This article explores the historical reasons behind the 50-ms requirement for convergence. as well as the problems service providers have in achieving it. This article also concludes that 50 ms, in itself, is not relevant to most of the applications running over Metro Ethernet Networks. Furthermore, as networks become more intelligent, other mechanisms can mitigate the effect of network outages.

1. Putting 50-ms In Perspective
by Lionel Florit
Abstract not going to be able to increase this time
easily based on a services argument. As a
ms CGA threshold reinforced the need for
50 ms APS switches at the DS transmis-
This article explores the historical reasons
consequence, 50ms is going to remain the sion level, to allow for worst-case reframe
behind the 50-ms requirement for con-
benchmark for some SP service types re- times all the way down the DS, DS2, DS1
vergence. as well as the problems service
gardless of the application’s requirements. hierarchy with suitable margin against the
providers have in achieving it. This article
20-ms CGA deadline. However, it was
also concludes that 50 ms, in itself, is not
relevant to most of the applications running
Where does 50 ms long since realized that a 20 ms CGA time
over Metro Ethernet Networks. Further- come from? was far too short. Many minor line interrup-
tions would trigger an associated switching
more, as networks become more intel- “The 50 ms figure historically originated
machine into mass call-dropping because
ligent, other mechanisms can mitigate the from the specifications of APS (Automated
of spurious CGA activations. As a result,
effect of network outages. Protection Switching) subsystems in early
the persistence time before call dropping
digital transmission systems and was not
Introduction actually based on any particular service
was raised to 2.5 +/- 0.5 s by ITU recom-
mendations in the 180s. Nevertheless,
Subscribers demand reliable services – as requirement. (Section extracted from
the requirement for 50-ms APS switching
they perceive reliability. Historically, service the book: Mesh based survivable net-
stayed in place, mainly because this was
providers (SP) have built their networks works Author: Wayne Grover ) Early digital
still technically quite feasible at no extra
with as much redundancy as they can af- transmission systems embodied 1:N APS,
cost in the design of APS subsystems.
ford in order to be as close as possible to which required typically about 20 ms for
a so-called 50-ms convergence time. On fault detection, 10 ms for signaling, and 10 The apparent sanctity of 50 ms was further
the surface, everything looks consistent. ms for the operation of the tail-end transfer entrenched in the 10s by vendors who
However, as we examine the problem more relay; consequently, the specification for promoted only ring-based transport solu-
closely, we will see that subscribers don’t APS switching times was reasonably set tions and found it advantageous to insist
really need the networks to converge in at 50 ms, allowing a 10 ms margin. Early on 50 ms as the requirement, effectively
50 ms. The 50 msec figure comes from generations of DS1 channel banks (from precluding distributed mesh restoration
historical requirements of a voice compo- the 170s) also had a Carrier Group Alarm alternatives that had been under equal
nent no longer in the network. What is more (CGA) threshold of about 20 ms. consideration at the start of the SONET era.
appropriate is to look at the convergence As a marketing strategy the 50 ms issue
The CGA is a time threshold for persis-
required by the application running on the thus served as the “mesh killer” for the
tence of any alarm state (such as loss of
network and provide the required service 10s [..].
signal or frame synch loss) on the trans-
to them. In most cases, it is extremely dif-
mission line side, after which all trunk On the other hand, there was also real
ficult for service providers to offer that level
channels would be busied out. The 20 urgency in the early 10s to deploy some
of availability and equipment vendors are
quick to confuse end-to-end convergence
– which is what is really needed – with
simple and bounded failure scenarios.
There is a great misunderstanding about
what people mean when they talk about
“50 ms”. In order to lift the fog, we look
at where this figure comes from, what it
means, where it applies, and which applica-
tions really need it.
