Communications Essays - Wireless VOIP

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Wireless VOIP

Now a days research in wireless VOIP is branch of a bigger research project that intend to increase the usability of VOIP applications on the wireless networks. If we talk about wireless networks then several wireless standards come in our mind. But in this research I will just focus my research on wireless LAN standard 802.11 because prior researches show that 802.11 standard is not specially designed for Real Time Applications. So the need raised to modify 802.11 with the capability to put up with voice traffic.

A several researches have been published to improve the Wireless LANs based fast handoff schemes but mostly researches were suitable for their own scenarios. However, previous researches shown that Layer 2 handoff is most important part of the handoff. The 90% of total latency belong to Layer 2 (Scanning New AP). Previous researches shown that mostly L2 based handoff schems cannot complete handoff process in 100 ms. However for Real Time Applications like VoIP requires a maximum end to end delay 50 ms. Mobile IP (MIPv4 or MIPv6) is using for mobility of wireless mediums(WiFi, Wimax, Cellular Networks) with no disbelief MIP is a better solution for client roams around. But still this solution is not good for Real Time Applications. Uniqueness of this research is to proposed fast handoff scheme with the improvement of voice quality for Real Time Applications. This thesis consist of L2 and L3 based handoff process which support to improve quality of voice for Real Time Applications. Many factors determine on voice quality just like voice codecs, delay, jitter and packet loss. Proposed solution will be combination of decreasing delay (like scanning delay) and minimization of packet loss (L3 QoS based)

As shown by previous researches one of the existing problem with voice traffic in wireless LANs is handoff or disruption happens when one client moves from one access point to other access point. There are two kinds of Real Time Applications, One way communication (like audio/video streaming) and Two way communication (VOIP applications). Ths research will focus problems for two way communications. If handoff is not fast and seamless or takes too long, call can be dropped or some part of the call could missed because of delay and packet losses. Some fast handoff schemes are suitable for just data files in which duplication of packets or packet losses do not matter. But my criteria to propose fast handoff is decrease the delay with minimization of packet loss.

To complete this project I need to review a lot of literature from different resources. The Part of literature Review is divided in poles apart following in which I will discuses the research relevant topics and cutting edge technologies and skills about my project. I will try to point toward the troubles which I can countenance during complete my project.

The following be the structure I will follow through discussing the theoretical aspects and literature of my project.

  • Back ground of Wireless LANs and VOIP Applications
  • Back ground of related work
  • Importance of QoS for wired and wireless mediums
  • Overview of other Handoff Techniques
  • Proposed Handoff scheme

Critical analysis of the literature will help me to understand how these techniques and technologies are suitable to complete this thesis

1.0 Back ground of Wireless Technologies and VOIP Applications

1.1 Wireless LANs

Wireless LANs is define in the sense when two or more computer equipment users chose to become mobile and they are connected each other without wires. This technology utilizes spread-spectrum or OFDM modulation technology on radio waves to keep communicates between each other. This technology gives users mobility with in a limited coverage area to keep connected to each other.

INTEL ANS 802.11

1.1.1 IEEE 802.11 family (WLAN Standards)

The IEEE 802.11 family is a set or Wireless LANs standards. Although the terms 802.11 and WiFi are often used interchangeably

802.11a - Up to 54 Mb/s in the 5 GHz band, using OFDM3 modulation scheme and WEP4 & WPA5 security;

802.11b - Up to 11 Mb/s in the 2.4 GHz band, using DSSS-CCK6 modulation, and WEP & WPA security;

802.11g - Up to 54 Mb/s in the 2.4 GHz band, using OFDM or DSS with CCK modulation, and WEP & WPA security.

1.2 Voice Over IP

Voice Over IP define as speech data is transformed into digital packets and sent over the Internet and then converted back into analog signals before reaching the phone receiver at the other end. [1] Vice Over IP has the intrinsic ability to decrease the whole total cost of of the network and to improve the overall efficiency of the network. As we know world moving to VOIP and its popularity continued to grow. The big advantage of VoIP is that over traditional telephony system is that voice sent on internet avoids the fixed circuitry of traditional telephony networks, so it saved the toll charges which gives to traditional telephony networks. That's why it gives cheap international calls[2]. On the other hand the biggest disadvantage of Voice Over IP is a (QoS) quality of service. Specially if VoIP migrate to wireless network, because 802.11x gives limited access to implement QoS on network. The new era of the communication is transform the VoIP applications to 802.11 networks to avoids the copper wires that secure residences and small business to serving telecom[1]

