tlabeosstandards(eos_ethernet over sdh)

8/3/2019 Tlabeosstandards(EOS_ethernet Over Sdh)

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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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Choosing the Best of Todays Ethernet-over-SDH Standards 2

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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)


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)


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)


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





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







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:

North America

Tellabs

One Tellabs Center

1415 West Diehl Road

Naperville, IL 60563

U.S.A.

+1.630.378.8800

Fax: +1.630.798.2000

Asia Pacific

Tellabs

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#43-02 Suntec Tower Two

Singapore 038989

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+65.6336.7611

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Europe, Middle East & Africa

Tellabs

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Tellabs

13800 North West 14th Street

Sunrise, FL 33323

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+1.954.839.2800

Fax: +1.954.839.2828

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

tlabeosstandards(eos_ethernet over sdh)

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