For example, in private line services, where
there is currently an SLA between the
SP and the customer for 50ms, the SP is
Figure 7 - Generic Metro Ethernet Network
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2. kind of fast automated restoration method. multipoint? Which type of failure does the is located at the HQ (right hand side of
This lead to the quick adoption of ring- 50 ms figure covers, fiber cut, port down, the diagram). If UNI B fails, the customer
based solutions which had only incremen- box down, POP down, CPE down? After equipment (CE) must make the decision to
tal development requirements over 1+1 a failure, are we allowed to drop less impor- switch to UNI D. In the best case scenario
APS transmission systems. However, once tant traffic in order to provide bandwidth for (as far as recovery speed is concerned),
rings were deployed, the effect was to only more important traffic? the CE is a single piece of equipment,
to reinforce further the cultural assump- either a router or switch. If it is a switch,
These questions are very difficult to
tion of 50 ms as the standard. Thus, as the CE is probably running Rapid Span-
answer. There is no common view on all
sometimes happens in engineering, what ning Tree Protocol (RSTP) to block one of
of them and it is easy to claim “50 ms” in a
was initially a performance capability in the two UNIs so a loop with the service
context and be less than stellar in another.
one specific context (APS switching time) provider is not created. Because the EVC
Let’s tackle the questions above using a
evolved into a perceived requirement in all is multipoint, RSTP has detected several
generic example. Consider the network
other contexts. “ neighbors and is now running with 0-s
diagram in Figure 7.
timers. If the CE is a router, the backup path
What do we mean by 50 This diagram represents a company’s will be used after the routing protocol con-
ms recovery? headquarters, HQ (right hand side), con-
nected to two branches offices over a
verges, which is likely longer than 50 ms.
The CE can’t take advantage of the 50 ms.
The expression “50 ms recovery” is overly
multipoint EVC with UNIs A, B, C, and D
used. What does it mean that “the network In the access and aggregation networks,
in that EVC. Can this network offer 50 ms
must converge in 50 ms”? Does it mean recovery is more complex. A frame
protection? Let’s take a closer look.
that a failure must be detected in less than coming from the branch offices going
50 ms and the recovery will take place The first thing we notice are the three loca- through Agg4 must now be directed to the
later? If a port fails, must a backup link be tions. This is a multipoint situation, there- other edge device (UNI D). This means all
brought up within 50 ms? Must end-to-end fore, one could use H-VPLS or Hierarchical headquarters’ MAC addresses mapped to
service must be restored in 50 ms? Is a VPLS (with L2 or MPLS in the access part UNI B must first be forgotten before the for-
service restored when the first frame of of the network) or L2 spanning tree end to warding tables can begin to be rebuilt. De-
that application makes it through the back- end (unlikely). pending on the size of the access network,
up path of the network or when the ap- this could easily take more than 50 ms.
Failure at the UNI
plication resumes its work? Does it apply
In our example, the only UNI protected Failure in the access layer
to all type of services, point-to-point and
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3. perspective, convergence is more compli-
cated than a simple link failover. Let’s now
turn our attention to the application layer.
Which application
needs 50 ms protection?
As mentioned previously, looking at protec-
tion from the application’s point of view
should be the ultimate goal. After all, net-
works ultimately carry application’s traffic. If
we thought the failure scenarios described
in the Sect 0 could be complicated, the
impact of a network failure on a user ap-
plication is even more complex to evalu-
ate. How the application behaves when
packets are lost and how this translates into
Figure 8 - A classification of applications user experience depends on many param-
eters: protocol behavior, computer speed
The failure of a link in the access layer of nodes would have to send their traffic to at each end, type of application (voice vs.
the network is probably what most people Agg. This could be challenging, because data), type of user (stock trader vs. instant
have in mind when they talk about 50 ms potentially a lot of addresses have to be messenger), and so on. Furthermore, it is
recovery. This event is not likely to involve mapped to different ports on Agg2 and nearly impossible to measure down to the
the CE and the failure can be dealt with Agg6. The EVC in our example is likely to millisecond the effect of a network outage
locally, between the aggregation node and share this network with many other EVCs on an application.
the edge node within the service provid- which will have to be moved away from Let’s adopt a commonly accepted sim-
er’s domain. This is the classic “backhoe” Agg4. This is a typical example of why the plification to this problem. Let’s assume
incident that causes a fiber cut. Equipment 50 ms context matters. It is one thing to ap- the application is considered to be fully
vendors claiming 50 ms recovery as a fea- ply this ideal delay to a single segment, recovered as soon as the first packet of
ture of their equipment are likely to protect quite another to apply it to an entire network. that application, after a failure, is transport-
against this type of scenario. If the access ed across the service provider’s network.
