UNDERSTANDING SONET BLSRs
ring is defined as a set of nodes interconnected to form a closed loop, where
links. There are two major types of
SONET rings: path-switched and line-switched SONET rings.
Line-switched rings use the SONET line level indications to initiate protection
switching. Line layer indications include line layer failures and APS signaling
messages that are received from other nodes.
A request for switching may also be initiated via an operations
interface. In a UPSR the traffic between two nodes is provisioned to travel
either clockwise or counterclockwise under normal conditions. A
connection/circuit on a UPSR uses capacity on the entire ring. If both
directions of transmission use the same set of nodes and links, the transmission
is said to be bidirectional. A connection/circuit on a BLSR uses capacity ONLY
between the nodes where
traffic is added and where it is dropped, see figure 1.
is much different than the UPSR where traffic between any two nodes consumes
bandwidth on the entire. Again
referring to figure 1. we see that if an STS-1 is provisioned between nodes A
and B on time slot 1 that time slot will also be available for use between nodes
C and D, (STS1-1, STS-1 number1). This
flexibility in the reuse of bandwidth is one of the major advantages of a BLSR.
The price you pay for this flexibility as a user is greatly increased
complexity in provisioning and troubleshooting.
Protection against fiber cuts and node failures is done by reserving
bandwidth on the ring strictly for protection.
In the BLSR OC-N network of figure 1. exactly one half of the bandwidth,
N/2 is available for working traffic between adjacent nodes.
The other N/2 of bandwidth is reserved for protection. In an OC-48 system time slots 1-24 are used for working
traffic and 25-48 are reserved for protection.
At first glance this would seem to be a waste of 1/2 of the system
bandwidth. Upon closer examination
it can be seen that this is not the case. Figure
2 illustrates this fact. Twentyfour
STS-1s are provisioned between node A and B node A’s East span and 24 STS-1s
are provisioned between node A and node C on node A’s West span.
Notice that node D is just a pass through node with no traffic being
added or dropped. In fact it would be impossible to add/drop and traffic at
node D because the maximum allowed bandwidth, N/2, is allocated as pass through
traffic. In this example of an
OC-48 ring 48 STS-1s (ring line rate) have been provisioned and there is no more
bandwidth left on the ring. This is
reminiscent of the UPSR case. In
fact in a hubbing scenario the capacity of a BLSR and UPSR are identical and
equal to the ring line rate, N.
traffic is not hubbed the BLSR can have a significant capacity advantage over
the UPSR. The extreme case of this
is when all traffic is between adjacent nodes.
Figure 3 illustrates this graphically.
The capacity advantage here is 2 times that of a UPSR, or 96.
the case of adjacent node traffic the capacity advantage of the BLSR grows
linearly with the number of nodes, however in most cases the traffic will likely
resemble a distributed logical mesh still providing a capacity advantage but not
quite as extreme as in the adjacent node traffic case.
Switching in The BLSR
illustrate the protection switching in a BLSR assume the traffic patter shown in
figure 4. STS-1 number 1 from node
A via short path to node B and STS-1 number 2 from node A to node C via pass
through node B.
a cable cut occurs as shown in
figure 5, both nodes adjacent to the cut detect an
initiate a message sent from a tail-end node, nodes A and B in this case to the
head-end node, nodes B and A respectively, requesting the head end perform a
bridge of the working channels onto the protection channels, i.e. bridge time
slots 1-24 onto time slots 25-48. This
is referred to as a bridge request. The intermediate nodes, on seeing a ring
bridge request not addressed to themselves, enter the pass-through mode. The
nodes to which the requests were addressed then perform ring switches. The
intermediate nodes continue to drop and insert traffic on the working channels
as normal. When the cut is repaired
the ring switches back restoring traffic to its original configuration.
order to perform a ring switch, the protection channels are essentially shared
span of the ring. Also, extra traffic may reside in the protection channels when
the protection channels are not currently being used to restore working traffic
transported on the working channels. With no extra traffic on the ring, under
certain multiple point failures, such as those that cause node isolation,
services (from the same time slot but on different spans) may contend for access
to the same protection channel time slot. This situation yields a potential for
misconnected traffic. With extra traffic on the ring, even under single point
failures, a service on the working channels may contend for access to the same
protection channel time slot that carries extra traffic. This situation also
yields a potential for misconnected traffic.
A BLSR prevents traffic from being misconnected by keeping track of the
connections in what is known as a squelch table.
fault-free conditions, it is possible to use the protection channels to carry
traffic. This additional traffic, which is referred to as extra traffic, has
priority than the traffic on the working channels and has no means for
protection. The extra
set up by provisioning the add and drop nodes for the traffic. Intermediate
ring are provisioned so that the protection channel STS-1s carrying extra
through the node. (Protection channels that are not carrying extra traffic are
the intermediate nodes.) Timeslot assignment of extra traffic on the protection
that support extra traffic, extra traffic is allowed on both sides of the node.
STS will be able to enter the ring at any node, and to exit the ring at any
are inserting, dropping, or passing through extra traffic indicate its presence
by inserting the Extra Traffic code in byte K2 bits 6-8. Note that extra traffic
lowest priority level, and will be pre-empted by any working traffic that
use of the
protection channels. The transmission of either Idle or Bridged code in byte K2
bits 6-8 is
an indication that extra traffic has been removed.
request of higher priority than the No Request priority is received by the node,
only if that
request is a ring request, or requires the usage of the protection channels
extra traffic, extra traffic is pre-empted and squelched on the spans whose
channels are required for the protection switch. When the affected nodes return
to the Idle,
K-byte pass-through, or span switching state, extra traffic (on spans whose
channels are not used for protection purposes) is restored. For Exerciser
is allowed to exist on the protection channels. Extra traffic is also allowed on
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