SONET
SYNCHRONOUS OPTICAL NETWORK
INTRODUCTION:
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In 1985 Bell Core proposed SONET as a solution for high speed transmission
over optical fiber
Later, ANSI standardized SONET in North America and ITU-T SDH in Europe.
SONET stands for “Synchronous Optical Network” and SDH stand for
“Synchronous Digital Hierarchy”
The term Optical was dropped by ITU-T because SONET was being transmitted
using other media such as Micro Wave
SONET Advantages:
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SONET uses synchronous multiplexing, that’s why STS-n (Synchronous
Transport Signals) are an exact multiple of STS-1 signal ( as opposed to DS1,
DS2, DS3 etc which are not).
Synchronous multiplexing allows adding and dropping channels without demultiplexing and then re-multiplexing the entire set of channels at switching
points.
SONET uses ADM ( Add Drop Multiplexers) which allow channels to be
switched and routed from one link to another within milliseconds as compared to
T1 signals which take a long time.
SONET is capable of Automatic Protection Switching (APS), which allows rerouting of traffic from a failed link to another active link without loss of
information.
SONET provides improved Operation, Administration, Management (OAM),
Diagnostics, Fault Detection and Correction, and Communication facilities.
SONET is an International Standard, whereas T3 is proprietary and therefore
customers can switch between vendors and manufacturers.
SONET Rates and SONET Hierarchy:
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Table below shows SONET Rates. STS designation stands for Electrical Signals
and OC stands for corresponding OPTICAL levels.
STM (Synchronous Transport Module) designation stands for the corresponding
SDH ( European ) signals. Notice that STM-1 starts at STS-3
Draw the Table in the space provide
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SONET TOPOLOGY AND IMPLEMENTATION:
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Consider the generic implementation shown in the diagram(p320B) as well as
ALCATEL implementation. Notice the existing payloads being aggregated by
various SONET equipment and transported over a SONET Ring. Draw a rough
sketch for your record in the space provided.
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Notice that SONET is capable of carrying all existing payloads such as DS0, DS1,
DS3, E1, E2, E3, ATM, FR, IP Packets etc.
Notice also that there are 3 major SONET devices
(a)
Terminal Multiplexer/Service Adapter /Access Node/STSMultiplexer (TM/SA/AN/STS-M): Performs following functions.
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- Maps various subrate payloads into SONET envelope
- Generates STS-1 Frame
- Can be a part of an ADD/DROP multiplexer, in that case it
multiplexes various STS-n signals onto a fiber channel
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(b)
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ADD/DROP Multiplexer (ADM):
Multiplexes various STS-n signals onto fiber channel i.e converts
electrical signals into optical signals
Inserts and removes ( Multiplexes and De-multiplexes OC signals
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(c)
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Digital Cross Connect ( DCS or DXC):
Acts as a HUB
Can ADD/DROP payloads
Makes two-way cross connection between different Carrier rates
SONET MULTIPLEXING:
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Consider the Fig shown (P322B). Notice how various existing payloads are fed to
the Service Adapters and mapped into STS-1 or STS-n frames. These are still
electrical signals. These various STS frames are then fed to a Multiplexer, which
generates a higher rate STS-n frame. Electrical to Optical conversion takes place
following the Multiplexer and finally, this very speed frame is transmitted through
the fiber.
What device would you use to carry out electrical to optical conversion? _______
SONET CONFIGURATION, EQUIPMENT AND LAYERS:
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Fig below shows SONET configuration (p323B, p595,596 F). Draw a sketch in
the space provided. Any SONET Network will be configured the way shown
below.
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A SECTION : is a portion of transmission facility between any two adjacent
Network elements
A LINE is an optical link between any two multiplexers.
A PATH is an end to end link between two STS-Multiplexers
SONET LAYERS: Fig below shows SONET Layers. Notice that the SONET layers
correspond to the SONET configuration defined above.
