 parity check
performed on the line overhead section and the
payload section. That is, it is performed on the entire
STS-1 frame except the section overhead bytes.
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26. The path overhead bytes
J1
J1
B3
C2
B3
C2
G1
G1
F2
F2
H4
Z3
Z4
H4
Z3
Z5
Z4
Z5
Location of the POH
Connection-Oriented Networks - Harry Perros
The POH bytes
26
27. • The following are some of the bytes that have
been defined:
– B3: This byte is similar to B1 used in the section
overhead and B2 used in the line overhead. It is used to
carry the BIP-8 parity check performed on the payload
section. That is, it is performed on the entire STS-1
frame except the section and line overhead bytes.
– C2: This byte is known as the path signal label and it
indicates the type of user information carried in the
SPE, such as, virtual tributaries (VT), asynchronous
DS-3, ATM cells, HDLC-over-SONET, and PPP over
SONET.
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28. The STS-1 payload
• The payload consists of user data and the
path overhead.
• User data:
– Virtual tributaries: sub-rate synchronous data
streams, such as DS-0, DS-1, E1, and entire
DS-3 frames
– ATM cells and IP packets
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29. Virtual tributaries
• The STS-1 payload is divided into seven
virtual tributary groups (VTG).
• Each VTG consists of 108 bytes (12 columns)
• Each VTG may carry a number of virtual
tributaries, i.e., sub-rate streams.
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30. • The following virtual tributaries have been
defined:
– VT1.5: This virtual tributary carries one DS-1
signal and it is contained in three columns, that
take up 27 bytes. Four VT1.5’s can be
transported in a single VTG.
– VT2: This virtual tributary carries an E1 signal
of 2.048 Mbps. VT2 is contained in four
columns, that is it takes up 36 bytes. Three
VT2’s can be carried in a single VTG.
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31. • VT3: This virtual tributary transports the
unchannelized DS-1 signal.
A VT3 is
contained in 6 columns that takes up 54 bytes.
This means that a VTG can carry two VT3s.
• VT6: This virtual tributary transports a DS-2
signal, which carries 96 voice channels. VT6 is
contained in 12 columns, that is it takes up 108
bytes. A VTG can carry exactly one VT2.
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32. ATM cells
4
1
2
90
10
Cell 1
Cell 2
Cell 2
Cell 3
3
POH
Cell 14
8
9
Cell 15
Cell 15
• Mapped directly onto the SPE. An ATM
cells may straddle two SPEs.
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33. IP packet over SONET
• IP packets are first encapsulated in HDLC and
the resulting frames are mapped into the SPE
payload row by row as in the case above for
ATM cels.
4
10
90
1
2
7E 7E 7E
3
POH
8
7E 7E 7E
9
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34. • IP packets can also be encapsulated in PPP
instead of HDLC.
• A frame may straddle over two adjacent SPEs, as
in the case of ATM.
• The interframe fill 7E is used to maintain a
continuous bit tstream
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36. • The channelized STS-3 frame is constructed by
multiplexing byte-wise three channelized STS-1
frames. As a result:
– Byte 1, 4, 7, … , 268 of the STS-3 frame contains byte
1, 2, 3, … , 90 of the first STS-1 frame.
– Byte 2, 5, 8, …, 269 of the STS-3 frame contains byte
1, 2, 3, … , 90 of the second STS-1 frame
– Byte 3, 6, 9, …, 270 of the STS-3 frame contains byte
1, 2, 3, … , 90 of the third STS-1 frame.
• This byte-wise multiplexing, causes the columns
of the three STS-1 frames to be interleaved in the
STS-3 frame
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37. • The first 9 columns of the STS-3 frame
contain the overhead part and the
remaining columns contain the payload
part.
• Error checking and some overhead bytes
are for the entire STS-3 frame, and they are
only meaningful in the overhead bytes of
the first STS-1 frame.
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38. SONET/SDH devices
• Several different equipment exist:
– Terminal multiplexer (TM)
– Add/drop multiplexer (ADM)
– Digital cross connect (DCS)
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39. The terminal multiplexer (TM):
• It multiplexes a number of DS-n or E1 signals
into a single OC-N signal
• It consists of a controller, low-speed interfaces
for DS-n or E1 signals, an OC-N interface, and a
time slot interchanger (TSI)
• It works also as a demultiplexer
DS-n
OC-N
...
