ceragon eth rings paper final

8/3/2019 Ceragon ETH Rings Paper Final

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Migration to Ethernet Rings:

High Capacity, High Availability Mobile Backhaul Aggregation Transport

Ceragon Networks September 2008

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Migration to Ethernet Rings:High Capacity, High Availability MobileBackhaul Aggregation Transport

Authors:

Ron Nadiv- Director of Technology, Ceragon Networks

Dudu Bercovich - Chief Architect, Ceragon Networks

September 2008

Ceragon Networks, CeraView, FibeAir and the FibeAir design mark are registered trademarks of CeragonNetworks Ltd., and Ceragon, PolyView, ConfigAir, CeraMon, EtherAir, QuickAir, QuickAir PartnerProgram, QuickAir Partner Certification Program, QuickAir Partner Zone, EncryptAir and Microwave Fiber aretrademarks of Ceragon Networks Ltd. All rights reserved.


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This paper provides guidelines for mobile operators who wish to deliver higher capacities

over their aggregation wireless backhaul networks. The paper will show how wireless

Ethernet Ring topologies facilitate much better availability than typical SONET/SDH and

how cost-per-bit reduction and improved overall user experience can be achieved. In

addition we will present three test cases for building cost effective aggregation rings that

take advantage of Ethernet features and capabilities. These will include:

IP/MPLS packet ring

Carrier Ethernet

Connection oriented Ethernet such as PBB-TE

Introduction:

Why TDM-based solutions are no longer enough

TDM-based SONET/SDH1

is the primary technology used today for carrying high

capacity traffic in mobile backhaul networks. Traffic that is initiated mainly as voice

calls at the base station is backhauled over PDH radios or leased lines and moves

towards the network core. Further along, T1/E1s are aggregated and transported

over fiber or high capacity microwave SDH links (from this point onwards we will refer

only to wireless backhaul networks over microwave).

Typically deployed in nxOC-3/STM-12

topologies (i.e. one or more 155Mbps links)

SDH rings were designed to ensure continuous high-quality voice services. However

as the focus of mobile users (and operators) shifts from voice to data, traditional SDH

may prove inadequate or at the very least, not as cost-effective as new alternatives.

Data services do not generate the same revenue-per-bit as voice, so the cost of

deploying additional TDM-based systems cannot be offset by revenue increase. In

other words, the old equation of matching demand and capacity bit-for-bit is no longer

economically viable.

A new service paradigm

Unlike SDH systems, Ethernet is more apt for handling data-centric mobile traffic.

Enabling statistical multiplexing and much higher granularity, Ethernet solutions can

1 SDH and SONET often use different terms to describe identical features or functions. Where SDH is mentioned in this paper the

reader can assume it also refers to SONET unless otherwise specified.2STM-1 is the SDH transmission standard carrying a same capacity of 155.52Mbps. In SONET rings the transmission standard iscalled OC-3.


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help decouple cost and capacity while ensuring carrier-grade quality of service for

premium real-time services.

Ethernet can be generated in two ways. First - at the base-station itself as in the case

of new EV-DO, HSPA and WiMAX (and even some new GSM base-stations). As the

mobile world moves towards LTE, we can assume that in the future, most (if not all)

base-stations would be Ethernet based.

The second and more common Ethernet scenario is the terminations of T1/E1 lines

over Ethernet either at the cell-site or further down the network at the aggregation or

hub site. The latter deserves additional discussion because the migration to Ethernet

is not always straight forward.

Current Situation E1/T1 are exploding

Historically, mobile cell sites were fed with a backhaul capacity of one or two

E1/T1lines. This was sufficient for mobile operators in order to build a scalable

backhaul network with inherent high availability, management and OAM features.

Obviously these 2G backhaul networks were built with a single class of service and a

single class of availability in mind. Though not inexpensive to deploy, T1/E1 based

backhaul networks delivered a premium service voice - which helped compensate

for high network costs.

But recently things are starting to change. With 3G technologies such as UMTS and

HSPA, data throughput becomes the main contributor to mobile traffic. Now more

and more E1/T1s are needed per cell site in order to support the increased traffic. In

some cases cell sites already require (and deliver) 8xE1 (or 16Mbps) of capacity and

theres even talk of having to double this capacity in the near future.

As this trend intensifies, operators quickly find out that simply adding T1/E1s to cope

with the growing demand for bandwidth raises other significant challenges. For one,

data service charges are much lower than voice service charges on a per-bit basis.

Hence, the increase in traffic can no longer justify the high costs of deploying and

managing a huge and growing number T1/E1s in the network.

