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1
This paperwill introduce the Ethernet standards
available to network operators today. It will
review each standard and conclude which standards are
best deployed as building blocks in a carrier-class
Ethernet service.
tellabs.com
Choosing the Best of Todays
Ethernet-over-SDH StandardsOctober 2003
ETHERNET OFFERS
KEY INGREDIENTS
FOR SUCCESS:
INEXPENSIVE
INTERFACES
HIGH BIT RATES
SUBSCRIBERS ARE
FAMILIAR WITH
ETHERNET AND
WILLING TO BUY
Executive SummaryEthernet technology dominates the Local Area Network (LAN) and is now
expanding rapidly into the Wide Area Network (WAN) space. Ethernet offers the
key ingredients for success, namely: inexpensive interfaces; high bit rates; and
many subscribers who are extremely familiar with it and willing to buy services
that use it. For these reasons, it makes sense to use Ethernet as a subscriber
interface. A judicious combination of standards is required for a network operator
to successfully deploy Ethernet as a WAN technology. Therefore, network
operators are faced with choosing the best standards from the many that are
available. This paper assumes that the network operator already owns and operates
an SDH network.
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Ethernet Servicesthat Subscriberswill BuyIt is believed that subscribers buy
Ethernet services because the operators
network behaves like a private Ethernet
network that is virtually built exclusively
for them. Privacy and security must be
assured. Spanning Tree Protocol (STP)
may be switched on or off as per
the subscribers specification. The
subscribers network may be a simple
point-to-point link or a complex mesh;
the operators network should supportboth. The operators network should
support IEEE 802.1Q VLAN tagging,
IEEE 802.1P prioriti-sation levels,
unicast, multi-cast and broadcast services.
This is all implied since the network must
appear to the subscriber as their own
private Ethernet switched network.
Subscribers may also choose the level
of protection they wish to purchase for
transport services.
The scope of an Ethernet service offering
should be as simple as this: if a private
Ethernet switched network can do it,
then a operators Ethernet service
should do it also. This implies a flexible
Ethernet service that deploys the most
common popular Ethernet standards
that are already deployed in most
subscribers LANs.
To assure privacy, all services may be
provided on a per-subscriber basis.
Ethernet StandardsAvailable TodayIn Table 1 you find a list of SDH
standards that often are portrayed as a
Table 1 Todays Ethernet Standards
ITU-T G.7041 Generic Framing Procedure (GFP)
ITU-T X.86 Link Access Protocol (LAPS)
ITU-T H.707 Virtual Concatenation (VC)
ITU-T G.7042 Link Capacity Adjustment Scheme (LCAS)
IEEE 802.1D Ethernet switching
IEEE 802.1Q/P Virtual LAN (VLAN) and Prioritisation
MPLS Multi-Protocol Label Switching
IEEE 802.17 Resilient Packet Ring (RPR)
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compulsory part of an Ethernet-over-SDH
service, but how do they fit together and
are they all really necessary? This paper
will review each of these standards and
conclude which standards are best
deployed as building blocks in a
carrier-class Ethernet service.
Choose the BestEthernet to SDHMapping TechniqueIn order to marry Ethernet and SDH, a
layer of elasticity is needed between
the two. SDH is like a conveyer belt,
always moving, never stopping. Ethernet
is more like trucks (i.e., frames), each
operating on their own, at liberty to
stop when there is no cargo and go when
there is cargo to carry.
A layer that marries SDH and Ethernet
needs to load the Ethernet trucks onto the
SDH conveyer belt. If a burst of trucks
arrives faster than the conveyor belt can
handle, the trucks need to be held in a
queue. Before the loading area fills to the
danger level, an 802.3x pause message is
sent back to the source of the trucks,
asking the sender to stop the flow of
trucks for just a moment. Conversely, if
the trucks are coming too slowly for the
SDH conveyor belt, then trucks filled
with packing material are loaded onto
the SDH conveyer belt because the SDH
conveyer belt must always be filled.
X.86 and GFP Frame
Alignment MechanismImagine an Ethernet frame being sent from the
subscriber network into the network operator's
Ethernet PHY interface. When the Ethernet frame
arrives, the 7 octet preamble and 1 octet start of
frame delimiter are scrapped since they can easily
be rebuilt at the other end of the SDH network.
The next step will vary depending on the use of
GFP or X.86. If the chosen interface is X.86, then
a hexadecimal 7E, called a flag, will be added to
mark the beginning of the frame (see Figure 1).
Additional fields will be added to the X.85
header, as shown in Figure 1. Since a 0x7E marks
the start of the frame, all occurrences of a 7E
must be removed from the payload, so all 7E
octets are replaced with 0x7D5E.
But this complicates things further because all 7D
octets must now be replaced with an 0x7D5Dstring, therefore, a packet laden with 7E and 7D
octets will cause this process to inflate the size of
the packet in preparation for transport. This is
called "packet inflation" and is the chief incentive
to avoid X.86 in favor of GFP.
On the other hand, X.86 came before GFP and is
nearly identical to a different standard, X.85,known as Packet-over-SDH (POS). When there
are no packets being received, X.86 fills the
SDH channel with back-to-back 0x7Ds. GFP
accomplishes packet delineation in a more deter-
ministic fashion. The limitation with GFP is that
it cannot operate in cut-though mode, but the
small amount of buffering delay to capture an
entire Ethernet frame is offset by the increased
throughput and furthermore, the Ethernet packets
coming from the end user are arriving at 100
Mbit/s and will readily keep the input buffer to
the SDH interface full, nearly eliminating the
value of cut-through processing in the first place.
A powerful advantage of GFP is its concise and
deterministic frame delineation. The mechanism
works like this. When an Ethernet frame is
encapsulated by GFP, its length is indicated in
the payload length indicator (PLI) field which
occupies the first two octets of the GFP header.