PATH LAYER: Performs following functions:
(a)
(b)
Maps user payload into STS-1 Frame
Inserts path level overheads that are carried end-to-end
LINE LAYER: Performs following functions
(a)
(b)
(c)
(d)
Synchronization and Multiplexing of STS-1 signals into STS-n signals
Error monitoring
Line Maintenance and Automatic Protection Switching
Inserts Line level Overheads
SECTION LAYER: Performs following functions
(a)
(b)
(c)
(d)
Framing
Error monitoring
Scrambling
Inserts Section Overheads
PHOTONIC LAYER: Performs following functions:
(a)
(b)
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Converts Electrical signals into optical signals and Vice-Versa.
Deals with signal levels, pulse shape, power levels, spectral characteristics
of 1310 and 1550nm wavelength and all other physical characteristics of
the fiber and the corresponding optical signals. No Overhead associated
with this layer
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SONET EQUIPMENT: Corresponding to above-mentioned layers a SONET Network
always deploys following equipment to carry out layer functions
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PATH TERMINATING EQUIPMENT ( PTE): Performs following functions:
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Maps user payloads into a standard STS frame format
LINE TERMINATING EQUIPMENT: Performs following functions
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Combines STS-1 frames into higher multiplexed levels at the transmit end
Converts STS-n signals into corresponding ON-n level
At the RX end, converts optical signals back to electrical and also demultiplexes STS-n into STS-1’s
SECTION TERMINATING EQUIPMENT: Performs following functions:
(a)
(b)
(c)
(d)
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Framing
Timing
Regeneration of Signals
Amplification
Notice that a Terminal multiplexer ( STS- Multiplexer/SA) incorporates path
layer functions
And an ADM/DCS incorporates Line Layer functions
A Regenerator incorporates Section and Photonic layer functions
SONET ENVELOP (SONET FRAME) :
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Consider the Fig shown (P328B). This fig shows serial bit stream of the SONET
FRAME. Draw the picture in the space provided.
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Notice the SONET envelop (Fig 15-18 T). Notice that although the transmission
is serial, the SONET frame is organized in the form of an envelope. This is called
an STS-1 Frame. Bits are transmitted from left to right, beginning with the first
octet of the first row, then the 2nd octet of the first row, and the 3rd and the 4rth
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and so on till the 90th octet of the first row. Then the first octet of the 2nd row, then
the 2nd octet of the 2nd row and so on until the 90th octet of the 9th row.
STS-1 envelope consists of ___Columns____Rows of ____ bits Octets with a
total of ______Octets 8bits/Octet___________bits transmitted in 125S
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SONET transmits 8000 such frames per second.
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Therefore, SONET STS-1 transmission rate will be
_______frames/second_______Octets/frame______bits/Octet_______Mbps.
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The first 3 columns (total of 27 bytes) are reserved for transport overheads(TOH).
First 3 bytes in the first 3 rows (total of 9 bytes) are reserved for Section
Overheads (SOH) and the remaining 18 bytes of the TOH are reserved for Line
Overheads (LOH).
The remaining 87 columns are called the Synchronous Payload Envelope (SPE).
One column of the SPE is reserved for Path Overheads (POH). The POH may be
located any where in the SPE, however it usually appears in the First column of
the SPE.
Therefore the remaining 86 columns are used to carry the actual Payload. Thus
the actual payload rate will be _____Columns_____ Rows _____bits=
_____Mbps
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An STS-3 envelope transmits 3 STS-1 frames in 125S giving a line rate of
_______________________Mbps.
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An STS-12 envelope transmits 12 STS-1 frames in 125S giving a line rate of
__________________= ___________Mbps.
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An STS-192 transmits 192 STS-1 frames in exactly 125S giving a line rate of
_______________= _________Gbps and so on.
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Notice that each higher-level STS frame is an exact multiple of STS-1 frame or an
exact multiple of any intermediate level. That’s why SONET is Synchronous as
opposed to DS levels that are not
The SDH envelope begins at STS-3 with a line rate of 155.52 Mbps.
The STS-3 frame can transport a broadband ISDN H4 channel directly in to the
Envelope. Fig shows the construction of STS-3 frame ( fig 16.15 H). Notice that 3
STS-1 frames are byte interleaved into the STS-3 frame.
Fig below shows the construction of STS-3c Frame ( c indicates concatenated). In
this case a user payload is large enough to be mapped into an STS-3 frame and
therefore requires only one set of overheads rather then 3 separate sets of
overheads.
