Survivability of a Integrated Fiber-Wireless (FiWi) Access Networks

Uilla Rathore Bhatt* , Tapesh Sarsodia, Raksha Upadhyay Department of Electronics &Telecommunication Engineering, Institute of Engineering & Technology,

Devi Ahilya University, Indore -452017, India 'Corresponding author- umarathore@rediffmail.com

Abstract - In past few years there is a huge development in the field of FiWi technology due to continuously increase in the

number of users of the internet. FiWi technology now a day's

proving itself a main tool in the field of telecommunication by its

own merits over the other existing technologies. The FiWi

technology provides huge amount of bandwidth and stability to

the network user in a flexible manner i.e. network provide

"Anywhere Anytime" connection service to the user at very high

speed. The existing technology viz PON offers higher bandwidth

and stability but at relatively higher cost due to the use of

expensive optical devices. On the other hand WMN technology

offers us communication among the users with a better flexibility

at relatively lower cost but its bandwidth is limited due to channel

interference. So, the efforts are put to combine the merits of both

the technologies resulting in a new technology named as "FiWi".

Various algorithms are proposed to enhance the performance of a

FiWi access network. In this paper, a detailed discussion is made

on survivability issues in FiWi networks against the different level

of failures in the network. This paper is basically a review paper

in which the main aim is to emphasize on the recent trends of

survivability in the FiWi networks.

Index Terms- Fiber-Wireless, Network Survivability, PON,

WMN.

Abbreviations

FiWi Wi-Fi PON OLT CO ONU RN WM

TDM WDM P2P

ACRONYMS

Full meaning

Fiber-wireless Wireless fidelity Passive optical networks

Optical line terminal Central Office Optical network units Remote node Wireless mesh network

Time Divison Multiplexing Wavelength Divison Multiplexing Peer to Peer Communication

I. INTRODUCTION

In past decade there is a huge growth in the field of broadband access technologies which enables users to access internet in a flexible manner. The broadband access networks basically requires higher bandwidth, better flexibility and lower cost. The technologies which are used for accessing the

internet are mainly classified as optical access networks and wireless networks. The PON gives higher bandwidth capacity and stability for accessing the internet but it fails to provide service to user in "Anytime Anywhere "manner and it also requires higher cost due to costly optical devices. On the other hand the most popular wireless access networks such as WIFI or WiMax provides services at lower cost and also serves better flexibility to the user. However, interference and low bandwidth limits its reliability. So, on integrating the technical merits of both the technologies a FiWi[ I]-[3] broadband access network was proposed to enable user to access internet at lower cost, with higher bandwidth and also providing better speed as we are approaching towards 4G communication level.

Since FiWi supports a huge data rate (typically of the order of Gbps), any kind of failure may results in huge data loss. Hence, survivability is one of the key issues in the FiWi network, because it is defined as the ability of the network to continue its services after a failure in the network. So it is essential to make a network more survivable in a such a manner that it protects huge loss of data after a failure. The failure in the optical network side i.e. at the back end of a FiWi network is more severe than the failure in front end because wireless front end has the property of self healing due its mesh topology which helps it to reroute its traffic on a failure but optical back end has a tree topology which has more probability of getting failured. The failure in the FiWi networks are categorized mainly in two parts: One is ONU level failure and another is segment level failure [4]. In ONU level failure which is caused due to failure of one of the distribution fiber cable in a segment which disconnects the corresponding ONU from the OLT as shown in Fig l(a). On the other hand, segment level failure is caused due to the failure of feeder fiber cable which disconnects the whole segment from the OLT as shown in Fig l(b). Energy Saving is also a related term to the survivability [4]. We can save energy of our network by putting some of the ONUs in the sleeping mode and transferring there traffic to the nearby ONUs as they are not utilizing their maximum capacity i.e. they are at very low load or no load conditions. Various algorithms are developed for energy saving issues in the FiWi technology but they are not of concern for this paper.

978-1-4799-2900-9/14/$31.00 ©2014 IEEE 238

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Fig. I (a) ONU Level Failure [5]

Fig. l (b) Segment Level Failure [5]

The rest of the paper is arranged as follows: Section 2 presents the background of the FiWi access networks and its related terminologies. In section 3, related literature works on the suvivabilty of a FiWi networks are discussed. Section 4 focus on the conclusion.