By reading the first two octets, the receiver knows
how long the frame is and therefore where it ends
and another begins. The problem for the receivingend is to locate and confirm the first two octets
are the PLI field. GFP solves this problem neatly
by making the third and forth octets of the GFP
header a mathematical function (CRC) of the first
two octets. During frame alignment, the receiving
end looks for two octets that are the CRC value
of the proceeding two octets. The search proceeds
octet by octet until two octets are found that are a
CRC value of the proceeding two octets. If it truly
is the header, then the first two octets must be a
payload length indicator and by counting forward
the indicated amount of octets, we should find
another PLI field which can be confirmed because
the two octets following the PLI are the CRC
value of the PLI. The process actually runs three
times before frame alignment is confirmed.
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When the trucks are loaded, they are
welded to the truck in front and behind,
hence there is no way at the receiving
end to tell where one truck ends and
another begins, so a special marker must
be inserted between trucks to delineate
them (i.e., frame delineation.) Loading
the Ethernet frames and delineating them
is the task of X.86 or GFP, but which
one is better?
Ethernet is asynchronous and SDH is
synchronous. Usually the SDH circuit
will operate at a different bit rate from the
subscribers Ethernet connection. In order
to make the two compatible, dynamic rate
adaptation and frame delineation (where
one truck ends and another starts) must be
deployed at the point where the Ethernet
physical layer ends and SDH begins
Thrown Away
Eth
ernetFrame
FCS of LAPS (4 octets)
MAC SA (6 octets)
Flag (0x7E)
SAPI 2nd octet (0x01)
SAPI first octet (0xFE)
Address (0x04)
Flag (0x7E)
Data (46 - 1500 octets)
FCS of MAC (4 octets)
MAC SA (6 octets)
PAD
MAC DA (6 octets)
Start of Frame (one octet)
Preamble (7 octets)
Length/Type
LAPSHeader
Length/Type
MAC DA (6 octets)
FCS of MAC (4 octets)
Add X.86Framing
The LAPS header and trailer are
added plus all occurrences of 7E
and 7D octets in the packet must
replaced with a 2 octet sequence,
inflating the size of the payload.
Control (0x03)
EthernetFrame
Figure 1 An Ethernet Frame May Be Inflated as it is Encapsulated
Into an X.86 Frame
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(PHY/SDH interface see Figure 1 and
Figure 2). Both GFP and X.86 accomplish
this but in different ways. X.86 inflates
the payload and is nondeterministic in its
behaviour. GFP accomplishes the task
with greater efficiency plus it is
deterministic. For details, see the text box
titled X.86 and GFP frame alignment
mechanism. Based on the nondeter-
ministic behaviour of X.86 and the
deterministic behaviour of Generic
Framing Proceedure (GFP), we conclude
that GFP is the better choice for Ethernet
to SDH rate adaptation.
Use VirtualConcatenation toMatch BandwidthMore Closelyto CustomerRequirementsVirtual concatenation combines multiple
slower bit rate SDH conveyer belts into a
single high-speed conveyer belt. Each of
the smaller conveyer belts must start and
end at the same point, but they may
follow completely different paths inside
the SDH network. This permits traffic
to be spread across different rings and
Payload Length Identifier
Thrown Away
EthernetFrame
Data (46 - 1500 octets)
FCS of MAC (4 octets)
MAC SA (6 octets)
PAD
MAC DA (6 octets)
Start of Frame (one octet)
Preamble (7 octets)
Length/Type
Data (46 to 1500 octets)
MAC SA (6 octets)
Type (2 octets)
cHEC (2 octets)
PLI (2 octets)
FCS of MAC (4 octets)
GFPHea
der
Length/Type
MAC DA (6 octets)
PAD
Add GFPFraming
tHEC (2 octets)
cHEC is simply a CRC
function of the PLI field
No inflation!
Figure 2 An Ethernet Frame is Mapped Directly Into GFP
Encapsulation. Overhead is Predictable and Minimal
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recombined at the other end in a
completely transparent fashion.
Prior to virtual concatenation, the SDH
conveyer belt would operate only at the
contiguous bit rates shown in Table 2.
Virtual concatenation can inverse
multiplex any VC-12, VC-3 or VC-4
channels into a single circuit. Virtual
concatenation will only combine similar
VC types. For instance, up to 63 VC-12s
can be combined to form a single
channel. This permits SDH to support
data transport in 2 Mbit/s increments
using VC-12s (see Table 2 for the exact
values). Virtual concatenation of VC-3
or VC-4 circuits is also supported,
permitting these circuits to be combined
into a single channel, permitting growth
increments in multiples of VC-3s
or VC-4s.
As a matter of history, prior to virtual
concatenation, contiguous concatenation
offered coarse growth increments that are
also illustrated in Table 2. Furthermore,
Contiguous Concatenation
X 149.76 where X = 1 to 255
Low Order
VC-4-Xv
VC-11
VC-4-64c
VC-4-16c
VC-4-8c
VC-4-4c
VC-4
VC-3
VC-12
1,6
9584,64
2396,16
1198,08
599,04
149,76
48,384
2,176
SDH Container
Virtual Concatenation
Type Payload capacity in Mbit/s
Low Order
High Order
High Order
High Order
High Order
High Order
High Order
Low Order
VC-3-Xv
VC-12-Xv
High Order
Low Order X 48.384 where X = 1 to 255
X 2.176 where X = 1 to 63
Table 2 Contiguous Concatenation and Virtual Concatenation Types
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contiguous concatenation requires that all
intermediate nodes support contiguous
concatenation while virtual concatenation
does not require any special capability of
the intermediate nodes.
There is more value to virtual concatena-
tion, but to see the big picture, we need to
dig deeper into the SDH protection.
Use LCAS forProtection and StopWasting Bandwidthon Traditional SDH
Protection OptionsSDH Automatic Protection Services
(APS) defines three kinds of channels in
ITU-T G.841.