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Mapping User Payload into SONET Envelope:
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Before we discuss how user payload is mapped into the SONET Envelope, we
must have a clear understanding of what the user payload might be. A user
payload might consist of:
(a)
Sub STS-1 rate signals such as
DS0 at 64 Kbps
DS1 at 1.544 Mbps
E1 ( CEPT-1 at 2.048 Mbps
DS1-c at 3.152 Mbps
DS2 at 6.312 Mbps
DS3 at 44.738 Mbps
ATM cells at 51.84 Mbps
FDDI
(e)
FR
(f)
(b)
©
(d)
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ISDN
(g)
IP Data
Most of the existing payload consists of lower rate Sub- STS-1 Signals. These
subrate signals are mapped into the STS-1 frame using a technique called Virtual
Tributeries (VT) in North America and Virtual Containers (VC) in Europe. Four
sizes of VTs have been defined in North America as follows.
VT1.5 at 1.728 Mbps (to carry DS1 load)
VT2 at 2.304 Mbps (to carry E1 or CEPT-1 load)
VT3 at 3.456 Mbps (to carry DS1-c load)
VT6 at 6.912 Mbps (to carry DS2 load)
DS3 signals at 44.738 Mbps can be mapped directly into the SPE without VT
groupings. Consider the Fig shown. (VT groupings P1 of your notes).
86 columns of STS-1 SPE
VT 1.5
SECTION
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VT2
SECTION
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VT3
SECTION
VT6
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3C9R
4C9R
6C9R
VT1.5
1.728
VT2
2.304
VT3
3.456
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12C9R
VT6
6.912
Furthermore these VTs are organized into 7 groups with each group consisting of
12 Columns by 9 Rows. Each group can carry only one type of VT. SPE consists
of 87 columns. This gives
__Groups__ Columns /group = __C+1C of POH+2 Empty Columns = 87 Columns,
which is the SPE capacity.
87C9 R=SPE
P GR 1
GR2
O 12C9R 12C9R
GR3
GR4
12C9R
12C9R
GR5
12C9R
GR6
GR7
12C9R
12C9R
H
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Since each VT group can carry only one type of VT, how many VT 1.5s or VT2s
or VT3s or Vt6s can we fit in one VT group? Look at the fig below
12C  9R
12C  9R
12C9R
12C9R
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In each VT group we can transport ___VT 1.5s or ___VT 2s or ___VT3s or _VT6
Therefore an SPE can carry any combination of VTs that will give us a total of
84columns. For example we can have all 7 groups carry VT1.5s for a total of
____ = ____VT1.5s = ___ DS0___ = ______ Voice channels. However, we
have _____ bytes available in a VT 1.5. Why? _________. . The remaining 3
bytes are used for VT 1.5 OH
Similarly, ___ ____ = ______ VT2s = ____ _____= ______voice channel can
be carried in an SPE.
OR ___ ___ = ____ VT3s = 48 DS1-c____ = _____ voice grade Channels
OR ___ ___ = ____ VT6s = __ DS2 _____ = ____voice grade channels
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OR an SPE can carry a combination of 4VT1.5groups+2VT2 groups+1 VT6
group or any other combination as long as total number columns occupied = ___
Furthermore, the VT Groups are assigned specific columns in the SPE for byte
interleaving (to mimic STS-3 structure). The reason for doing that is to multiplex
byte by byte each source until an entire row is filled. The result is that the
multiplexing process creates a column-by-column structure where each column
belongs to a specific source and always occupies the same position in the SPE.
This makes synchronous Multiplexing, De-multiplexing, Identification and
Switching of individual VTS extremely fast. And it also makes the adding and
dropping individual channels at various switching points extremely easy, fast and
efficient.
Look at fig 7-2 , p211 of the handout .
VT’s within a group is also interleaved for easy identification and switching for
example, a VT Group can carry 4 VT 1.5 as follows
VT 1.5 (A)
VT 1.5 (B)
VT 1.5 (C)
VT1.5 (D)
1
5
2
3
4
A
B
9
C
D
A
6
10
B
C
D
7
11
A
B
C
8 12
D
Complete the Map yourselves.
