9/12/2006 1

What do you expect from a Cellular service Provider

- LOW SUBSCRIPTION FEE

- HIGH QUALITY CONNECTION

9/12/2006 2

Outline

• Review of the wireless network operation

• Limitations of the current network architecture

• Description of the new architecture and benefits

• Backhaul cost reduction: Analysis and results

• Increased availability: Analysis and results

• Conclusions

• Further research topics

9/12/2006 3

A Simple Wireless NetworkMobile Data Set

PSTN

PacketNetwork

MobileVoice Unit

Base Transceiver System (BTS)

Base Station Controller

(BSC)

Mobile Switching

Center (MSC)

Packet Inter-Working Function

Challenge is to keep connection and not loose any data during handoff operation

9/12/2006 4

The Components

• BTS– BTS consists of one or more transceivers placed at a single location.

The BTS terminates the radio path on the network side.

• BSC– Provides allocation and management of radio resources. – SDU: Selection and distribution unit. Also responsible for handoff

coordination

• MSC– Provides and controls mobile access to the PSTN. Interprets the

dialed number, routes and switches call to destination number. Also manages mobile’s supplementary services. Maintains a register of visitors operating within the coverage area of the MSC’s connected BTSs.

• PDSN: Packet data service node is basically a packet router.

9/12/2006 5

MSC PDSN

BSC(SDU)

BSC(SDU)

BSC(SDU)

BTS BTS BTS BTS BTS

 

- Backhaul cost is by $$$/mile- 10-100 miles between BTS and BSC- Voice or data use one DS0 channel at a time- BTSs are located in the tower- BSC and MSCs are located

in the central office

Current Wireless Network Architecture

BSC

BTS

24xDs0in T1

PacketsTDMchannels

TDMchannels

9/12/2006 6

Soft Handoff between two BTS

Handoffs == ( Hard || Soft )

Handoff: A handoff mechanism is needed to maintain connectivity as devices move, while minimizing disruptions to ongoing calls. This mechanism should exhibit low latency, incur little or no data loss, and scale to a large network.”

9/12/2006 7

SDU and soft handoff

  

  

 

 

SDU - 1

WR-A

 

 

SDU-1 -

   

SDU - 2

WR-B

-

 

- 3 to 6 BTSs involved in soft handoff- SDU changeover due to weak signal from primary BTS- BTS forwards even corrupted radio frames to the SDU for selection

SDU - 2-

BTS-1

BTS-2

BTS-3

9/12/2006 8

Problems with the current architecture

• Duplicate traffic on the links: Frame selection is done at the BSC (One frame is generated for each soft handoff leg.) This results in duplicate traffic flow at the backhaul

• No traffic aggregation: Each call is allocated DS0 capacity. Even when there is no activity on the call, the DS0 is reserved. At this rate, currently each BTS can support only around 20 calls per sector (normally 3 sectors per BTS). So, no traffic aggregation – does not utilize statistical multiplexing (results in inefficient backhaul link provisioning)

• If six BTS, then more overhead: Seventy percent (70%) of wireless operators expenditure is on RAN. Around 30% expenses are backhaul cost. Only 15% of the BTS-BSC traffic is payload and rest is overhead. If six BTS are involved in the soft handoff then the overhead will be lot higher.

• Carries dead payload: BTS forwards even error frames to BSC. Because the selection is done at the BSC. This means we are carrying dead payload to BSC.

9/12/2006 9

Problems with the current architecture (cont..)

• Uneven utilization of links: For data services as well as voice, IP networks overlay on top of current wireless networks. This is very inefficient, not cost effective, and very difficult to deploy the new services.

• Performance: Less propagation delay. Currently, for some of the BTS-BSC configurations, the links run 100 miles, it means several milliseconds of delay. This creates problem during soft handoff.