1. Working channel (i.e., protected
service) This is a working circuit. If
this circuit fails, traffic is diverted to a
protection channel within 50 milli-
seconds. The protection channel is
defined next.
2. Protection Channel (i.e., pre-emptible
service) This is the protection circuit
that carries traffic when the working
channel fails. This circuit is wasted
until it is needed as a redundant path
when the working channel fails.
Hopefully the ring performs well and
the protection channel is rarely needed.
If this is true, then it may make sense to
put the protection channel to work
rather than waste it when all systems
are functioning normally. Therefore,
the G.841 standard permits this channel
to carry extra traffic when the
working channel is operating. Any
traffic that uses the protection channel
is operating at some peril, because
when the working channel fails, the
extra traffic is pre-empted so that the
protection channel can perform the job
for which it was intended, which is to
carry the load of the working channel
during a failure. It seems foolish to use
the protection channel for anything
other than protection, so we conclude
immediately that the option to use a
pre-emptible channel for working
traffic is too risky.
3. Non-pre-emptible Unprotected (i.e.,
unprotected service) This is a
working channel that does not have any
protection available in case of a failure.
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This type of channel has several
positive characteristics. First, it has the
advantage of not wasting bandwidth on
protection since protection channels are
not used. Secondly, several unprotected
channels can be grouped together, each
operating on different paths so that if a
failure occurs, not all of the channels
will be affected.
To understand the advantage of this third
option for data traffic, see Figure 3. In
this example a VC-4-4V is being carried
using an unprotected channel. Service
remains after the fibre cut because the cut
did not affect the facilities being used to
transport this circuit.
Now look at Figure 4 and this time the
fibre cut occurs in the VC-4-4vs section
and as expected, the VC-4-4v is now
down since the service is unprotected.
However, an alternate path is known by
the affected device and traffic is rerouted
with no assistance from SDH. We
conclude that unprotected service is best
for technologies that already supply their
own protection path(s) and do not want to
pay for SDH protection bandwidth.
It is this logic that drives a powerful
NE
NE
NE
NE
Failure!
STM-16 Ring
VC-4-4v Channels Unprotected
Figure 3 Non Pre-Emptible Unprotected Services Will Not Be
Affected By Fibre Cuts Elsewhere in the SDH Ring
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application for virtual concatenation that
specifically serves data services,
especially Ethernet-over-SDH. Virtual
concatenation can be used to concatenate
any VC-12, VC-3 or VC-4 SDH
channels. The channels can traverse
different rings, some protected, some
unprotected or even all of them
unprotected. The requirement is that the
channels be of the same VC-type and that
they drop to a single NE tributary card.
The SDH equipment along the path from
point A to Z is oblivious to the presence
of virtual concatenation.
Link CapacityAdjustment SchemeIf a failure of a single unprotected VC-12
occurs within a group of n VC-12s,
the entire virtually concatenated group
will fail. Instead, what should happen is
that when part of a virtually concatenated
group fails, data is diverted to the
remaining channels in the group that
are still working. This is a primary
role of Link Capacity Adjustment
Scheme (LCAS).
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Failure!
STM-16 Ring
VC-4-4v Channels Unprotected
Altern
atePa
thKnow
n
ToThi
sDevi
ce
Service Down
Figure 4 Non Pre-Emptible Unprotected Services are Desirable When
Protection is Available From a Higher Layer
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LCAS is a new kind of protection for
data services operating over SDH. It is a
protocol that can sense a failure on a
virtually concatenated member and drop
it from service while keeping the working
members carrying traffic. LCAS itself
can only sense a problem. It then notifies
virtual concatenation of the failure and
virtual concatenation automatically load
balances traffic across the remaining
working members.
Remember, LCAS is strictly for data
services and is a perfect fit for serving
Ethernet-over-SDH. The technology that
adds the elasticity to the changing data
rates is GFP. Recall that GFP performs
dynamic rate adaptation at the Ethernet
PHY/SDH interface. This means that the
Ethernet PHY can operate at any rate it
chooses and/or the SDH transport
channels can be dynamically changed and
GFP will dynamically adjust to the new
rate. GFP rate adjustment is
instantaneous. Data will be lost, however,
from the instant of failure until LCAS
senses the failure. The momentary outage
can be as short as instantaneous to as
long as 64 milliseconds for VC-4
concatenations and 128 milliseconds for
VC-12 concatenations. This is why
virtual concatenation and LCAS are
often mentioned together. They are
complementary and ultimately a mutual
GFP
100 Base-T
Failure!
GFP
100 Base-T
VCG
VC-12 Unprotected
1
32VCG
4
Figure 5 LCAS will Remove Member 1 and Balance the Offered Load
Across the Remaining Three VC-12s
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in many applications can multiply the
sellable bandwidth many times over. To
accomplish statistical gain, plus deliver
Virtual Private LAN Service (VPLS), we
need to add additional technology to the
Ethernet Private Line basic design.
Several technologies may perform this
function, which are MPLS, 802.1D,
802.1P/Q and IEEE 802.17 RPR.
Offer the EthernetService that theSubscribers Want:802.1D/Q/PThe IEEE 802.1D MAC bridges standard
defines Ethernet switching based on the
use of source and destination MAC
addresses. For the sake of this discussion,
an Ethernet switch is simply defined as
a multi-port Ethernet bridge. Ethernet
permits two MAC clients to communicate
via an Ethernet network by sending an
Ethernet frame that contains both source
and destination MAC addresses.
An Ethernet switch learns the MAC
addresses of each connected client by
looking at the source address field and
recording from which direction it came.
Each time an Ethernet frame is sent,
the source MAC address identifies the
identity of the sender and the Ethernet
switch uses this information to associate
MAC addresses with the inbound switch
port. In this fashion the switch quickly
associates MAC addresses with each of
its switch ports. Once a switch learns the
port location of a MAC address, it
forwards the Ethernet frame out the
appropriate port only.
