SONET Frame Structure
STS-1 STS-1 STS-1 STS-1
Map
STS-1 STS-1 STS-1 STS-1 Byte STS-3
Map Interleave
STS-1 STS-1 STS-1 STS-1
Map
Incoming Synchronized new
STS-1 frames STS-1 frames
• When n STS-1 signals are multiplexed, they are first
synchronized to the clock of the multiplexer. Need to be
done at the boundary of SONET. In the SONET, we
assume all streams are synchronized.
SONET Frame Structure
• Negative byte stuffing:
When the payload stream is faster than the frame rate,
H3 is used to transmit an extra SPE byte from time to
time. The H1H2 pointer is decreased by one in the next
frame.
• Positive byte stuffing:
When the payload stream is slower than the frame rate,
the byte immediately follows the H3 byte is used as a stuff
byte (byte with dummy information) from time to time.
The H1 H2 pointer is increased by one in the next frame.
1
Negative & Positive Stuff
Frame Frame
k Pointer k Pointer
First octet First octet
of SPE of SPE
Stuff byte Stuff byte
Frame Frame
k+1 Pointer k+1 Pointer
First octet First octet
of SPE of SPE
(a) Negative byte stuffing (b) Positive byte stuffing
Input faster than output Input is slower than output
Send extra byte in H3 to catch up Stuff byte to fill gap
Speed Mapping
1. Virtual tributary: 84 = 7 groups of 12 columns in a SPE.
– Each group is a virtual tributary.
– 12 x 9 x 8000 x 8 = 6.912 Mbps
– Can accommodate 4 T1 signals. 4 x 1.544 < 6.912
2. a single SPE can handle one DS3 signal. (44.736
Mbps)
3. concatenated STS-1 frames
– E.g. STS-3C., carries only one column of path overhead.
– So:
• STS-3C: 87 x 3 – 1 = 260 columns of user data.
• STS-3: 86 x 3 = 258 columns of user data.
2
4.3 Transport Networks
• Transport Network:
– Form the backbone of multiple, independent networks.
– Need to be designed to be very resilient with respect to faults.
1 STS-1 = 783 voice calls; 1 OC-48 = 32000 voice calls
Telephone
Switch
Router
Router
Transport Network
Telephone
Switch
Telephone
Switch
Router
4.3.1 SONET Networks
• “Asynchronous” multiplexing systems (prior to
SONET): requires the entire multiplexed stream
to be de-multiplexed to access a single tributary.
• SONET: add-drop multiplexer (ADM)
– can insert and extract tributary streams without
disturbing tributary streams that are in transit.
MUX ADM DEMUX
Remove Insert
tributary tributary
3
Linear SONET Networks
• Linear SONET Networks.
– Fig 4.18. (a). (b)
(a) Physically linear topology
(b) logically: fully connected mesh
– SONET ADMS can be used to create different “Virtual”
topologies
1 2 3 4
2
1 3
Automatic Protection Switching
– Provide linear protection against failures at the line layer
– Fig 4.19: A working line and a protection line
• Operate in parallel to provide 1+1 (one plus one) linear APS
protection
• inefficient: use twice the bandwidth
W
T R
Bridge Selector
T R
P
4
Automatic Protection Switching
– Fig 4.20: 1:1 linear APS protection (one for one)
• The signal is only transmitted in the working line during
normal operation. the protection line can be used to carry
extra traffic
• The extra traffic is pre-empted when there is a failure. Need
more time to recover from failure than 1+1. but more efficient
in bandwidth usage.
Switch Switch
W
T R
APS signaling
T R
P
Automatic Protection Switching
– Fig 4.21: 1:n linear APS protection
Switch Switch
W1
T R
W²
T R
…
…
…
Wn
T R
P
T R
APS signaling
5
Ring Networks
– Fig 4.22: ring topology networks
– Fig 4.23: logically fully connected topology.
– Self-healing rings: line level, path level
(a) (b)
a a
OC-3n
OC-3n
b c
c
OC-3n
Three ADMs connected in Logical fully connected
physical ring topology topology
Ring Networks
• UPSR: unidirectional path switched ring provides
protection at the path level
– Fig 4.24: two-fiber ring.
• Working traffic: clockwise
• Protection traffic: counter clockwise
– 1+1 protection at path level
– Each exit node monitors the two received path signals and
selects the better one
– Fast path protection but inefficient in bandwidth usage. Used
widely in the lower speed rings in the access portion of networks
6
UPSR path recovery
1
4 2
W = Working line
P = Protection line
3
Ring Networks
• BLSR: bidirectional line switched ring provides protection
at the line level
– Adjacent ADMs are connected by a working fiber pair and a
protection fiber pair
– Fig 4.27: the working pair fails between node 2 and 3
⇒ switch both working channels to the protection channels
span switching
– Fig 4.28: both working pair and protection pair fail.
Use the protection pair in the direction away from the failure
ring switching
– More efficient than UPSR: traffic can be routed along the
shortest path, the protection fibers can be used to carry extra
traffic when no failures.
– BLSR is preferred in high-speed backbone networks
– Disadvantage: requires complex signalling
7
BLSR Span Switching
1
W
Equal
delay
P
zSpan
Switching 2
4
restores
failed line
Fault on
working
links
3
BLSR Ring Switching
1
W
Equal
delay
P
zLine
Switching 2
4
restores
failed lines
Fault on
working and
protection
links
8
Interconnected ring networks
– Fig 4.29
– To provide protection against faults, rings may be
interconnected using matched inter-ring gateways:
primary gateway
secondary gateway
– Ring networks are difficult to manage in as environment of
rapid growth. To increase the capacity of a single span in
a ring. All the ADMs have to be upgraded.
Managing 1 ring is simple; Managing many rings is
very complex
Backbone Networks consist of
Interconnected Rings
Regional Metro
ring ring Interoffice
rings UPSR
OC-12
BLSR
OC-48, UPSR or
OC-192 BLSR
OC-12,
OC-48
9
Mesh Topology Networks using
SONET Cross-Connects
• Cross-Connects are nxn switches
• Interconnects SONET streams
• More flexible and efficient than rings
• Need mesh protection & restoration
Router
B A
C
D
Router F Router
G E
Router
4.3.2 Optical Transport Networks
Provide optical wavelength connections between
attached clients
• OADM: optical Add-drop multiplexer (WDM
system)
– Ideally, all processing in OADM is performed in the
optical domain. expensive optical-to-electrical
conversion is avoided
– Fig 4.31 (WDM linear and ring networks)
– (similar to SONET networks)
– In WDM, each wavelength is modulated separately,
can carry different transmission format. e.g. one
wavelength for SONET, one wavelength for Gigabit-
Ethernet
10
4.3.2 Optical Transport Networks
• Optical cross-connect and optical fiber switching
– Optical fiber switching: transfer entire-multi-
wavelength signals from input ports to output ports
without WDM de-multiplexing
– Optical cross-connect: switch individual wavelength
signals.
• Fig 4.32: all optical fiber switch and cross-connect
• The cost of demodulating a single WDM signal and
processing its components in the electronic
domain is extremely high
⇒ keep WDM signals in the optical domain as they
traverse the network
Optical Switching
…
Optical
fiber switch
…
…
DeMUX
MUX
Output Input
WDM
…
WDM
Wavelength
cross-connect
WDM
…
…
WDM
Dropped Added
wavelengths wavelengths
11