• Availability: Less availability due to single point of failure (in terms of base stations, base station controllers and links connecting between them)

9/12/2006 10

Review

• Simple wireless network operation

• Different components in the network

• Wireless network topology

• Mobility and soft handoff

• Problems with the existing architecture

9/12/2006 11

33.1

33.2

33.3

33.4

33.5

33.6

33.7

33.8

33.9

34

-85.2 -85 -84.8 -84.6 -84.4 -84.2 -84

Typical US city BTS Map

30x30 miles

Urban

Rural highway

9/12/2006 12

2G/3G RAN Network (Traditional)

CO

BTS BTS BTS BTS BTS BTS

CO CO CO

BSC

Interoffice distance (costs per mile) cost + Fixed Cost

ChannelTerminationCost

ChannelTerminationCost

MSC

9/12/2006 13

What configuration is best in terms of cost and availability

• Cost Reduction: How and where to place wireless router in the RAN network with respect to network-level backhaul cost and availability. For example, existing WR can be part of GSR (high end router) ? How close it to BTS

• Higher Availability: Distributed IP-RAN for backhaul cost savings and higher availability

Variables in analysis – Different kinds of transport cost structures

– Different types of links (T1/T3/Microwave etc.)

– Different types of connectivity between BTS and wireless router

– Number of carriers supported supported by BTS

9/12/2006 14

Cost Model• Commercial BTS network topology. BTS are connected to BSC using the

chain of Central Offices.

• Real ILEC cost structure is assumed for the T1 leased lines from urban and rural areas to the BSC.

• 20-30 Node network in urban area and 10 BTS in rural areas.

• Soft hand factor of 2.0: How many BTS are in soft handoff

• 1.5 T1 per BTS per carrier

• BTS Network Regions: I) Dense Urban, ii) Urban iii) Suburban iv) Rural

• Cost models– Channel termination costs

– Interoffice fixed costs

– Per mile costs: Transport cost changes according to distance and the type of transport (T1/T3/OC3)

9/12/2006 15

Configurations

Four BTS network configuration are considered:1. Traditional BTS-BSC network (Config-1, fully star, call it

“Traditional”)

2. One Wireless Router supporting multiple BTSs (10-30). For example, existing WR can be part of GSR (high end router) ? How close it to BTS (Config-2, call it “star”)

3. Meshed Wireless Routers (Config-3, one wireless router per BTS, full mesh within the central office region. Call this “WR”)

4. Meshed Wireless Routers (WR) and each WR connected to multiple Gateway Routers for higher availability (Config-4, with connection to all the nearby high end routers. Call this “WR-HA”)

9/12/2006 16

2G/3G RAN Network (Traditional)

CO

BTS BTS BTS BTS BTS BTS

CO CO CO

BSC

Interoffice distance (costs per mile) cost + Fixed Cost

ChannelTerminationCost

ChannelTerminationCost

MSC

- Around 150 BTS per BSC

9/12/2006 17

WR supports multiple BTS (10-20). Selection and distribution is done in the WR. WR collocated with CO

WR supporting multiple BTSs (Star Topology)

WR

WR-BTSlinks

BTS BTS BTS BTS BTS BTS

WR

WR-BTSlinks

BTS BTS BTS BTS BTS BTS

CO CO

9/12/2006 18

Solution: Distributed Control

GRGateway Router

WR

WR

WR

WR

WR

WR

WR

WR

WR

WR

WR

WR

WR

WR

WR

WR

WR

WR

GRGateway Router

Radio framesEmbedded in IP packets

Each packetContains severalradio frames.

Wireless Router is an IP router with RF termination. Functions include BTS, SDU, power control, and handoff control

Gateway Router is a IP router connected to the internet core

9/12/2006 19

WR-WRLinks

WR-WRLinks

WR

WR/BTSWR/BTS

WR/BTSWR/BTS

GRGateway Router

WR

WR/BTSWR/BTS

WR/BTSWR/BTS

GRGateway Router

GR-GR links

WR-GR links

Solution (cont.)