Overview H.323 Standard

The H.323 standard is an umbrella recommendation from ITU-T for vendors who develop equipments for voice transmission over internet (VOIP). It provides the mechanism for Real Time Applications over LANs but without guaranteed quality of service (QoS) over network. This standard was designed specially for multimedia conferencing but shortly it become a standard of Voice Over IP (VOIP). First version of this standard was available in 1996 and up gradation in this version came later and still research is on going for the latest version. H.323 is one of the VOIP standard like Session Initiation Protocol (SIP) H.323 suite include several protocols that why it know as an umbrella from ITU-T. Voice communication related important parts are include RAS (registration admission and signalling), RTP (Real Time Transport Protocol) and audio codecs, Theses codec are using in VOIP to maintain voice quality over internet like G.711, G.723.1, G.728, etc

H.323 Components

H.323 standard consist of four components. Terminal, Gateways, Gatekeepers and Multipoint Control Units

Terminal

Role of terminal in H.323 provides real bidirectional two way communications. It is also known as client end points on LAN. It support and audio and video communication support protocols in H.323 suite like H.245, RAS, RTP and also support some optional protocls like T.120 data and MCU and audio video Codecs. Role or terminal in VOIP is highly important and highlighted

Gateway

Role of gateway is actual communicate with two different networks like PSTN or some extra features like call forwarding, call transfer etc. Gateway uses H.245 and H.225 protocol. Gateway can be use for translation between audio and video and data format and Role of gateway is optional in the VOIP network

Gatekeepers

Gatekeepers have strong appearance in the VOIP network. It use for several advance functions which are available in normal PSTN. Gatekeeper actually performs call services and it is a central point of all calls with in its defined zones. Major functions of the gatekeeper is call forwarding, call transfer, bandwidth control, call management, admission control, call authorization and most important zone management

Multipoint Control Unit (MCU)

Role of MCU is establishing multipoint conference. MCU is consist of mandatory Multipoint Control (MC) and optional zero or more Multipoint Processors(MP). One of the important role of MCU is using as a bridge between centralize and decentralized mixed multipoint conferences

H.323 Protocol Stack

Application

Data

T.20

Call Control and signalling

Audio/Video

Registration

H.225 Call signalling

H.245

RTP/RTCP

H.225 RAS

TCP

UDP

Network Layer (IP)

Session Initiation Protocol (SIP)

As we discussed above about H.323 is a one of the VOIP standard but H.323 has some complexities. To overcome these complexities Voice Over IP needs a new standard which can overcome those complexities. In 1999 IETF proposed a new standard to over come problems of H.323 and that new standard called Session Initiation Protocol (SIP). SIP is an application layer protocol using for call setup management. This is a single module protocol and most important thing of SIP can also work with existing Internet Application and also works with the other existing protocols like User Datagram Protocol (UDP), Transport Control Protocol (TCP), Real Time Transport Control Protocol (RTCP), Session Announcement Protocol (SAP), Session Description Protocol (SDP) etc.

SIP architecture consist of two components

.        User Agent (Divided in further two parts)

User Agent Client

User Agent Server

.        SIP Server (SIP components)

Registration Server

Proxy Server

Redirect Server

Six basic SIP methods

INVITE = Invite is used for session creation with the end points ACK = ACK is a acknowledgement to confirm INVITE request BYE = BYE is used for call termination CANCEL = CANCEL is used for cancellation of pending requests REGISTER =RIGISTER is used for registration of user location OPTIONS = OPTION is used to know capabilities of the calling and receiving phones

2.0              Background work for VOIP and 802.11 based Wireless LANs

2.1 Why Voice Over Wireless LANs

We can see that companies have been installing and using WLANs more than a decade, which shows that importance of WLANs in the industry. Now it can be seen WLANs every where like homes, schools, universities, hospitals, airports and business. Importance and rapid growth of WLANs can see in figure 2

On the other hand no body can deny importance and rapid development of Voice over IP applications in the IT industry in last few years. Now we are towards the inside in the new era of technology with the emerging market of Voice over Wireless LANs. After some success and advancement in QoS (quality of service) on Wireless LANs (802.11 based LANs) organizations and companies are encouraging to emerge the Real Time Applications (voice/audio or internet telephony) over the Wireless LANs with the replacement of traditional communication system, which were relatively expensive and costly. [3]

2.1.1 Voice Over WLANs benefits

A significant usage of Voice Over WLANs is emerging of voice and data with the facility of mobility and make use of common infrastructure. The emerging of voice and data is bit simpler and less expensive as compare to support both separate entities. Usage of Voice Over WLANs can avoid the headache of additional wires for data and telephony systems and also off course less expensive to install and support than wired based data or voice network [3]

2.2 Technological Background of Wired/Wireless VOIP

Instead of Voice over WLANs are less expensive as compare to wired base network. It has their own pros and cons as well. The most significant draw back of Voice over WLANs are roam delay between one access point to another access point. IEEE still could not define a standard algorithm to minimize this delay. Cause of handoff delay on Wireless network can tolerate network performance and specially it become more worst when Real Time Applications (VOIP) are running on Wireless LANs. There are several cause of handoff issues to use evaluate performance of wireless network, which are based on jitter, packet loss and buffering.