This means that a MAC client must
send frames for a switch to learn its
MAC address. Until a MAC client sends
Ethernet frames, the switch has not
yet learned its MAC address and this
particular clients MAC address is
unknown to the switch. When a switch
receives a frame with an unknown
destination MAC address, it must flood
the frame out all of its ports. A response
to the flooded frame will reveal the
location of the unknown MAC address
and the switch will stop flooding frames
for this particular MAC address given
that its location is now known.
In this example, the discovery of the
unknown MAC address occurs when the
destination MAC client receives the
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flooded Ethernet frame and responds back
to the source, placing its own MAC
address in the source field of the Ethernet
header. The switch instantly learns where
the previously unknown MAC address
was. Now that the switch has learned the
location of the MAC address, Ethernet
frames to this destination are only
forwarded to the appropriate Ethernet
switch port rather than flooding the
entire Ethernet network.
The simplicity of the 802.1D protocol
makes it compelling for LANs, but if
several subscribers share the same
Ethernet switch here are a few of the
many problems that could happen:
All subscribers will receive each others
broadcasts, multi-casts and flooded
frames.
One subscriber could broadcast
incessantly and fill all other subscribers
networks with broadcast traffic
(in addition to their own of course).
Ethernet switching requires that each
MAC client posses a unique MAC
address, but there is no guarantee that
two subscribers may through accidental
or intentional means use the same
Ethernet MAC address and cause
conflicts or serious security issues.
A hacker can learn MAC address of
other subscribers by sniffing broadcast
and flooded traffic. Once a victims
MAC address is known it is then
possible to use that MAC address to
appear as the victim and access
information that is not intended to be
shared. Automation exists for hackers to
systematically learn and hack each
learned MAC address, therefore creating
a security nightmare.
There are many more examples, but this
evidence is sufficient to conclude that
802.1D switching must be partitioned so
that each subscribers private Ethernet
network appears as a unique instance of
an 802.1D switch. This does not mean
that there need to be separate physical
Ethernet switches, but that each Ethernet
switch needs to appear as a separate
logical Ethernet switch to each subscriber.
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IEEE 802.1QVirtual BridgedLocal NetworksThe IEEE 802.1Q standard describes
how to logically partition a single
physical Ethernet network into separate
logical Ethernet networks called Virtual
Local Area Networks (VLANs). Inbound
traffic from each LAN is tagged with a
unique VLAN identifier which the
Ethernet switched network uses to assure
that no traffic on any particular VLAN
leaks onto any other
VLAN. While this works quite well, there
are limitations to the IEEE 802.1Q VLAN
approach. First, there are only 4096
VLAN IDs which is woefully inadequate
for carrier-class networks that would
possibly require millions of VLAN IDs in
the future. Furthermore, most subscribers
are already using IEEE 802.1Q VLANs in
their own networks and they expect to
maintain complete control of this.
Therefore, VLAN tags may not be
referenced by the Ethernet network
operator to partition different subscribers
traffic from each other. Instead, the
Ethernet network operator must process
the VLAN tags on a per subscriber basis
because the subscriber is already using
802.1Q VLAN
802.1P Priority
802.1D Bridge Protocol and Spanning Tree
802.1W Rapid Spanning Tree802.3U 10/100Base-T (auto negotiation)
802.3Z Describes a Gigabit physical layer
1000 - Base-SX (770-860 nm optical Layer)
1000 - Base-LX (1270-1350 nm optical layer)
1000 - Base-LH (Not an IEEE standard, normally uses
1310 nm of the DWDM C and L bands)
802.3AE 10 Gigabit Ethernet physical layer
802.3AH Ethernet in first mile
802.3AB 1000Base-T (Gigabit Ethernet over copper)
802.3X Flow Control
Table 3 Some of the More Popular IEEE Ethernet Standards
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the VLAN tags within their organisation
to partition their own traffic. For instance,
a certain subscriber does not want the
sales department to have access to the
personnel departments network so they
will put both departments in separate
VLANs. Often a subscriber uses routing,
packet filter or firewall services to control
traffic flow between the two VLANs.
IEEE 802.1PSupplement to MediaAccess Control (MAC)Bridges: Traffic
Class Expeditingand DynamicMulticast FilteringThe IEEE 802.1P standard defines how
real-time frames are tagged and
forwarded in an Ethernet network. In
order to offer 802.1P services, an 802.1Q
header is required. Part of the 802.1Q
header is a priority field that identifies the
priority of the Ethernet frame. By simply
reading the 802.1P priority field, the
Ethernet switch knows how the frame is
to be treated and forwards the frame with
the indicated level of priority. The 802.1P
field is only important when there is
congestion in the network and frames
are being queued. If there is so much
bandwidth available that congestion never
occurs, then obviously 802.1P is not
needed. However, the likelihood of
bandwidth availability always being
greater than bandwidth consumption
cannot be guaranteed so support for
802.1P is a very good idea. Since voice
and video over IP are growing in
importance, support of the 802.1P field
may become mandatory for Ethernet
network operators if they wish to
be competitive.
IEEE 802.1D,802.1Q, and 802.1PConclusionsThe simplicity and effectiveness of these
standards has made them ubiquitous.
They exist for the sake of controlling a
subscribers private LAN environment.
If an Ethernet network operator tries to
use the same technology without making
any additional provisions to partition
subscribers, a security nightmare will
result. Therefore, a carrier must deploy
some additional technology beyond
these IEEE standards to partition each
subscribers network from that of
other subscribers.
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Guarantee aSecure Network ByUsing MPLSMulti-Protocol Label Switching may be
used as the workhorse of the Ethernet
network operators network since it has
the advantage of offering end-to-end
services while still delivering statistical
gain, partitioning of subscriber traffic and
meeting the delivery specification spelled
out in subscribers Quality-of-Service
(QoS) Service Level Agreements (SLAs).