Some questions are left unanswered. In
is a ring, technologies such as SONET Assume, furthermore that, after the failure,
Ethernet technologies, flooding is a com-
(Note that standard SONET performance it doesn’t matter whether the packets are
mon mechanism for recovering from a
figures are given in the context of a single out of order, or even if some of them are
failure. After detecting a failure, network
ring with 16 nodes, and with adequate un- missing . Finally, assume that there is only a
elements flood unicasts until they learn
used bandwidth reserved for protection.) single failure event.
the location of the source unicast and stop
or Cisco REP ( will switch traffic to the other
flooding at that point. Because flooding With this in mind, let’s try to make sense of
side of the ring very quickly. If the access
causes frames to be multiplied, conges- the application space and find which appli-
network is hub and spoke, then MPLS FRR
tion may occur in other parts of the network cation really needs 50 ms. Figure 8 shows
(MPLS FRR: MPLS fast reroute (MPLS-FRR)
that were not affected by the failure in the a possible way to classify some applica-
mechanisms deviate the traffic in case
first place. However, traffic is indeed flow- tions. The leaves of this tree show applica-
of network failures) will also switch traffic
ing end to end. When do we decide the tions commonly thought to be candidates
quickly. As long as the data path doesn’t
network has converged? Can we stop the perceived to require a maximum 50-ms
have to use a different aggregation node,
clock that measures when we hit the “50 outage. The list is not exhaustive.
recovery will be prompt.
ms mark,” even if new congestion is intro-
Failure at the aggregation Layer duced in other parts of the network? The application space can be divided into
A failure of the aggregation node itself is three categories: data, voice and video.
As we see, we are still far away from the The diagram shows “mission-critical” as a
more involved. In our example, if Agg4
point-to-point context described in the first classification. A mission-critical application
were to fail altogether, all other aggregation
section. Observed from a network-wide
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4. is viewed by the subscribers as important tions. Clients using lower-quality clocks window of optimum trading opportunity
enough that they would be willing to pay must poll more frequently than well-syn- is increasingly measured in milliseconds.
more to the service provider in order to chronized clients. If a packet is lost, no This is the world of zero packet loss (and
guarantee fast recovery. retransmission takes place, the stations big dollar loss). 50 ms outage is not accept-
simply wait for the next update. Therefore, able. Uptime is essential. If a single packet
Data oriented applications
with NTP a network outage could last as is lost, a transaction may not take place,
In the data space, unicast and multicast ap-
long as several seconds, with the loss of money will be lost.
plications are the two main subcategories.
only a single timing packet, with little effect
We will not talk about applications run- Most market feeds are point-to-point T1s.
on accuracy.
ning over TCP. These applications expect Metro Ethernet providers may be used
packet loss and are adapted to retransmis- IEEE1588 offers much better accuracy as backup to T1 lines. Banks and trading
sion mechanisms built into TCP. They will than NTP. Typically, 1588 will generate a companies will constantly monitor the
not be affected by short network outages. few packets per second. During a network performance of their backup connections.
outage, the client clock would drift for the However, these applications use other
Unicast applications
duration of the outage (depending on the networks than Metro Ethernet.
The unicast applications of interest run
precision and quality of the clock, the drift
over a connectionless protocol. Therefore, Voice applications
will vary). However, the short-term drift of a
network outages have a greater impact be- Voice applications are very interactive.
Stratum clock is less than .7 x 10-7 in 24
cause packets can’t be retransmitted. Let’s There are two broad categories: Voice over
hours. This amounts to approximately 255
look at three categories: military, industrial IP and Circuit Emulation over Packet.