Similarly, VT2 following columns in Groups A, B, and C
A
1, 4, 7, 10
B
2, 5, 8, 11
C
_, _, _, __ ( Fill in the blanks )
Show the chart for VT3 and VT6
DS3 signal can be mapped directly into the STS-1 Frame.
I will show you ATM cell mapping into the SONET envelop when we cover Atm
in the class.
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SONET OVERHEAD BYTES
SONET OH bytes perform following functions.
(a)
(b)
(c)
(d)
(e)
(f)
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Synchronization and Multiplexing
Alarms and Supervisory reporting
Provisioning commands
Emergency Restoration Instructions
Voice Circuits ( popularly called Order Wiring)
Provides Data communication channels for interoffice communications
The STS-1 frame structure is reproduced below to show the location of OH bytes
STS-1 Frame- 90 Bytes
TOH = 27 Bytes
S
O
H
SPE
POH
Framing
Framing
STS-ID
TRACE
A1
A2
C1
J1
B
BIP-8
B1
Order wire USER CH
E1
Datacom Datacom
D1
D2
Pointer
H1
BIP-8
B2
Pointer
H2
APS
K1
BIP-8
F1
B3
Datacom
D3
Signal
Label
C2
Path
Status
G1
Pointer
Action
H3
APS
K2
User
Channel
F2
Multi
Frame
H4
Growth
Z3
Datacom Datacom Datacom
D4
D5
D6
L
O
Datacom Datacom Datacom
D7
D8
D9
H
Datacom Datacom Datacom
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D10
D11
Growth
Z1
Growth
Z2
Growth
Z4
D12
Growth
Z3
Growth
Z5
Fill out the rest of the bytes yourselves as we discuss them in the class.
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PATH OH: Path OH stays with the payload up to the receiving node. That’s why it is
called Path OH
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J1 Trace: Used to transmit 64 Kbps signal to verify connection. The contents of
this channel is user programmable at both TX and RX end
B-3 ( BIP-8): Bit Interleaved parity –8: This byte is used for Path Error
monitoring. A Block Check Character is formed by generating an even parity at
the transmit end over all previous SPE by first forming a block consisting of one
bit from each byte in the part of the frame to be checked. An even parity BCC is
then generated.
C2 ( Signal Label): Indicates the type of payload in SPE ( DS1, DS2, DS3,
ATM, FR, IP , SMDS , FDDI etc)
G1 ( Path Status): This bytes contains performance monitoring information such
as maintenance an diagnostic signals for example indication of block errors. It
informs the upstream receiving node that a down stream node has detected a
failure indication.
F2 (Channel Path ): User Channel: Reserved for communication between Path
Terminating Equipment.
H4 ( Multiframe Indication): Indicates more than one type of payloads.
Currently being used to indicate the frame position of VT or ATM. It can also be
used to show a DS0 signaling bit.
Z3, Z4, Z5 ( Growth): Presently undefined. Allocated for future use.
SECTION OVERHEADS:
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A1, A2 Framing: The Framing pattern to allow the Receiver to lock on to the
125ms frame. The pattern is A1A2= F628
C1 ( STS-ID): ID # assigned to STS-1 frame and stays with the frame until
decoded beginning with 01H for the first frame, 02H for the 2nd and so on.
B1 (BIP-8): Bit Interleaved Parity calculated over all bits of the previous SPE for
performance monitoring.
E1 ( Order Wire) : Voice channel for voice communication between STE’s
F1 (User Channel): Reserved for User
D1-D3 (Data Communication): 192Kbps message based channel for alarms,
maintenance, control, monitoring, administration between STE’s
LINE OVERHEADS:
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B2 ( BIP-8): Calculated over the Line OH and SPE of the previous frame for
performance monitoring.. Only LTE processes Line BIP –8
K1, K2, ALAMRMS & APS ( Automatic Protection Switching): The SONET
provides for Automatic Protection Switching to a parallel system in case of failure
of the working system. It monitors the BIP-8 of the working system and if BIP-8
indicates a failure of the working system or signal degradation beyond a pre-set
value, it automatically switches to a parallel system with minimal loss of
customer information.