MPLS creates virtual pathways called
Label Switched Paths (LSPs). To
understand the mechanism of MPLS and
in order to give a dry topic some levity,
an analogy is presented here. Assume you
are giving a customer direction to your
workplace. You could simply give the
postal address, including street address
and tell your customer to find their own
way. Instead you decide to make it easy
for your customer and you give directions
in a totally different fashion. First, you
post a person (smiley) to stand at every
21299
70
22
125
WORKPLACE
In = 125Out = 99Direction = East
In = 22Out = 125Direction = North
In = 70Out = 22Direction = North
In = 99Out = 212Direction = East
Figure 6 An MPLS Label Switched Path
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intersection. You give each smiley a label
switching table that you have already
configured, as shown in Figure 6. You
then tell your customer to start walking
north, carrying the number 70 and to look
for a bright yellow smiley face. The first
smiley takes the number 70 and in this
case gives back a number 22 and tells
your customer to keep walking north. At
the next intersection, the smiley takes the
number 22 in exchange for a number 125
and tells your customer to keep walking
north. At the next intersection, the
number 125 is exchanged for the number
99 and your customer is told to walk
east. The number 99 is exchanged for
number 212 at the next intersection and
told to continue walking east. Standing
outside your office, you see a bewildered
person walking towards you carrying the
number 212 and you know that this must
be your customer.
This simple analogy points out some very
important issues. First, the street is a
common pathway that can be used by
lots of other traffic. The differentiator
from one traffic flow to another is the
label number itself. There is nothing in
this analogy keeping us from designing
millions of Label Switched Paths by
adding more information to the label
switching table at each smiley face. Since
the LSP is virtual, not physical, the
network can be made controllable by
software, so the analogy needs to be
carried a bit further.
Obviously it is too tedious to compute
the best LSP through the city streets and
then to update each smileys LSP table
manually. It is much better if a map of
the city is available to you and all you
have to do is click the A and Z points so
that the management system would
calculate the best path and update each
smileys LSP table. Furthermore, you
should be able to identify the QoS level
of each LSP so that each smiley face
could manage the relative importance of
each packet attempting to pass through
the intersection.
To see how this analogy applies to
real-world networks, consider that the
smiley faces are MPLS switches which
are fully contained on a single card in an
SDH network element. The streets are
Virtually Concatenated Groups (Groups)
that interconnect each MPLS switch. We
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now have common data highways to carry
multitudes of statistically multiplexed,
QoS protected subscriber traffic and a
new diagram is required to see this
enhancement over EPL.
Provide True QoSUsing MPLSMulti-Protocol Label Switching offers
another key feature for network operators,
that is, the ability to guarantee
Quality-of-Service on a per subscriber
basis. Rather than simply providing
connectivity, a network operator can offer
(and charge) for different levels of
service. The service levels are based on
two components: bandwidth class and a
service grade.
The bandwidth class specifies the
sustained and peak bandwidth guarantees.
The service grade determines the delay,
jitter, and drop precedence aspects. The
QoS experienced by the customer is a
function of both bandwidth class and
service grade. Some generic examples of
service classes are as follows:
Real-time or Expedited Forwarding:
This class is for applications like video
and Voice-over-IP where jitter must be
tightly controlled. This class may not be
over-subscribed and bursted traffic is
discarded. This service compares with
ATM Constant Bit Rate (CBR) or leased
line service.
Business Data With or Without Burst:
Also called assured forwarding, this class
is for business data traffic and is subject
to minor queuing. Lower priority traffic
is pushed out of the way when present.
This is similar to ATMs Variable Bit Rate
(VBR) or RPRs Class B-CIR
(Committed Information Rate)
Best-Effort:
This is for low priority traffic that may
be able to tolerate widely varying queuing
delays, such as internet traffic. This is
similar to ATMs Unspecified Bit
Rate (UBR).
Consider a subscriber that wishes to
operate video conferencing over a
network operators Ethernet network.
One way to accomplish this is to
configure a new LSP in parallel with an
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existing best-effort LSP. The appropriate
LSP is selected based on the priority of
the Ethernet frame being sent. A more
elegant method which will avoid
additional label switched paths for each
grade of service uses the EXP bits in the
MPLS header. The three bits can be used
in a fashion similar to the 802.1P field
and can specify the service level within a
given label switched path.
Using this approach, when the
subscribers Ethernet frames carrying
video traffic are sent into the operators
network, the subscriber tags the Ethernet
frames with an 802.1P field that indicates
a high priority level. The network
operators Ethernet card will forward the
frame over the appropriate label switched
path and set the EXP bits to high priority.
Subsequent MPLS switches read the
EXP bits and forward the Ethernet frame
accordingly.
Using separate label switched paths for
each subscriber assures that each
switching point along the way never
loses track of each subscribers traffic.
The ability to individually manage each
specific subscribers traffic along the
entire path of the network is the basic
premise of end-to-end (E2E) service.
Increase UtilisationBy an Order
of MagnitudeWith MPLSBased StatisticalMultiplexing
Multi-Protocol Label Switching may be
used to add statistical multiplexing
capabilities to our EPL design. Recall the
Layer 1 EPL design shown in Figure 5.
This design could only support a single
subscriber per virtual concatenation. By
adding MPLS, it is now possible to
operate many subscribers across the same
virtually concatenated group. In Figure 7,
a VC-12-4V is being used to interconnect
two different subscribers. Separate MPLS
LSPs are assigned per subscriber in
order to assure that both subscribers
networks are partitioned from each other.
The Ethernet interface, MPLS switching,
GFP encapsulation, and virtual concatena-
tion are all included in a single Ethernet
services card as illustrated in figure 7.