- Radio frame routing using IP routing- Radio neighbors exchange resource info. using IP routing protocols- Mobility is handled through IP signaling protocols- Radio resource management is handled by IP traffic management- WR assumes SDU function

Transit traffic management is handled by IP routing, QoS

9/12/2006 20

Solution (cont.)

– Distributed SDU. Distributed bearer and control– Radio Routing Protocol: IP routing merged with Radio frame

routing for soft handoff and mobility. Wireless extensions of OSPF with radio neighbors.

– Radio Discovery Protocol: Discovering Radio Neighbors– Radio Resource Management: IP traffic management

merged/enhanced with Radio Traffic Management. Radio Power Control integrated and aligned with IP QoS

– RSVP extensions for Radio: IP resource management merged/enhanced with Radio Resource Management. For example, Radio Resource Management and soft hand off signaled with RSVP.

– IP transport in Radio Access NetworkDisruptive Technology

9/12/2006 21

Benefits

• Cost Reduction: Efficient use of backhaul links and aggregation. Only 15% of the BTS-BSC traffic is payload and rest is overhead. Around 30% expenses are backhaul cost. Objective is to reduce the backhaul traffic and small number of high speed backhaul links.

• Scalability: – Separation of call processing and bearer paths– Distributed SDU and Distributed Control– Ability to provide coverage and capacity during peak hours

• Redundancy and Availability: Due to meshed architecture, network is robust and works around the failures.

9/12/2006 22

Benefits (continued)

• Marriage of IP and wireless protocols: Seamless operation of

IP-Network-Layer with Radio Control. • Reuse already deployed routers in the Central Offices.• New wireless services: Automatic Reconfiguration of Radio Access

Network. Expand cell attributes to provide more capacity• Performance: Less propagation delay. Currently, for some of the

BTS-BSC configurations, the links run 100 miles, it means several milliseconds of delay. This creates problem during soft handoff. Due to short lengths between base stations, the delay will be negligible.

9/12/2006 23

WR collocated with BTS (WR)

WR-GRlinks

WR

WR/BTSWR/BTS

WR/BTSWR/BTS

Central Office

WR-WRLinks

GRGateway Router

WR-GRlinks

WR

WR/BTSWR/BTS

WR/BTSWR/BTS

Central Office

WR-WRLinks

GRGateway Router

* WR-GR link (primarily) used for backhauling selected traffic to the destination* WR-WR link (primarily) is used for selection and distribution traffic between two wireless routers. * GR-GR links are links between two IP routers. These routers do not distinguish between wireless and wireline traffic

GR-GR links

9/12/2006 24

WR collocated with BTS with HA (WR-HA)

WR-GRlinks

WR

WR/BTSWR/BTS

WR/BTSWR/BTS

Central Office

WR-WRLinks

GRGateway Router

WR

WR/BTSWR/BTS

WR/BTSWR/BTS

Central Office

WR-WRLinks

GRGateway Router

GR is collocated in the the closest COConnectivity to two (atleast) GRs is established for higher availability. The second GR is collocated in the neighboring Central office.

9/12/2006 25

Traffic Models

scheduler

Overhead /container

Overhead/ stream

Overhead/ stream

voicestream

voicestream

multiplexer

packetizert

multiplexer

packetizert

Overhead/ stream

Overhead/ stream

datastream

datastream

Segment. Segment.

Traffic Mix

• 100% voice + 0% data• 100% data, 14.4K, 64K and 144Kbps• 80 % voice + 20% data, 14.4K, 64K and 144Kbps• 20 % voice + 80% data, 14.4K, 64K and 144Kbps

9/12/2006 26

Cost Comparisons—Urban (30 node network)

0

100000

200000

300000

400000

500000

600000

700000

800000

1 3 5 7 9 11

WR (one per BTS)

WR with HATraditional

Star (10 BTS per WR)

No. of Carriers (increased bandwidth at BTS, 1carrier requires ~~2 Mpbs)

$$$$$

9/12/2006 27

Cost Comparison—Rural (around 10 nodes)

0

100000

200000

300000

400000

500000

600000

1 2 3 4 5 6 7 8 9 10 11 12

WRWRHATradStar

No. of Carriers (increased bandwidth at BTS, 1carrier requires ~~2 Mpbs)

$$$$

9/12/2006 28

Why less cost ??