II. BACKGROUND

1) Optical Access Network

As PON provides larger bandwidth capacity, lower

transmission loss and better tolerance to interference than any

other existing wired technology such as Digital Subscriber

Line(DSL) and Cable Modem(CM).Basically PON has tree

topology, in which OLT in CO connected with multiple ONUs

via RN, feeder fiber and distribution fiber as shown in

Fig.2

ONUs RN

OLT I� User1

0 User 2

User 3

Feedcrfiber DiBlributioofiber Fig. 2. PON Network

On the basis of various technologies used in PON network the different methods available for PON are TDM-PON, WDM­PON and Hybrid TDM/WDM-PON [4]. In TDM-PON, for upstream (ONU to OLT) and downstream (OLT to ONU) channels a separate wavelength was assigned. It supports point-to-multipoint(P2MP) communication which means all �NU's shares same wavelength channel as per TDM method. Whereas in WDM-PON each ONU's has its own upstream channel and downstream channel over the same fiber. Hence it supports point to point(P2P) communication and gives larger bandwidth capacity then TDM-PON. Apart from the above two methods, one more method is used in PON networks which is hybrid version of TDM-PON & WDM-PON named

as hybrid TDM/WDM-PON. The hybrid TDM/WDM-PON [1] was achived in four different ways : one is TDM assigned for upstream services and WDM for downstream services. Another method is vice versa of the above. In next multiple TDM-PONs of distributed fiber side shares same feeder fiber through WDM technology and another one is vice versa of these method.

Therefore PON provides larger bandwidth capacity and better stability but at higher cost due to the use of costly optical devices.

2) Wireless Access Network

Wireless access network gains attention due to its low cost,

easy deployment and flexible access. There are three major

technologies which are available in present world, supporting

wireless networks are WIFI, WiMax and Cellular Technology.

(a) WIFI - It enables users to connect to the internet via

wireless access points in a limited range and also helps to

communicate between two different users communicating in

local area network. It will be operated in infrastructure mode

and adhoc mode. In infrastructure mode, there is access point

(AP) through which users will communicate. In adhoc mode

users can communicate directly to each other for a short period

of time like P2P.

The existing WIFI standards which are approved by IEEE

Standards are shown below:

Standard Speed Freq band • 802.11 2 Mbps 2.4 GHz • 802.11a 54 Mbps 5 GHz • 802.llb 11 Mbps 2.4 GHz • 802.llg 54 Mbps 2.4 GHz

(b) WiMax - WiMax (Worldwide Interoperability for

Microwave Access) is a standards-based technologywhich

helps users to access internet in a better way then cables and

DSL. WiMax supports fixed and mobile wireless broadband

services without the direct line-of-sight (LOS) by the help of

base stations which are located in different geographical areas.

Mobile network deployments are expected to provide up to 15

Mbps of capacity within a typical cell radius deployment of up

to three kilometers.

2014 International Conference on Issues and Challenges in Intelligent Computing Techniques (ICICT) 239

WiMax Standards are shown below:­

� IEEE 802.16 o Single carrier PHY layer and TDM MAC layer

� IEEE 802.16a o Include NLOS application o OFDM PHY layer and OFDMA MAC layer

� IEEE 802.16d o"Fixed WiMax" oCombine previous versions

� IEEE 802.16e o "Mobile WiMax" o Add mobility support o Scalable OFDM PHY layer and Scalable

OFDMA MAC layer

(c) Cellular Technology - It is widely used in mobile

communication, in which a base station in each cell is used to

communicate between two different users and also support

voice and low rate data application. The advancement in

cellular mobile technology is termed as "Generations" and

presently four generations are revealed till date and they are

described below in shortly:

• I G (first generation) - voice-oriented systems based

on analog technology; ex.: Advanced Mobile Phone

Systems (AMPS) and cordless systems.

• 2G (second generation) - voice-oriented systems

based on digital technology; more efficient and used

fewer spectrums than IG; ex.: Global System for

Mobile (GSM) and US Time Division Multiple

Access.

• 3G (third generation) - high-speed voice-oriented

systems integrated with data services; ex.: General

Packet Radio Service (GPRS), Code Division

Multiple Access (CDMA).

• 4G (fourth generation) - still experimental, not

deployed yet; based on Internet protocol networks

and will provide voice, data and multimedia service

to subscribers.

3) Fiber- Wireless Access Network

FiWi technology [4] was proposed after combining the merits of optical and wireless access technologies i.e. larger bandwidth, high stability and flexibility at relatively higher data rates. Hence FiWi provides different network services to the user in an "Anywhere Anytime manner".

o IC> "" .t .& OLT wireless splitter ONU user

rouler end

Fig. 3 FIWI architecture including two segments [6]

The architecture proposed for a FiWi network as shown in fig.3 has a combination of tree and mesh topologies. The architecture was further classified as front end and back end. Front end consist of a wireless network which has a mesh topology on the other hand the back end consist of passive network which was implemented through a tree topology. The whole network was divided into various segments, each consisting of pairs of ONU' s and wireless routers connected to the users locating in different geographical areas. The ONU is a key device in the network because it act as a interpreter between front end and back end. Its function is to convert optical signals into wireless signals and vice-versa. A wireless gateway is connected with each ONU through a wire, which helps to scatter wireless signals in the channel. The users which are located in different areas can access internet through different pairs of wireless routers and ONU's. Now If somone wants to access the internet, initially he has to send his data to its corresponding wireless router, and then further router transmits it to its primary ONU following a multi hop wireless path via wireless gateway. After reaching to one of the ONU's the data was then forwaded to OLT through the fiber cable and then this data is injected into the internet back bone. Then internet gives service to the user in the reverse fashion as stated ahead.