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The Ethernet services card maps customer
A1 traffic to an LSP that transports
subscriber As Ethernet frames between
locations A1 to A2. Likewise, the same
service is offered between subscriber Bs
B1 and B2 locations. In order to ensure
that no customer may utilize more band-
width than they have purchased, the
Ethernet services card restricts the bit rate
according to the bandwidth class and a
service grade that was sold to the
customer. This is explained in greater
detail in the MPLS QoS section.
A Layer 2 Ethernet Private Line makes
sense for:
1. Low-cost, two locations only: a
subscriber has only two locations and
does not wish to purchase higher priced
Layer 1 EPL services.
A1Subscriber
100 Base-T 100 Base-T
Ethernet Services Card10/100/1000 Base-X Plugs
Subscriber B1
A2
B2
Subscriber
Subscriber
MPLS
LSPs
VC-12-4V
12
GFP VCG
MPLS
EmbeddedEthernet
switch
Figure 7 A Layer 2 Ethernet Private Line Combines Virtual Concatenation,
LCAS and MPLS to Enable Statistical Multiplexing
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2. Hub and spoke networks: a
multi-location subscriber network
where all traffic must flow back to a
common hub location such as a
corporate headquarters.
Use Ethernet PrivateNetworking WithDistributed Switchingto Support MeshApplications, AvoidBack-Haul andProvide ResiliencyWhen subscribers wish to operate a mesh
design which allows information to flow
directly between each of their multiple
locations rather than back-hauling the
data to a common point, a more advanced
service is required.
The Ethernet services card may be added
to an SDH network element wherever
Ethernet services are needed. This card is
capable of IEEE 802.1D/Q/P Ethernet
switching in addition to MPLS switching,
and Ethernet to SDH rate adaptation as
shown in Figure 8. Furthermore, the
Ethernet switching function of the
Ethernet service card represents a unique
instance to each subscriber, creating a
single virtual Ethernet switching function
which acts as a private Ethernet switch
per subscriber. The physical interfaces to
the Ethernet services card are
10/100/1000 Mbit/s Ethernet on one side
and SDH on the other. Rate adaptation
and protection technologies are also
included on this single card.
Interconnecting Ethernet services cards in
the core of the network forms the core of
an Ethernet services network as seen in
Figure 8, Central Office A. The
Ethernet services cards are interconnected
using low-cost Gigabit Ethernet links.
Since GFP dynamically rate adapts to
whatever bandwidth traverses these
Gigabit links, we can use all or any
portion of the Gigabit Ethernet link to
support Ethernet traffic between the two
SDH rings. The traffic that traverses the
Gigabit Ethernet links is partitioned by
MPLS LSPs so the links are really
common statistically multiplexed
highway between SDH rings.
Furthermore, cross-connecting different
subscribers additional networks are now
added using MPLS LSPs which are virtu-
al, not physical. The conclusion here is
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that MPLS not only solves the problem of
partitioning different subscribers traffic,
it does so in a virtual fashion that greatly
reduces port counts in the core of the
network and eases the effort involved for
provisioning Ethernet services using
existing SDH rings.
In Figure 9, a subscriber is added to our
Ethernet-over-SDH network. Since we
have the core already built, Ethernet
services cards F, G, H, I and J must be
added. Ethernet services are then
physically connected to the subscribers
sites 1-8. Next, virtual concatenation
groups are added to interconnect the new
Ethernet services cards.
Once the physical layer is completed,
five label switched paths are configured
as seen in Figure 10. Ethernet services
card C is elected as the core Ethernet
switch and LSPs one to five are built to
connect this subscribers traffic to the
Ethernet Services Card10/100/1000 Base-X Plugs
Gigabit Ethernet links interconnect Ethernet
services cards within the same Central
Office, creating the inter-ring links
Central Office B
Central Office A
MPLS
GFP
GFP
GFP
Embedd
edEthernet
switch
VCG
VCG
VCG
D
E
C
A
B
Figure 8 Adding Ethernet Services Cards in the Core Network Elements
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core Ethernet switch. As an example, look
at LSP #1. It originates at switch F,
passes through switch B and terminates
in switch C.
Since all Ethernet service cards are
capable of Ethernet switching and MPLS
label switching, the same cards can be
used to support many subscribers. Since
any Ethernet services card can act as a
virtual Ethernet switch and/or MPLS
switch, we choose C as the best choice
for the core virtual Ethernet switch for
this subscriber. Ethernet services card I
will switch traffic between subscriber
locations 6 and 7, passing traffic to any
other destinations through LSP #3.
Ethernet services card H will locally
switch 4 and 5 and pass any other traffic
through LSP #4 respectively. The core
switch C examines the MAC address or
VLAN tag (subscribers choice) and
chooses the appropriate LSP, forwarding
the packet directly to the destination.
We conclude that the network seems
versatile so far, but what happens when
B
D
E
C
A
F
J
4
H
G
I
2
5
8
7 6
4
3
1
Virtually concatenated Group (VCG)
5
31
2
Figure 9 Virtual Concatenated Groups are Configured to Interconnect Ethernet Service Cards
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another subscriber is added? The answer
is that the same infrastructure is reused,
potentially adding no additional capital
expenses (CapEx) to the project if the
new subscribers locations are already
near an SDH NE that is equipped with an
Ethernet services card.
This design methodology avoids the
n-squared problem of trying to build
this same network using an LSP mesh.
For example, if a subscriber would
interconnect all eight locations using a
standard mesh network, 28 label switched
paths are required to interconnect all sites
according to this formula: n(n-1)/2 or
8(8-1)/2 = 28. Rather than 28 LSPs, this
architecture accomplished the same by
configuring only five LSPs yet delivering
a virtual mesh service.