TM
DS-n
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39
40. The add/drop multiplexer (ADM)
• It is a more complex version of the TM
• It receives an OC-N signal from which it can
demultiplex and terminate (i.e., drop) any
number of DS-n or OC-M signals, where M<N,
while at the same time it can add new DS-n and
OC-M signals into the OC-N signal.
OC-N
OC-N
ADM
...
DS-n. OC-M
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41. SONET rings
ADM
1
OC3
OC3
ADM
4
ADM
2
OC3
OC3
ADM
3
• SONET/SDH ADM devices are typically connected to
form a SONET/SDH ring.
• SONET/SDH rings are self-healing, that is they can
automatically recover from link failures.
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42. An example of a connection
TM
1
A
DS1
ADM
1
OC12
ADM
2
OC3
OC12
OC12
DS1
OC3
ADM
4
OC12
Connection-Oriented Networks - Harry Perros
ADM
3
TM
2
B
42
43. • A transmits a DS-1 signal to TM 1
• TM 1 transmits an OC-3 signal to ADM 1
• ADM 1 adds the OC-3 signal into the STS12 payload and transmits it out to the next
ADM.
• At ADM 3, the DS-1 signal belonging to A
is dropped from the payload and
transmitted with other signals to TM 2.
• TM 2 in turn, demultiplexes the signals and
transmits A’s DS-1 signal to B.
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44. • Connection setup:
– Using network management procedures the
SONET network is provisioned appropriately.
This is an example of a permanent connection.
– It remains up for a long time.
• The connection is dedicated to user A
whether the user transmits or not.
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45. A digital cross connect (DCS)
• It is used to interconnect multiple SONET rings
• It is connected to multiple incoming and outgoing OC-N
interfaces. It can drop and add any number of DSn and/or
OC-M signals, and it can switch DSn and/or OC-M
signals from an incoming interface to any outgoing one.
ADM
ADM
ADM
Ring 1
DCS
ADM
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Ring 2
ADM
ADM
45
46. Self-healing SONET/SDH rings
• SONET/SDH rings have been specially
architected so that they are available 99.999% of
the time (6 minutes per year!)
• Causes for ring failures:
– Fiber link failure due to accidental cuts, and
transmitter/receiver failure
– SONET/SDH device failure (rare)
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47. Automatic protection switching (APS)
• SONET/SDH rings are self-healing, that is, the
ring’s services can be automatically restored
following a link failure or degradation in the
network signal.
• This is done using the automatic protection
switching (APS) protocol. The time to restore the
services has to be less than 50 msec.
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48. Protection schemes: point-to-point
• Schemes for link protection
– dedicated 1+1
– 1:1
– Shared 1:N
Working
ADM
ADM
Protection
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49. Working/protection fibers
• The working and protection fibers have to
be diversely routed. That is, the two fibers
use separate conduits and different physical
routes.
• Often, for economic reasons, the two fibers
use different conduits, but they use the
same physical path. In this case, we say
that they are structurally diverse.
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50. Classification of self-healing rings
• Various ring architectures have been
developed based on the following three
features:
– Number of fibers
• 2 or 4 fibers
– Direction of transmission:
• Unidirectional bidirectional
– Line or path switching
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51. Number of fibers: 2- or 4-fiber rings
1
ADM 1
ADM 2
ADM 1
ADM 2
ADM 4
ADM 3
5
4
8
6
2
7
ADM 4
ADM 3
3
Two-fiber ring: fibers 1, 2, 3, and 4 are
used to form the working ring (clockwise),
and fibers 5, 6, 7, and 8 are used to form
the protection ring (counter-clockwise).
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52. 1
ADM 1
ADM 2
ADM 1
ADM 2
ADM 4
ADM 3
5
4
6
8
2
7
ADM 4
ADM 3
3
• In another variation of the two-fiber ring, each set of fibers
form a ring which can be both a working and a protection
ring. In this case, the capacity of each fiber is divided into
two equal parts, one for working traffic and the other for
protection traffic.