On the Aggregation segment, SONET/SDH rings offer little relief. SONET/SDH

solutions are ideal for aggregating T1/E1 traffic, but have no means of performing

statistical multiplexing and thereby reducing the number of channels across the

network. This is where Ethernet rings would come into their own.

* * *


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Termination of E1/T1s onto Ethernet

Let us now go back now and look at how Ethernet traffic is generated.

1. Ethernet base-station: New 3G and WiMAX base stations (as well as future

LTE) are already Ethernet based. Hence all traffic generating from these

base-stations is already Ethernet.

2. Cell-site termination: Making use of Cell Site Gateways operators can

terminate some or all of the E1/T1s at the cell site itself over Ethernet. Here a

Generic Interworking Function (GIWF) device performs the conversion from

TDM to Ethernet.

3. Aggregation site Gateway: Aggregation sites concentrate large quantities

of E1/T1s into a single location using a higher capacity GIWF device.

Figure 1: Termination of E1/T1s onto Ethernet

The case for Wireless Ethernet Rings over SONET/SDH

So what is the best way to transport packets in a protected and manageable way

using high capacity Microwave?

As already mentioned above the major change in mobile operators current service

mix is that data services do not generate the same revenue-per-bit as voice. But

theres an up side too. In the data centric world, not all services are created equal.


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For example, most data services do not require the lowest latency and the highest

levels of availability. Todays backhaul networks, and to a greater extent those of

tomorrow, could utilize a more flexible approach - one that would maintain the

reliability, durability and predictability of SDH, but bring about the decoupling of cost

and capacity. This approach will be carried out with Ethernet rings.

Protection - use the standby capacity

Legacy SONET/SDH solutions use an n+1 protection scheme. Protection capacity in

this case is used forprotection only. For example, in a 1+1 configuration as shown in

Figure 2 below, one SDH channel is used to carry service while the other channel

remains unused.

Only when the primary channel fails, does the secondary kick into action. In short,

SDH protection is about protecting the traffic at all costs. And the cost in this case isan entire channel standing by unused for the odd chance of failure.

Figure 2: SDH Ring/Ethernet Ring Protection Modes

In Ethernet Rings, protection works differently. Here the concept is not to protect all

traffic at all costs, but rather define which type of traffic needs to be protected all the

time, and which can have a lower level of protection. For instance, an Ethernet ring

built in 2+0 configuration will utilize the entire available capacity (two channels in this


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case) in a normal work mode. In case of failure along the network, a protection

algorithm is activated giving high-priority to premium services like voice over non-

real-time data. This is explained in more detail in the example below.

Let us assume that voice makes up for 30% of the traffic in a certain network and that

capacity is now cut to 50%. The capacity degradation over the ring activates QoS

mechanisms that ensure the delivery of high priority traffic without delay by using

strict priority schemes. In this case, all voice packets will continue to flow

uninterrupted. Low priority traffic will be delayed and, if the problem persists, perhaps

even dropped using drop algorithms at the edge of the network.

True, the above scenario does not offer a 100% protection scheme like in the SDH

world. It does however offer 100% protection level for premium (i.e. revenue

generating) traffic - while leaving some headroom for low priority service so as to

avoid starvation. One should also keep in mind that a 50% capacity loss due to

fading conditions is rare and when it occurs it seldom lasts more than a few seconds.

At any other time the network can utilize all the available channels and provide higher

capacities at a much better cost-per-bit ratio than SDH.

Granularity- Utilize all available capacity

SONET/SDH operates in hierarchies -155Mbps, 622Mbps etc. This rigid structure is

not designed to cost-effectively handle data traffic. Ethernet, on the other hand, has

port hierarchies (10/100/1000Mbps) but traffic can be of any granularity. So, while

SONET/SDH microwave systems are restricted to the protocol hierarchy, Ethernet

solutions can utilize the entire available bandwidth.

For example, a microwave radio system based on SDH can deliver a 155Mbps

service. Yet under similar power, channel-bandwidth, antenna size and other

parameters, an Ethernet based solution will deliver between 200Mbps and 250Mbps

(the actual capacity depends on compression and coding techniques). So, by simply

allowing the use of Ethernet radio, a microwave system at a mobile operators service

may deliver much more capacity than in a SDH/SONET scenario.

Availability

The SDH world operates in an all-or-nothing mode - you either have the link or you

dont. But Ethernet microwave, that can also employ Adaptive Coding & Modulation

(ACM), gives operators the freedom to transmit in a range of service levels. For

example, a 155Mbps SDH pipe delivers 99.999% availability. Using the same


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equipment and frequency channel an Ethernet link can be used to transmit an

additional 50Mbps at 99.99% availability. But theres more.