As additional subscribers are added to this
network, new label switched paths must
be created for each subscriber in order to
Core Virtual Ethernet Switch
Edge Virtual Ethernet Switch Locally
Switches Traffic Between 6 and 7
Edge Virtual Ethernet Switch Locally
Switches Traffic Between 2 and 3
Edge Virtual Ethernet Switch Locally
Switches Traffic Between 4 and 5
Configure additional LSPs to operate Bas a secondary "Core" for resiliency
B
D
C
A
F
J
4
H
G
I
2
5
8
7 6
4
3
1
3
2
5
E
Figure 10 Label Switching Paths are Configured to Create a Private Virtual Ethernet Service Offering.
Additional LSPs would Normally Be Added for Redundancy, But are Not Shown in the
Figure for the Sake of Clarity
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partition each subscribers traffic from the
others. When an Ethernet card receives an
Ethernet frame, it references the MPLS
label in order to determine to which
subscribers network the frame belongs.
Since the Ethernet card maintains separate
MAC address tables for each subscriber,
MPLS labels serve an important function
of discriminating which subscriber a
particular Ethernet frame belongs to.
With MPLS, complete partitioning of a
subscribers traffic from the other
subscribers traffic is guaranteed.
Resilient Packet RingResilient Packet Ring establishes a
new MAC layer, designed to run autono-
mously on its own ring, providing its
own protection. It is important to note
that RPR does not define a new Layer 1
and normally would use Gigabit Ethernet,
10 Gigabit Ethernet or virtually
concatenated SDH channels as the spans.
Ringlet 1Ringlet 2Domain All stations belonging to this RPR.
RPR Station An RPR node- 255 stations maximum- Each stations identity = it s MAC address
RPR Span The Physical Layer- The link between stations- All spans must use the same bit rate
- An RPR span may traverse multiple SDH rings- Gigabit or 10 Gigabit Ethernet PHY can serve as the link- RPR does not define interconnection of multiple RPR domains.
Figure 11 RPR Ring Architecture
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The general topology of the RPR
architecture is illustrated in Figure 11.
The RPR network elements, called
stations, are interconnected to adjacent
stations via RPR spans. Since RPR is
Layer 1 agnostic, an RPR span could
theoretically be anything although
practicality dictates otherwise so
RPR currently spells out either SDH
or Ethernet PHY as the preferred
Layer 1 technology.
There are two limiting factors regarding
the spans themselves. First, they all must
be the same bit rate, so if stations one
and two are connected via a VC-4-5v,
they all must be connected at that rate
regardless of situational necessity. This
means that RPR requires a fixed amount
of bandwidth between every SDH
network element on the SDH ring if we
assume that RPR stations are plug-in units
in each SDH network element. Second,
RPR provides its own protection scheme;
therefore, RPR should be assigned
virtually concatenated unprotected
channels to meet the subscriber
requirement for bandwidth and levels of
protection. When allocating RPR
bandwidth for subscriber utilisation, it
is up to the management software to
calculate how much protection bandwidth
will be available to RPR in the event of a
failure and make sure protected services
are not over committed.
RPR Need MPLSSimilar to 802.1D switching, RPR must
depend on MPLS or some other means to
identify and partition each subscribers
traffic. When traffic is inserted onto an
RPR ring, RPR encapsulates the frame
and assigns appropriate source and
destination MAC addresses. These source
and destination MAC addresses are of the
RPR stations, not the subscriber.
Consider this example while viewing the
RPR topology shown is Figure 11.
1. Two subscribers Ethernet frames are
inserted at station A and stripped at
station D.
2. RPR station A individually
encapsulates both Ethernet frames but
both carry the same RPR MAC
addresses as follows:
From: Station A
To: Station D
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3. The frames pass through Station B
which cannot discriminate between the
two subscribers since both RPR frames
contain the same source and destination
RPR MAC addresses.
4. The frames pass through Station C with
the same limitation explained in step 3.
5. The frames arrive at station D where
the RPR header is stripped off and the
contents of the RPR frame are revealed.
6. If MPLS was used, then the MPLS
label could now be referenced in order
to properly forward the frame.
7. If MPLS or some other function is not
used to identify the ultimate source and
destination of the frame, then the
identity of the frame is lost.
Therefore, we conclude that RPR is not
an end-to-end service and cannot be used
for that particular purpose. RPR service
begins when traffic enters a ring and
immediately ends when the frame is
stripped from the ring. A service like
MPLS is still required unless the entire
RPR ring is intended for only a
single subscriber.
Service ClassesWhen traffic enters an RPR ring, it must
be assigned one of three user classes of
service, some of which are divided into
subclasses so we can say there are really
five distinct levels of service if the entire
RPR specification is deployed. When
packets are inserted onto an RPR ring,
they must be marked according to the
type of treatment they require when
traversing the RPR ring. Each service
class is now explained in detail here:
Class A0 and A1 is for applications such
as video and Voice-over-IP. Class A0
traffic may not be over-subscribed and if
the allocated bandwidth for A0 is not
used, it cannot be reclaimed by lower
priority services and it is simply wasted.
Bursted traffic is discarded.
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Class B-CIR (Committed Information
Rate) is for business data traffic and is
subject to minor queuing but will push
lower priority traffic out of the way
when present.
Class B-EIR (Excess Information Rate)
allows business traffic to take a weighted
fair share of the available unallocated
bandwidth plus any reclaimed bandwidth
from other subscribers that are not using
their Class A1 or Class B-CIR traffic at
this particular instant.
Class C is for low priority traffic that
may be able to tolerate large amounts of
queuing delays.
RPR ProtectionRPR can provide protected services
through one of three different means,
either wrapping, steering or pass though.
The appropriate method depends on what
was deployed and the type of failure. If
the station itself fails, it may take itself
off line and simply pass data through as if
it were a repeater. This is better then
breaking the connection all together. If
RPR senses a fibre cut or catastrophic
station failure steering or wrapping may
be deployed but not both. Regardless of
the protection method, the ring just lost
50 per cent of its capacity and traffic must
be routed the opposite way around the
ring, approximately similar to SDH.