• In a four-fiber SONET/SDH ring there are two working
rings and two protection rings, one per working ring.
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53. Direction of transmission
• Unidirectional ring:
– signals are only transmitted in one
direction of the ring.
• Bidirectional ring:
– signals are transmitted in both directions.
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54. Line and path switching
• Path switching: Restores the traffic on the
paths affected by a link failure (a path is an
end-to-end connection between the point
where the SPE originates and the point where
it terminates.)
• Line switching: Restores all the traffic that
passes through a failed link.
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55. Based on these three features, we have the
following 2-fiber or 4-fiber possible ring
architectures:
–
–
–
–
Unidirectional Line Switched Ring (ULSR)
Bidirectional Line Switched Ring (BLSR)
Unidirectional Path Switched Ring (UPSR)
Bidirectional Path Switched Ring (BPSR)
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56. Of these rings the following three are
used:
– Two-fiber unidirectional path switched ring
(2F-UPSR)
– Two-fiber bidirectional line switched ring
(2F-BLSR)
– Four-fiber bidirectional line switched ring
(4F-BLSR)
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57. Two-fiber unidirectional
path switched ring (2F-UPSR)
1
A
ADM 1
ADM 2
B
5
Working ring
4
6
8
2
Protection ring
7
ADM 4
ADM 3
3
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58. • Features:
– Working ring consists of fibers 1, 2, 3 and 4,
and the protection ring of fibers 5, 6, 7, and 8.
– Unidirectional transmission means that traffic
is transmitted in the same direction. A
transmits to B over fiber 1 of the working ring,
and B transmits over fibers 2, 3, and 4 of the
working ring.
– Used as a metro edge ring to interconnect
PBXs and access networks to a metro core ring
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59. • Self-healing mechanism:
– Path level protection using the 1+1 scheme. The
signal transmitted by A is split into two. One
copy is transmitted over the working fiber 1, and
the other copy is transmitted over the protection
fibers 8, 7, and 6.
– During normal operation, B receives two
identical signals from A, and selects the one
with the best quality. If fiber 1 fails, B will
continue to receive A’s signal over the
protection path. The same applies if there is a
node failure.
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60. Two-fiber bidirectional line switched
ring (2F-BLSR)
ADM 1
1
ADM 2
ADM 3
2
B
A
8
7
6
9
12
3
10
11
C
ADM 6
5
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ADM 5
4
ADM 4
60
61. • Features:
– Used in metro core rings.
– Fibers 1, 2, 3, 4, 5, and 6 form a ring, call it ring 1, on
which transmission is clockwise. Fibers 7, 8, 9, 10, 11,
and 12 form another ring, call it ring 2, on which
transmission is counter-clockwise.
– Both rings 1 and 2 carry working and protection traffic.
This is done by dividing the capacity of each fiber on
ring 1 and 2 to two parts. One part is used to carry
working traffic and the other protection traffic.
– A transmits to B over the working part of fibers 1 and
2 of ring 1, and B transmits to A over the working part
of fibers 8 and 7 of ring 2.
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62. • Self-healing mechanism:
– The ring provides line switching. If fiber 2 fails
then the traffic that goes over fiber 2 will be
automatically switched to the protection part of
ring 2.
– That is, all the traffic will be re-routed to ADM
3 over the protection part of ring 2 using fibers
7, 12, 11, 10, and 9. From there, the traffic for
each connection will continue on following the
original path of the connection.
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62
63. Four-fiber bidirectional line switched
ring (4F-BLSR)
ADM 1
ADM 2
ADM 3
A
B
Working rings
Protection rings
ADM 6
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ADM 5
ADM 4
63
64. • Features
– Two working rings and two protection rings.
The two working rings transmit in opposite
directions, and each is protected by a
protection ring which transmits in the same
direction.
– The advantage of this four-fiber ring is that it
can suffer multiple failures and still function.
In view of this, it is deployed by long-distance
telephone companies in regional and national
rings.
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65. • Self-healing operation (span switching):
– If a working fiber fails, the working traffic will
be transferred over its protection ring. This is
known as span switching.