Ethernet and network timing applications:
Military applications
As seen in RFC 167, the Navy’s High-Per-
formance Network (HPN) working group
has studied the requirements of mission-
critical applications on Navy platforms.
However, these applications are deployed
on submarines, on aircrafts, on ships, and
on bases – and the military owns the WAN.
This is typically not a service provider play.
Industrial Ethernet applications
Very tight time synchronization among ma-
Figure 9 - CESoP Loss of one frame
chines is needed (below 1 ms, IEC 61850
part 5). This is not a WAN application, but
rather a LAN application. It is out of the frame slips in 24 hours while the system Voice over IP
scope of this document. is holding. An outage as long as 500 ms There are two classes of traffic: the voice
– 10 times our archetypical 50-ms factor! signal itself (bearer traffic) and the out of
Time synchronization – won’t introduce a significant drift. band call-signaling traffic. If an outage oc-
protocols (NTP) Multicast and unicast data applications
curs during a conversation, the end-user
may lose contact for the duration of the
The synchronization accuracy of a WAN There are a few data oriented applications
outage. If the outage lasts less than a few
using NTP is typically within the range of which have more stringent requirements.
seconds, the call itself is not dropped.
10 to 100 ms; on a LAN, this is typically Two examples are: Real-Time Distributed
If the outage occurs during a call setup
a few milliseconds. A broadcast server Applications and trading applications.
phase, it takes longer to set up the call, or
sends out a packet about every 64
seconds. A non broadcast client/server Trading Applications: the user might simply have to dial again.
VoIP deployments over networks designed
requires 2 packets per transaction. When
first started, the transactions occur about
the race to the best price to converge in 800 ms or more are
Automated order routing systems and the very common.
once per minute, increasing gradually to
dawn of algorithmic trading mean that the
once per 17 minutes under normal condi-
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5. Circuit Emulation over Packet interactive and unidirectional Let’s use a common MPEG2 stream for
Of all the applications we have seen so far,
this one is the most challenging. Circuit
• Video on demand (VoD), which is non-
the basis of the discussion. Video streams
travel compressed over data networks.
real-time, interactive (VCR-like controls),
Emulation Service over Packet (CESoP) is A data packet contains a certain amount
unidirectional
defined by the IETF, Metro Ethernet Forum, of information describing a video frame.
MPLS Forum, and ITU-T. The entire T1 • Near VoD, network personal video There are about 0 video frames (or im-
(framing, signaling, and payload) is carried recorder (PVR) ages) per second. There are different kinds
transparently across the Ethernet network.
If a packet gets dropped (or excessively • Security applications – surveillance
of video frames: I, B and P frames. I, P and
B frames are assembled into a Group of
delayed) in the network, its content in the • Internet streaming to PC desktop Pictures (GOP). A GOP is typically bound-
egress data stream is replaced with a con-
figurable idle pattern, as shown in Figure . • Broadcast contribution and TV produc-
ed by I frames and 12-15 frames long but
it can vary with frame rate, content com-
tion networks
plexity, and encoder implementation. An I
When a lot of packets are lost, an alarm
These applications have different re- picture is a reference picture containing all
is sent towards the packet source and
quirements and addressing all of them is the pixel information needed to represent
the destination sees all 1. In the case of
beyond the scope of this paper. However, accurately the picture. A P picture (also
unframed service, all 1 is an alarm in itself.
when people talk about video quality, called predicted picture) contains all the
However in case of framed service the
they often use the example of to the final motion vectors to describe the new posi-
framing is preserved and only the payload
touchdown of a Super Bowl game, the last tions of the macroblocks, along with the dif-
is replaced by FFs. As a result, the destina-
tion will not see an alarm at all. More than
50 ms (or even 200) will not cause calls to
be dropped.