Merits of a FIWI networks are [4]:

(1) FIWI provides larger bandwidth, better stability, reliability and flexibility as compare to WMN and PON networks which helps user to access channel at higher data rates with low blocking rate.

(2) If one of the ONU's fails suddenly then the traffic will be rerouted to other ONU in the same segment via wireless multi hop paths and if one of the segment loses its contact with the OLT then its traffic will be rerouted to other segment in the network through a pair of backup ONU's interconnecting different segments through a back up fiber. Hence due to the

240 2014 International Conference on Issues and Challenges in Intelligent Computing Techniques (ICICT)

fault management capability of a FiWi network we got a more suvivavble network as compare to WMN and PON networks.

III. LITERATURE REVIEW

As PON is the backbone of the FiWi network and also it has a tree topology (which is less reliable), hence survivabilty is the mandatory issue for a passive optical network. Therefore various algorithms was proposed for making a PON network more survivable which are discussed below. The author [7] evaluated two schemes for protection against failure in WDM optical Networks. The two schemes are link protection and path protection. The path protection was further divided into two categories: 1) Dedicate path protection 2) Shared path protection. For making more survivable multi-domain optical networks the author [8] proposed an algorithm DDP (Differentiated Domain protection) which supports survivability within domain and between two domains and was implemented in three schemes viz. Dedicated protection, Shared protection and No protection using VGT (Virtual Graph Topology) method and then the author revised its literarture work in LBDDR[9] (Load Balanced Domain-by-Domain Routing). As DDP algorithm, which is based on virtual topology fails due to two problems. One is that there is a lot of signals which are to be updated time to time by means of costly equipments which increases overall cost and on the another hand if the periodical update is not up to time then the inter-domain routing will not be efficient. In LBDDR, for finding efficient inter-domain survivable routes, the domain-by-domain routing (DDR) method is proposed which can fmd the intra-domain sub­working path and sub-backup path in each single-domain to form the inter-domain working path and backup path for each demand in the network, also for reducing the blocking probability, the author proposed load balanced routing method which helps to distribute the traffic uniformly on the links with more free wavelengths.

The problem of delay among different successive packets was occurring frequently in the network so for reducing it the author [10] explained an algorithm delay-aware routing algorithm (DARA) that minimizes the average packet delay in the wireless front-end of a WOBAN. In DARA they modeled wireless routers as M/MIl queues and predict wireless link states periodically and by the help of this predictions the packet delay will be minimized. Results shows that DARA achieves less delay and congestion, and improved load balancing compared to traditional approaches.The author further carried their work to design a network which reduces packet delay as well as having capability of predicting the different risks occurring in the network for protecting data loss against them. So for doing such, author proposed RADAR[ll] ("Risk-And-Delay Aware Routing algorithm") algorithm for WOBAN. RADAR minimizes packet delay in the wireless front end of WOBAN and reduces packet loss for multiple failure scenarios: gateway failure, ONU failure, and OLT failure.

Taiming Feng and Lu Ruan [12] proposed an algorithm for protection of WOBAN which is more cost effective then RADAR algorithm by making some new assumptions. In this algorithm a new solution for MPMC (Maximum Protection with Minimum Cost Problem) was proposed by the help of ILP (Integrated Linear Programming) model and heuristic algorithm. In this algorithm if a segment fails the traffic of that segment will be rerouted to the neighbour segment ONU through backup fiber among randomly selected ONUs .