As already mentioned, operators can increase the capacity of a microwave link

simply by switching from SDH to Ethernet. Taking advantage of the now available

granularity features, operators can split capacity according to service type. For

instance, a 200Mps link can utilize as follows: 50Mbps high-priority real-time services

at 99.999%; 100Mbps data services at 99.99% and 50Mbps for low-priority traffic at

99.9%. From a first glance it may seem like we have reduced availability, but in truth,

the system ensures that premium types of service never fail and have a guaranteed

channel regardless of any other traffic (this scenario assumes that the ACM between

modulations shift is performed in a hitless/errorless manner and therefore has no

negative impact on the smooth and continues delivery of high-priority traffic).

The solution described above makes perfect economic sense. Operators protect and

improve the availability of their revenue generating services to ensure high-quality,

uninterrupted user experience. The trade off is lower protection and lower availability

of low-revenue services but only in extreme - and rare - cases of congestion or

network failure.

Statistical Multiplexing dont overpay for capacity

Statistical multiplexing gives operators a tool with which to cope with, and indeed

control the number of T1/E1s in their network. Statistical multiplexing is based on the

assumption that not all channels are 100% utilized at all times. Hence, 32 E1s may

not require 64 Mbps of backhaul capacity at all times.

Based on statistical calculations operators can groom or aggregate a number of

T1/E1 lines onto a single pipe and significantly reduce the complexity of the network.

Statistical multiplexing will play an important role in the transition from voice-centric to

data-centric services over mobile networks.

* *

Topology considerations

With Ethernet, adding links to cater high capacities and increase availability at a cell

site location is more straight-forward than in the SDH world. It also involves moving

from a Tree topology to a Ring or even Mesh topologies.

As cost per bit is reduced, operators can benefit from adding protection mechanisms

(as shown above) and enjoy the statistics of a large ring instead of sticking to the


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more careful and risky planning of Tree topologies. Nowhere is this added benefit

more evident than in the process of planning a chain of links because in the data-

centric world, the longer the chain, the higher the probability of failure along the

network.

Path Protection and Fast recovery

New Standards such as ITU G.8031 or PBBT-TE, and the commonly used IP/MPLS,

offer reliable ways to manage end to end transport such as protection on a per-flow

basis. Since Packet Rings support higher capacities and offer better performance

than SONET/SDH, the transition towards Ethernet makes sense. These

implementations are further discussed in the following chapter.

* *

Case studies

Ethernet Rings is not only a good concept in theory. The case studies below, based

on interoperability tests and real-life deployments, show how it can be successfully

implemented in the field.

IP/MPLS Aggregation Backhaul

The diagram below shows a solution combining a high-capacity, IP/MPLS-aware

microwave and a powerful mobile backhaul-focused Ethernet switch. The solution

aims at replacing SONET/SDH systems in aggregation sites. Using similar

architectures, mobile operators can employ new Ethernet paradigms in the

aggregation layer. Connecting switch-routers to service aware microwave link

endpoints, allows operators to maximize their networks efficiency. This coordinated

solution also ensures delivery of high priority, real-time services - while enhancing

capacity to cater for additional data services.

A solution such as the shown in Figure 3 can deliver two to three times more capacity

than comparable SONET/SDH solutions - as well as higher availability. It can also

ensure predictable performance in multi-service environments, under fading

conditions and during congestion.


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Figure 3: IP/MPLS aware Ethernet microwave as alternate/primary path in a network protection scheme

* *

Backhaul Aggregation over a Resilient Ethernet Ring

The following case study features an advanced native Ethernet microwave combined

with Carrier Ethernet switch routers to aggregate traffic across resilient Ethernet

rings.

This architecture, shown in Figures 4 and 5 below, allows service providers to

seamlessly migrate their transport network to IP by introducing Carrier Grade

Ethernet in the Access and Aggregation layers.

Delivering improved over-the-air bandwidth granularity, scalability and availability, a

combined microwave-switch solution can also allow operators to direct data traffic

originating at new WiMAX or HSPA cell sites, to new Carrier Ethernet transport

solutions in the aggregation layer. Connecting switch routers to service-aware

microwave link endpoints facilitates an extremely low cost-per-bit transport

mechanism. With the evolution of mobile specifications to be all IP, these advanced

Carrier Ethernet solutions can also serve to replace legacy SONET/SDH while

offering superior availability mechanisms and overall capacity planning.

In order to achieve Carrier class Ethernet service performance, operations,

administration and maintenance (OAM) functions must be readily available to ensure

QoS and meet provisioned Service Level Agreements (SLAs). Native Ethernet fault

management protocols are essential for the delivery of high quality end-to-end

services.