ADM 1
ADM 2
ADM 3
Normal operation
Connection-Oriented Networks - Harry Perros
ADM 1
ADM 2
ADM 3
Span switching
65
66. • Self-healing operation (ring switching):
– Often, the working and protection fibers are
part of the same bundle of fibers. When the
bundle is cut the traffic will be switched to the
protection fibers. This is known as ring
switching.
ADM 1 Working ADM 2
ADM 1 Working ADM 2
ADM 3
ADM 3
A
A
Protection
Protection
B
B
ADM 6
ADM 5
ADM 4
Connection-Oriented Networks - Harry Perros
ADM 6
ADM 5
ADM 4
66
67. Generic Framing Procedure (GFP)
• This is a light-weight adaptation scheme
that permits the transmission of different
types of traffic over SONET/SDH and in
the future, over G.709.
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68. • GFP permits the transport of
a) frame-oriented traffic, such as Ethernet, and
b) block-coded data for delay-sensitive storage
area networks (SAN) transported by networks
such as Fiber Channel, FICON, and ESCON
over SONET/SDH and G.709.
• GFP is a result of joint standardization
effort by ANSI committee T1X1.5 and ITUT.
• It is described in ITU-T recommendation
G.7041
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68
69. Existing and GFP-based transport options
for end-user applications
Voice
Data (IP, MPLS, IPX)
Private
lines
Ethernet
Frame Relay
SAN
ESCON
FICON
Video
Fiber
Channel
DM
POS
HDLC
ATM
GFP
SONET/SDH
WDM/OTN
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70. The GFP stack
Ethernet
IP over PPP
SAN data
GFP client-dependent aspects
GFP
GFP client-independent aspects
SONET/SDH
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G.709
70
71. GFP frame structure
Core header
Payload length
Payload length
Core HEC
Core HEC
Payload header
Payload
Payload
•
GFP core header
– Payload length indicator
(PLI) - 2 bytes. It gives the
size of the payload.
– Core HEC (cHEC) - 2
bytes. It protects the PLI
field. Standard CRC-16
enables single bit error
correction and multiple bit
error detection.
Payload FCS
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72. The GFP payload structure
Payload type
Payload type
PTI
PFI
EXI
UPI
Type HEC
Payload header
Payload
Type HEC
0-60 bytes
of
extension header
Payload FCS
Payload FCS
Payload FCS
Payload FCS
Payload FCS
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73. GFP payload header
variable-length area from 4 to 64 bytes.
•
Payload type - 2 bytes
–
PTI
Payload type
PFI
EXI
Payload type identifier (PTI) - 3 bits.
Identifies the type of frame:
•
–
UPI
Payload type
–
Type HEC
–
Type HEC
Payload FCS indicator (PFI) - 1 bit.
Identifies if there is a payload FCS
Extension header identifier (EXI) - 4 bits.
Identifies the type of extension header.
User payload identifier (UPI) - 8 bits.
Identifies the type of payload
•
•
•
•
•
•
0-60 bytes
Of
Extension header
•
Connection-Oriented Networks - Harry Perros
User data frames , Client mgmt frames
Frame-mapped Ethernet
Frame-mapped PPP (IP, MPLS)
Transparent-mapped Fiber Channel
Transparent-mapped FICON
Transparent-mapped ESCON
Transparent-mapped GbE
Type HEC (tHEC) - 2 bytes. It protects the
payload header. Standard CRC-16.
73
74. GFP payload trailer
Payload header
•
Payload
Payload FCS
Optional 4-byte FCS.
– CRC-32
– Protects the contents of
the payload
information field.
Payload FCS
Payload FCS
Payload FCS
Payload FCS
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75. GFP-client independent functions
• The client independent sublayer supports
the following functions:
–
–
–
–
Frame delineation
Client/frame multiplexing
Payload scrambler
Client managment
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76. Frame delineation
• The frame
delineation
mechanism is similar
to the one used in
ATM.