The mobile backhaul applications add a
little twist to this equation. Mobile opera-
tors want an end-to-end delay of less than
10ms so phone calls are not dropped
during tower site hand offs. This implies
a de-jitter buffer size of less than 10 ms,
which means a 10 ms network outage will
cause an alarm. However, once the alarm
is raised, nothing else should happen if
the outage is less than 500 ms. From the
user-experience perspective, there will be
a glitch, more or less noticeable depending Figure 10 - Effect of a loss
on the length of the outage.
penalty kick of soccer’s World Cup final, ference data that must be added to those
Note that running CES over IP allows more a brain surgeon using an HD video feed macroblocks. P pictures require approxi-
flexibility in terms of packet loss and com- to stitch synapses on a patient located on mately half of the data of an I picture and
mon implementations can accommodate a another continent, or the president having are based on the previous picture (I or P).
500ms outage. a video conference with his generals to
The B pictures are based on past and
order (or not) the launch of a nuclear strike.
future I and P pictures and are not derived
Video applications We can imagine the consequences of a
from each other. Like P pictures, they
Video applications over data networks can 500-ms glitch in these situations. “Did he
contain vectors and difference data. They
have many different forms: say launch or not?” We can’t say that these
usually require about a quarter of the data
situations will never happen, but one can’t
• Video conferencing, which is real time, design a network based on these require-
of an I picture.
interactive, and bidirectional ments either! Nonetheless, let us closely Because video frames are linked to the
• Broadcast content (TVoDSL) to living examine the effect of a network outage on
a video stream.
each other, the loss of consecutive packets
translates into a bi-dimensional effect, in
room TV which is near real-time, non-
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6. space and time, as shown in Figure 10
One of the most visible effects of a frame
loss occurs when some data of an I frame
is not received by the decoder. Typically,
20% of the packets of a video stream carry
information used to construct an I frame.
A network outage of less than 500 ms
affects up to two consecutive GOPs. The
user will see pixilation, but the video pro-
gram resumes after the network recovers
without the need for user intervention. It is
important to note that the duration of the
artifact on the video screen will be longer
than the duration of the network outage
itself. No matter how short the network out-
age is, as long as frames are lost, there is a
possibility to see an artifact on the screen.
Several schemes exist to compensate for
the network as the only requirement will not
prevent artifacts due to a network failure.
Error Repair
The client waits for missing RTP sequence
numbers. If packets have been dropped
and remain uncorrected following FEC
repair, the client requests retransmission
of the packets from its designated VQE
server . Before they are handed off to
an MPEG demux, retransmitted packets
are re-sequenced and de-jittered in the
client’s network. A single RCTP message
may request the retransmission of multiple
contiguous or non-contiguous packets.
Figure 11 - Retransmission of lost video packets
Live-live protection
packet loss: some of the most efficient are ed by the client, which uses received FEC
In certain cases such as head-end redun-
forward error correction, error repair and Protection Packets. Any missing RTP pack-
dancy, a lossless delivery may be required.
live-live protection. ets beyond FEC coverage are forwarded to
In this situation, we need to achieve protec-
an error repair function.
Forward Error Correction (FEC) tion against a single network failure of any
A FEC capable client receives FEC repair FEC is a good way to improve the quality length. A solution consists of sending two
packets and searches for missing Real- of experience but introduces overhead, copies of the multicast video stream on two
Time Transport Protocol (RTP) sequence latency, and subsequently cost. The more physically separated paths. The last core
numbers encountered across a FEC-pro- we budget for overhead and latency, the edge router or a VQE element receives the
tected block period of N-packets. FEC longer network outage we can absorb two copies but passes only one. Such a
protection periods are determined at the without seeing an image artifact. FEC can design protects against a single failure of
headend by the definition of the FEC block handle a network outage of 50 or 100 ms or any length and makes the 50 ms discus-
size. Missing RTP packets within the FEC more. FEC budget and network design go sion irrelevant.