Y. Liu et. al. proposed an OBOF (Optimizing Back-up ONUs selection and back fibers deployment) algorithm [6] which is better then RADAR. The algorithm was implemented in two different steps viz. Backup ONUs selection and Backup fibers deployment. For selecting backup ONUs SA algorithm (Simulated Annealing) and for selecting back up fiber deployment EGCE (Enhanced Greedy Cost Efficiency) algorithm was proposed. The EGCE algorithm was further classified as RBS (Remote Backup Segment) method which efficiently utilize the residual capacity of the segments and the other is BLB (Bound on Length Backup-Optical-Path) method which limits the increase in recovery time induced by RBS. Simulation results proved that the algorithm is much better than other algorithms for the higher traffic demand networks. For making more cost effective network then OBOF the author extends his work with new concept of graph theory and abbreviated as AGP (Auxiliary Graph Based Protection) algorithm [5] for selecting Backup ONUs and reducing backup fiber deploymen cost. The algorithm concerning mainly about Maximum Protection and Minimum Hops Number (MPMHN) problem and Maximum Protection and Minimum Backup Fibers Length (MPMFL) problems to improve the network results and minimum cost.

The algorithms proposed so far are suitable only for single feeder fiber cable and if multiple segment fails simultaneously then above algorithms will fail to survive. Therefore for multiple failure the author [13] proposed RPMF](Ring based Protection considering Multiple Failures) algorithm and also demonstrated the way in which traffic can be transferred to others if the interconnecting backup fibers fails as well.

In order to improve RPMF algorithm, the author of [14] proposed an algorithm for protection against both the level of failures occurring in a FiWi networks i.e. OLT level failure and ONU level failure. For ONU level failure, SBR(Sharing Backup Radios) algorithm is proposed in which each ONU will connect to its partner ONU with a radio backup to reroute the traffic on a failure at wireless end i.e. it provides a wireless-backup-path with minimum hope number. On the other hand for OLT level failure, SPR(Shortest Protection Ring) algorithm is evaluated which will be applied in two steps (1) GA(Genetic Algorithm) to cluster all segments in the network and (2) Backtracking method to find the shortest path in each cluster to deploy backup fibers with minimum cost among the backup ONUs in each cluster. Table 1 summarizes the various algorithms which are discussed so far, for easy understanding of their merits and limitations in comparison with other available algorithms.

20J4 Internationai Conference on Issues and Challenges in Intelligent Computing Techniques (ICICT) 241

Algorithms

Link protection and path

protection

DDP

LBDDR

DARA

RADAR

MPMC problem solved

by ILP model and

Heuristic algorithm

OBOF

AGP

RPMF

SBR and SPR

TABLE I Comparison of various algorithm for survivability in a FiWi Network

Characteristics

Moderate

Good

Improved over DDP

Better

Extension of DARA

Better than RADAR

Advantage

Efficient in case of single domain

WDM network.

Suitable for multi-domain failure

Lower blocking probability.

Better load balance.

Less delay and congestion.

Improved load balancing

Improve average packet delay. Reduce packet loss in multiple

failure scenarios.

Communication between routers is

possible through backup fibers.

Cost efficient and reduce running

time.

Limitation Ref.

Not suitable for multi-domain failure... [7]

Virtual topology increases overall cost [8]

because periodical up gradation is

compulsory which includes additional

devices.

Not suitable in case of multi-link failure in [9] a single domain.

Not suitable for multiple failure scenario. [10]

Assumption made that algorithm is self [II] protecti ng.

Not possible to reroute if wireless router are

at long distance.

Ignore optimizing the selection of backup [12]

ONU which affects overall cost.

Better than above Optimally selection of backup Not suitable in case of simultaneous failure

algorithm ONUs. of segments. [6]

Reduces backup fiber deployment

Improved over OBOF

Advancement to

OBOF and AGP

Improved over RPMF

IV. CONCLUSION

cost.

Better performance in higher traffic

demand.

Cost efficient as compare to OBOF. Not valid for multiple segment failure.

Cost efficient.

Suitable for multiple failures.

More cost efficient than RPMF.

[5]

[13]

[14]

In this paper, a detailed discussion is made on the different broadband access technologies along with FiWi technology. Since FiWi network carries huge amount of data over the network at relatively high speed, so protection against failure is a mandatory issue to make a network more survivable. As survivability is a main issue in a FIWI network, a detailed discussion was made in this paper on various algorithms available for making a network more survivable. It is examined from the literature survey that the best survivability scheme available till now for a FiWi network is RPMF (Ring

based Protection considering Multiple Failure) which supports multiple failures in a network. It is noticed from the literature survey, that various topologies are used for interconnecting different segments with each other for transferring data at the time of feeder fiber failure. Most of them uses only single fiber connection between OLT and a remote node but a topology shown in fig.4, proposes a tree topology with a redundant trunk (which is not implemented till now) which uses an extra fiber cable between OL T and remote node.

242 2014 International Conference on Issues and Challenges in Intelligent Computing Techniques (ICICT)

ONU2

OLT

ONUS

Fig. 4 Tree with redundant topology [18]

Although the topology uses an extra fiber, which increases the overall network cost but it will make a more reliable network and will increases survivability of an OLT level failure.

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