• The cHEC is used to
assure correct frame
boundary
identification
Connection-Oriented Networks - Harry Perros
hunt
Non-correctable
core header error
Sync
No 2nd
cHEC
Correct
cHEC
2nd
cHEC match
Presync
76
77. • Operation:
– Under normal conditions, the GFP receiver
operates in the Sync state. The receiver
examines the PLI field, validates the cHEC,
and extracts the framed higher-level PD. It
then moves on to the next GFP header.
– When an uncorrectable error in the core
header occurs (i.e., cHEC fails and more than
one bit error is detected), the receiver enters
the Hunt state.
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78. • Hunt state:
– Using the cHEC it attempts to locate the
beginning of the next GFP PDU, moving one
bit at a time (Same as in ATM - see Perros “An
introduction to ATM networks, Wiley 2001.
– Once this is achieved it moves to the Pre-Sync
state, where it verifies the beginning of the
boundary of the next N GFP PDUs.
– If successful, it moves to the Sync state,
otherwise it moves back to the hunt state.
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79. Frame multiplexing
• Client data frames and client management
frames are multiplexed, with client data
frames having priority over client
management frames.
• Idle frames are inserted to maintain a
continuous bit flow (rate coupling)
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80. GFP client-specific functions
• The client data can be carried in GFP
frames using on of the two adaptation
modes:
– Frame-mapped GFP (GFP-F) applicable to
most packet data types
– Transparent-mapped GFP (GFP-T) applicable
to 8B/10B coded signals
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81. Frame-mapped GFP
• Variable length frames such as:
– Ethernet MAC frames,
– PPP/IP packets
– HDLC-framed PDUs
can be carried in the GFP payload.
• One frame per GFP payload.
• Max. size: 65,535 bytes
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82. Transparent-mapped GFP
• Fiber Channel, ESCON, FICON, Gigabit
Ethernet high-speed LANs use 8B/10B
block-coding to transport client data and
control information.
• Rather than transporting data on a frame-byframe basis, the GFP transparent-mapped
mode, transports data as a stream of
characters.
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82
83. • Specifically, the individual characters are
de-mapped from their client 8B/10B block
codes and then mapped into periodic fixedlength GFP frames using 64B/65B block
coding.
• This reduces the 25% overhead introduced
by the 8B/10B block-coding.
• Also, transparent mapping reduces latency,
which is important for storage related
applications
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83
84. • The first step, is to decode the 8B/10B
codes. The 10 bit code is decoded into its
original data or control codeword value.
• The decoded characters are then mapped
into 64B/65B codes. A bit in the 65-bit
code indicates whether the 65-bit block
contains only data or control characters are
also included
• 8 consecutive 65-bit blocks are grouped
together into a single superblock.
• A GFP frame contains N such superblocks.
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85. Data over SONET/SDH (DoS)
• The DoS architecture provides an efficient
mechanism to transport data coming from
interfaces such as: Ethernet, Fiber Channel,
ESCON/FICON over SONET/SDH.
• It relies on a combination of
– GFP,
– Virtual concatenation, and
– Link capacity adjustment scheme (LCAS)
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85
86. Virtual concatenation
• This procedure maps an incoming traffic stream
into a number of individual sub-rate payloads.
• The sub-rate payloads are switched through the
SONET/SDH network independently of each
other
• At the destination, they are used to reconstruct the
original traffic stream.
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87. Example
• Let us consider the case of transporting the
1 GbE signal over SONET/SDH.
• According to the specifications, an STS48c (2,488 Gbps) has to be used, thus
leaving a lot of unused capacity.
• Using the virtual concatenation scheme, 7
independent STS-3c (7x155,520 = 1,088)
can be employed to carry the 1 GbE signal
at full rate.
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88. This works as follows:
• At the transmitter the incoming stream is
de-multiplexed and distributed in some
fashion over 7 different payloads, each an
STS-3c.
• Intermediate SONET/SDH nodes only see
different payloads and they are not aware
of the concatenation
• At the destination, the seven flows get
multiplexed into the single original GbE
stream.
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88
89. Link capacity adjustment scheme
(LCAS)
• This scheme permits to dynamically adjust
the number of sub-rate payloads allocated
to a traffic stream, whose transmission rate
may vary over time.
• LCAS can be also used when re-routing
traffic due to a failure.
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89