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K1: contains signals requesting service action as well as identifies the requesting
line
K2: Identifies the failed line ( working line that has failed)
D4 –D12:(576 Kbps Data Communication Channels): These channels are used
for OAMP ( Operation, Administration, Maintenance and Provisioning)
exchanges between large Networks. Not accessible by STE’S and not used for
small Networks
E2 (Order Wire): Voice Circuits for voice communication between LTE”S
Z1, Z2 ( Growth) : Not defined. Reserved for future use.
H1, H2,(STS Pointers), and H3( Pointer Action ) Bytes:
One of the most important advantages of SONET is that it permits asynchronous
and synchronous data to be carried synchronously. It does this through the use of
pointers and pointer action bytes. Since an SPE can begin anywhere in the frame
(except where OH bytes reside) and also it can begin in one frame and end in
another frame, pointers bytes are needed to indicate exactly where payload is
beginning and where it is ending. Consider the fig shown.
8
7
6
5
4
0
1
1
0 0
3
2
1
0
I
D I
NDF - H1
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8
7
6
5
4
D
I
D I
3
D I
2
1 8 7 6 5 4 3 2 1
D
H2-10 Bit Pointer
H3-Pointer Action
H1 & H2 Bytes: Indicate the location of the SPE within an STS frame as shown
in the Fig above. Usually, the SPE begins right after the H3 byte. In that case 10
bit pointer value will be 0 0 0 0 0 0 0 0 0 0.
H1
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H2
H3
1
2
3
1
2
3
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If 10 bit pointer value was 0 0 0 0 0 1 0 0 0 0 (16), then the SPE starts at the 16th
byte after H3 counting across the row and down the column as shown in the fig
below
If pointer value is 0 1 0 0 0 0 0 111 ( decimal 263) then the SPE begins at the 3rd
byte in the 3rd row immediately after H3.
Since the SPE contains a maximum of 783 bytes, the largest allowable pointer
value is 782. This mechanism allows transportation of floating SPE’S.
FREQUENCY JUSTIFICATION:
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Because of slight timing differences, an SPE may be running slightly faster or
slower the transport overhead.
Consider the fig shown below:
F2
West
SONET
TM
F1
F2
SONET
ADM
40 Km
F1
EAST
SONET
TM
Let n = 1.5 (index of refraction), c = 300,000000 m/s (Speed of light), S = 40 Km
If a frame starts from West TM, calculate how long will it take to arrive at the ADM?
ANS: ______________
Notice that in the mean time ADM is sending a frame continuously at 125s intervals.
200s
125s
250 s
375s
This means that the ADM will have to buffer the frame for 50s before it can load it
into the next frame. However that would require (at OC-48 rate) a buffer size of app
125 Megabytes at every Equipment along the way, which is very expensive to
implement. Problem is solved by frequency justification technique as follows.
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POSITIVE FREQUENCY JUSTIFICATION ALSO CALLED POSITIVE
STUFFING:
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If the SPE was loaded into the frame one byte later (i.e if SPE is running
slower then the frame the TOH), I bits are inverted. Receiver compares the I
bits of the previous frame with the I bits of the current frame. If it finds I bits
inverted, it knows the SPE begins one byte later. This is called positive stuff
i.e the byte after H3 byte is empty as shown in the fig below.
If the payload is running faster then the TOH, by a maximum of 1 byte, then
the pointer action byte is used to contain data of the previous SPE. This
happens if the SPE was loaded into the STS-1 frame 1 byte too soon. In this
case H1 and H2 bytes will be decremented to notify the receiver of this action.
This process is called Negative Stuffing as shown in the Fig
If the location of the SPE changes by more then one byte, the entire pointer
value is changed. This is indicated by the first 4 bits of H1 byte. These 4 bits
are called NDF ( New Data Flag). Normally NDF is set to 0 1 1 0. It is
inverted ( changed to 1 0 0 1) to notify the Receiver that the pointer value
indicates a new beginning location of the SPE.
1
2
3
90
87
Data begins in byte 1 for Positive stuff
H1
H2
H3
0
1
Positive Stuff-Dummy value inserted
SPE begins in byte 1
Negative Stuff-H3 byte contains Data
from the previous SPE
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