• Backhaul cost reduced with WR mesh architecture. Customer saves $$$$ (per mile backhaul cost)

• Why is the backhaul much less with WR? What is new with WR?– Frame Selection is done at the WR. No duplicate traffic

after the selection is done– The aggregation of voice and data traffic from multiple

WRs enables better Statistical multiplexing and reduces the backhaul requirement. This also enables customer to use less costly T3 and saves them more $$$$

– Flexible and more reliable traffic routing.

9/12/2006 29

Review and Conclusions

• Statistical multiplexing and compression techniques are not accounted in the results described. If counted, more savings are realized

• Cost savings for one carrier are not much but substantial for multiple carriers.

• Star at the near-by-CO with a high speed (e.g., DS3/OC3) uplink is the optimum but without higher availability.

• Mesh is ideal for higher availability and cost savings

• All the WR cases win over traditional deployment

• Ongoing work: Different kinds of transport cost structures and different types of links (T1/T3/Microwave etc.)

9/12/2006 30

Service Availability

CO CO CO CO

BSC

MSCBTS BTSBTS BTS BTS

BTS WR

WR

WR

WR

WR

WR

WR

WR

WR

- Rerouting around failed links/nodes- Rerouting around congested links/nodes- Wireless router is also IP router hence no need to deploy full mesh- If BTS fails, neighboring resources can be used to complete the calls.

-Single point of failure at BTS- Single point of failure at BSC- No rerouting around congested nodes possible.

9/12/2006 31

Availability Model (example)

Tower

WR

WR

WR

WR

WR

WR

WR

WR

GR

GR

Backplane

Line Card

Line card

ControlProcessor

ControlProcessor

SW

SW

Calculate MTBF and MTTR of all the components in each element

9/12/2006 32

Annual Down Time Comparison

Link Avail .99 .999 .9999 .99999

Traditional (Hours)

174 17.77 2.01 0.438

WR-HA (Minutes)

5.25 5.25 5.25 5.25

BTS Availability: 0.99999

GR Availability: 0.99999

Link availability is varied from 0.99 to 0.99999 and downtime is computedFor traditional and WR-HA network topologies.

9/12/2006 33

Annual Down Time Comparison

Link Avail .99 .999 .9999 .99999

Traditional (Hours)

174 17.77 2.01 0.438

WR-HA (Minutes)

5.85 5.78 5.78 5.78

Assumptions:

BTS Availability: 0.99999

GR Availability: 0.99

Link availability is varied from 0.99 to 0.99999 and downtime is computedFor traditional (2G/3G) and WR-HA network topologies.

9/12/2006 34

RESULTS

• In conventional case, when a link fails, the call level reliability is low because the call is not redirected without dropping. However, in mesh architecture, the call can still be maintained and the rerouting takes place at the frame/packet level.

• Though GR’s availability is only 0.99, the overall service downtime is not impacted due to the fact that there are multiple paths from BTS to another GR.

9/12/2006 35

Review and Conclusions

• Distributed RAN architecture saves backhaul cost (less than half of the cost of existing architecture )

• Distributed RAN architecture supports 99.999% service-level availability compared to conventional network. In fact, full mesh is not required for realizing the 99.999% availability. Even partial mesh can achieve this level of availability

• Distributed architecture increases the complexity of control and requires working prototype for understanding the new protocols.

• Disruptive technology: Required new protocols design and approval from vendor and hence may take long time to get to the field (this is weakness in this architecture).