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Figure 4: Ethernet Aggregation using xSTP and fast recovery of an integrated Carrier Ethernet switch

Figure 5: Resilient Ethernet Aggregation Using an External Carrier Ethernet Switch

* *

PBB-TE-based Mobile Aggregation Backhaul

This scenario combines a high-capacity, PBB-TE-aware microwave and a PBB-TE

solution to replace TDM-based aggregation backhaul.


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PBT (Provider backbone transport) also known as PBB-TE (Provider Backbone

Bridge Traffic Engineering), is an ongoing project in IEEE standard which provides

enhancements to Ethernet and supports traffic engineering within Provider Backbone

Bridge Networks (P802.1ah). PBB-TE brings control to data paths within a large

carrier network, enabling QoS and the ability to set aside specific paths for specific

traffic types.

PBB-TE makes packet networks predictable, allowing operators to efficiently manage

their packet network resources and maximize their performance, while ensuring the

superb QoS needed for delay sensitive, real-time services such as video and voice.

Bringing Carrier-Grade services over microwave paves the way to migrating existing

networks to cost-efficient Ethernet. Figure 6 below shows how PBB-TE is now

accessible across the entire network and can be implemented by mobile and fixed-

line operators.

Figure 6: Predictable Packet-based Aggregation Backhaul with PPB-TE Resiliency

Protection in a PBB-TE ring is service aware as well. The radio link drops all the

traffic belonging to the right trunk allowing the switches at the nodes to switch to an

alternate trunk. In Figure 6 (above) a high priority and low priority trunk defined for

traffic flowing from site A (Aggregation site) to site C (Core site) use a primary path

which is direct. In case of fading conditions or radio failure in a multi radio link, the


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lower priority traffic is re-switched to a pre-defined alternate path. This is a longer

path crossing the 2nd

aggregation site (site B in the diagram). If the entire link is

unable to sustain the high priority traffic requirements, the high priority trunk will be

switched on as well. Obviously, in case of a failure, ports are shut-down at both ends

so that all traffic may be instantly switched to the alternate path.

* *

Conclusion

The proliferation of T1/E1 lines in mobile operators access and backhaul networks,

calls for a more flexible and cost-effective solution for handling legacy voice and new

bandwidth intensive data services. Ethernet Ring topologies facilitate much better

availability than typical SONET/SDH and can help to reduce cost-per-bit and improve

overall user experience.

While the migration is only in its initial phases, forward thinking equipment vendors

are already offering a range of solutions to help operators achieve smooth and pain

free transformation of their networks to Ethernet.

For more information about wireless Ethernet solutions and to learn more about Ceragon and its broad

portfolio of Ethernet and TDM high-capacity backhaul product lines, please visit our website at:

www.ceragon.com


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Appendix A:

Access and Aggregation Microwave Backhaul Networks - Characteristics

Aggregate Switch

POP

Base Station

Aggregation Backhaul BS Backhaul

Control

Access Backhaul Aggregation Backhaul

Figure 7: Mobile Backhaul over Microwave

Ceragons FibeAir IP product line covers the entire microwave for Mobile backhaul

addressable market, both in the Access and the Aggregation segments. Access

backhaul refers to the access of the base-station, whether in a Tail, Chain or Hub

site. Aggregation backhaul refers to the network element that collects Access traffic,

concentrates it and delivers it to the core of the mobile network. Table 1 illustrates the

main difference between the two network segments

Access Aggregation

Currently typical 1,2 x E1/T1 nxSTM-1

Near future (HSPA) 8 x E1/T1 or 10-20Mbps NxSTM-1 (N>n) or 200Mbps-1GbE

Future (LTE) 30-100Mbps nxGbE

Scale 000s of base stations 00s of ringsAvailability requirements Medium to high High to highest

Spectral Efficiency Highest Medium to high

Networking functions Integrated Part of a greater network concept

Legacy Support Self contained Hybrid models (Native2) Usually on a different network or

Mapping NG-SDH or PW

Table 1: The Access and Aggregation backhaul segment - differentiation

The Access and Aggregation segments of the mobile backhaul network today are

mainly served by low-capacity PDH and SONET/SDH respectively. Mobile operatorshave come to trust and depend on these two technologies over the course of the past

two decades and until recently, there was no reason to consider their replacement.

Yet with the advent of data-centric 3G networks and services operators may need to

revisit their wireless backhaul strategy and consider Ethernet as a cost efficient

alternative to PDH/SDH rings.

ceragon eth rings paper final

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