Spatial ReuseSpatial reuse only has meaning to people
familiar with FDDI and token ring. When
a packet is placed on a FDDI or token
ring, it would block access to the entire
ring while it traveled the entire
circumference of the ring to be stripped
by the station that sent the packet in the
first place. RPR is different from those
LAN topologies. With RPR a packet can
be sent from station A to station B while
at the same time a packet is being sent
from station B to station C. To an SDH
engineer this is standard operating
procedure but nevertheless, we cannot
discount spatial reuse because the old
ways like FDDI and token ring wasted
lots of bandwidth. Compared with a pure
MPLS network that does not include
RPR, spatial reuse does not deliver
anything especially useful or new.
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Adding RPR tothe EthernetServices CardThe same Ethernet services card we
studied earlier now has a new RPR layer
sandwiched between MPLS and GFP
(see figure 12). This is the RPR layer
services the RPR ringlets and serves the
MPLS client.
In comparison to the network shown in
Figure 10, an RPR enabled Ethernet card
is now added to show a different
implementation. Shown here in Figure
13, we see the same network deployed
using RPR with MPLS supplying the
end-to-end services.
The first thing we notice is that we used
a lot more bandwidth on our individual
rings to create the four RPR rings
required to deploy this service. We also
notice that there are five LSP required
since RPR does not support end-to-end
services. Since our Ethernet services card
supports Ethernet switching in addition to
RPR, cards G, H and I perform local
switching between 2 and 3, 4 and 5,
6 and 7 respectively rather than placing
the packets on RPR. When packets are
destined for other locations, it is MPLS
that really does all the work; RPR just
forms an additionallink layer (which is
superfluous) between Ethernet services
GFP VCG Ringlet 1
MPLS
10/100/1000 Base-X Plugs
RPR
GFP
EmbeddedEthernet
switch
Ringlet 2VCG
Figure 12 Ethernet Services Cards With RPR Stack Included
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cards which is already adequately
handled by GFP and MPLS.
In Figure 13, the network using RPR
looks much like it did in Figure 10. The
difference is that additional bandwidth is
consumed on each ring to support RPR
ringlets. The tough question is now asked,
What is RPR doing? It appears that it is
using up bandwidth and adding cost. Over
an SDH network, it seems to add an
additional layer without eliminating
anything or providing any new services.
We can accomplish the same thing at a
much lower cost and achieve better
bandwidth efficiency with virtual
concatenation, LCAS, 802.1D/Q/P
switching, and MPLS.
So Where does RPR
Make Sense?If we eliminate SDH from the equation,
and use native Gigabit Ethernet as the
PHY between RPR stations, then RPR
could make sense. In this case, LCAS,
virtual concatenation, SDH clocking and
SDH itself are eliminated from the
equation. All of this assumes that network
Virtually concatenated Group (VCG)
Label switched Path (LSP)
Core Virtual Ethernet Switch
Edge Virtual Ethernet Switch Locally
Switches Traffic Between 6 and 7
Edge Virtual Ethernet Switch Locally
Switches Traffic Between 2 and 3
Edge Virtual Ethernet Switch Locally
Switches Traffic Between 4 and 5
B
D
E
C
A
F
J
4
H
G
I
2
5
8
7 6
4
3
1
2
3
5
Figure 13 Ethernet Service Using RPR
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operators are planning on running a
data-only network with no legacy
equipment that requires an SDH network.
For network operators who already have
an SDH network, the new next-generation
features make RPR a hard sell.
RPR-over-SDH was a good idea when
RPR development first began because
LCAS and virtual concatenation did not
then exist.
ConclusionFor network operators who already
operate an SDH network, adding Ethernet
services to existing SDH network
elements is now a reality. A powerful
combination of virtual concatenation,
LCAS, GFP and MPLS all packed into
a single Ethernet services card is a
compellingly simple solution.
Adding RPR to the equation offers no
value unless SDH can be removed from
the picture entirely and that does not seem
plausible in the near future given the stateof the world economy and the fact that
most organisations are looking for ways
to capitalise on the investments they have
already made rather than buying all new
equipment and building an overlay net-
work. Therefore, the conclusions on each
of the following technologies are
as you see on the next page:
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How to reach us:
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Tellabs
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Fax: +1.630.798.2000
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Use SDH as the Layer 1, which is already deployed in
most carriers networks.
Use standard 10/100/1000 Ethernet to interface with
the subscriber.
Use GFP rather than X.86 to interface Ethernet
(asynchronous) and SDH which is synchronous.
Use virtual concatenation to provide physical
connectivity growth in 2 Mbit/s increments.
Use LCAS for protection services that will direct traffic
only on the working channels and stop traffic from
flowing on failed channels. LCAS also offers the benefit
of not wasting bandwidth on protection channels.
Use a hybrid card that plugs directly into the SDH
network element which provides virtual 802.1D/Q/P
switching services with a separate logical Ethernet
switched network per subscriber.
Use MPLS to partition subscribers traffic, offer virtual
end-to-end services and meet Service Level Agreements
involving best-effort, assured forwarding and expedited
forwarding over a statistically multiplexed backbone.
Therefore the recommended protocol stack is as follows:
The subscriber sends Ethernet frames, with or
without VLAN tagging.
Which are encapsulated within an MPLS frames.
Which are encapsulated within GFP frames.
Which are load-balanced across multiple
virtual concatenated channels using virtual
concatenation.
LCAS provides protection services in case a
particular channel fails by forcing traffic to flow
only on the remaining working channels.
SDH provides the Layer 1 connectivity, similar
to plumbing between water fixtures.
The following trademarks and service marks are
owned by Tellabs Operations, Inc., or its affili-
ates in the United States and/or other countries:TELLABS, TELLABS and
T b l d T b l
Any other company or product names may be trademarks of
their respective companies.
2003 Tellabs. All rights reserved.
74.1412E Rev. A 10/03