block coverage are automatically correct- hand in hand. A 50 ms or 100 ms limit on
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7. Summary of Video Applications
As we saw at the introduction of this sec-
Conclusion We have also encountered examples
of applications that will be affected by a
As the context in which 50-ms has been
tion, there are numerous video applica- 50-ms outage. Trading applications can’t
reflexively used has expanded, the defini-
tions, each with its own requirements. accept any loss. For circuit emulation over
tion of failure and recovery has become
Some applications may require zero loss, Ethernet, a 10 ms loss could have an effect
less meaningful. A common trap is to look
others may accommodate some loss. A (alarms) but this doesn’t mean catastrophic
at localized failure scenarios and claim to
network outage of any length will cause consequences. Finally, packet loss will
achieve “50 ms” for all cases. When we
packets to be lost and a 50-ms or longer create video impairments. There are ef-
look at failures across the entire network,
network outage can create visible video fective mechanisms beyond fast network
we see it is very difficult for a service pro-
artifacts. The faster a network converges, convergence that can be used to comple-
vider to design a network that will accom-
the more satisfactory the user experience. ment a given network’s performance. Such
modate 50-ms recovery for any possible
However, aside from network convergence, mechanisms are FEC, VQE, live-live and
failure. We have also established that the
there are other mechanisms that improve time-shifted streams. It is up to the service
concept of recovery itself, when viewed
video delivery. These new tools should providers to balance the investment be-
network wide, is not well defined. When
be built into the video delivery solution tween fast network convergence and error
exactly, after a failure, a network has recov-
to improve overall the performance of the corrections based on the level of quality of
ered is up to debate.
network. Examples of such tools are FEC, experience they want to achieve.
repair packets, live-live feeds, time-shifted We have reviewed a set of applications
Service providers and vendors continue
streams and so on. These mechanisms perceived to be very sensitive to packet
to strive to provide solutions that recover
can correct the degradation of quality loss. We have established that, in most
from failures as quickly as possible. How-
resulting from a network failure. Relying cases, these applications don’t mandate a
ever, trying to achieve an artificial goal of
solely on a 50-ms convergence require- hard 50-ms figure. Most of the time, they
50 ms is likely to affect the affordability,
ment is not likely to lead to the most satis- can cope with much a longer outage. Ex-
scalability and flexibility of the solution.
factory solution. amples of such applications are Voice over
Deciding when we reach the right balance,
IP, time synchronization, real-time distrib-
using sound technical and economic jus-
uted systems.
tification, will serve the interest of both the
provider and the consumer.
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Americas Headquarters Asia Pacific Headquarters Europe Headquarters
Cisco Systems, Inc. Cisco Systems (USA) Pte. Ltd. Cisco Systems International BV
San Jose, CA Singapore Amsterdam, The Netherlands
Cisco has more than 200 offices worldwide. Addresses, phone numbers, and fax numbers are listed on the Cisco Website at www.cisco.com/go/offices.
CCDE, CCENT, CCSI, Cisco Eos, Cisco HealthPresence, the Cisco logo, Cisco Lumin, Cisco Nexus, Cisco Nurse Connect, Cisco Stackpower, Cisco StadiumVision, Cisco TelePresence, Cisco WebEx, DCE, and Welcome to
the Human Network are trademarks; Changing the Way We Work, Live, Play, and Learn and Cisco Store are service marks; and Access Registrar, Aironet, AsyncOS, Bringing the Meeting To You, Catalyst, CCDA, CCDP, CCIE,
CCIP, CCNA, CCNP, CCSP, CCVP, Cisco, the Cisco Certified Internetwork Expert logo, Cisco IOS, Cisco Press, Cisco Systems, Cisco Systems Capital, the Cisco Systems logo, Cisco Unity, Collaboration Without Limitation,
EtherFast, EtherSwitch, Event Center, Fast Step, Follow Me Browsing, FormShare, GigaDrive, HomeLink, Internet Quotient, IOS, iPhone, iQuick Study, IronPort, the IronPort logo, LightStream, Linksys, MediaTone, MeetingPlace,
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All other trademarks mentioned in this document or website are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (0903R)