Voice over WLANs based on totally different architecture as compare to wire based network. Fixed network has no any problem of handoff delay so evaluate the VOIP network performance is quite easier as compare to Voice over Wireless LANs.

2.3 Voice over Wired LANs

Network architecture of Voice over Wired LANs is quite simpler in which all VOIP devices (mobile nodes) are connected with Ethernet switches through routers. VOIP on wired based network works under concept of separate voice and data VLAN segmentations. Cisco based VOIP supported switches are easily manage and programme able. So there are not roaming issue in wired network and VOIP performance is mostly rely on end to end QoS. Which also easy to programme on wired based switches or router.

Voice over Wireless LANs

As we know that architecture of Voice over Wireless LANs is quite different than wired based network. Wireless LANs have limited management. VOIP and Wireless LANs they both have their own technical issues. Major issue for VOIP applications are to maintain voice quality (QoS) over network but on the other side Wireless LANs biggest issue is delay while mobility. Uniqueness of this proposal and research is propose a method which can overcome the both issues

Mobility Issues in Cellular Network

Mobility is one of the foremost difficulties in Cellular and Wireless Networks. Mobility issue raised first in cellular networks like 3GPPP, GSM and UMTS, where each cell managed by based station (BS) and from one cell to another cell and call such cell care connected with BSS (Base Station subsystem) and Network Subsystem. In cellular network mobility issue is try to avoid call termination during mobility of mobile node from one cell to another cell. This process is know in the world of voice communication is handoff

There are two kinds of handoff in cellular networks.

Hard Handoff, When mobile node transfer from one cell to another cell and call drop it is called hard handoff it is also known as break before make because in this case connection to source cell is broken before make connection to target cell. On the other hand

Soft Handoff when mobile node transfers from one cell to another cell with out call dropping known as soft handoff. It is also know as make before make, because in this case connection to next cell is established before the connection to the source is broken

Mobility in Voice Over Wireless LANs

As discussed above mobility is a biggest issue in voice communication over wireless networks. But this issue massively highlighted when voice communication happens over 802.11 based wireless networks. Even if 802.11 based WLANs were planned to support roaming and handoffs under 802.11 a, b and g versions, but it were for just only data supported scenario. Which caused handoff delay too long during Real Time Applications like audio/video streaming and VOIP applications. This situation goes worst when VOIP applications are using on 802.11 based WLANs. Current handoff delays on 802.11 based WLANs are more than hundred of milliseconds. This can cause to Real Time Applications loss of connectivity and poor voice quality. Need of fast handoff raised over 802.11 based WLANs for voice applications with maintain of voice quality

Handoff in Wireless Technologies

As discussed above, handoff is one of the major problems in Wireless Networks (IEEE 802.11 (wifi), 802.16 (wimax) and Cellular Network) especially in 802.11 based Wireless LANs. In 802.11 based Wireless LANs, a mobile node leaving an AP raised a need of handoff process for finding the new AP and establishing a link with new AP. Like previous studies shown authors proposed Layer 2 (MAC Layer) or Layer 3 (Mobile IP) based handoff solutions. This paper will actually focused to improve quality of voice on Wireless LANs mean giving fast mobility solution for VoIP applications.

Once Radio Signal Strength (RSS) decreased and its truly to lose connectivity, Mobile Node begins a handoff procedure. For any successful handoff following described handoff steps included

Challenges Of Handoff Management

As discussed above biggest issue for Wireless LANs and Real Time Applications (like VOIP) are handoff difficulties. The major issue in handoff management is to maintain a tolerable quality of voice during fast handoff. It also includes minimizing the layer 2 and layer 3 based handoff latency and packet drops to avoid degrade quality of service for Real Time Applications. Handoff management also includes endorsing intelligent handoffs to avoid unnecessary handoffs and to make sure that the finest available connection is used at a time of handoff [4]

To measure quality of service (QoS) during handoff targets can be prioritized by bandwidth usage, latency, packet collisions error rates. Several fast handoffs mechanism have been proposed by different authors and these handoff mechanism makes tradeoff between these measures when selecting the handoff target. QoS has become most efficient handoff tradeoff for Real Time Applications. This research paper is basically to propose such a handoff mechanism, which improve QoS for Real Time Applications. As we know that VOIP application cannot afford latency and packet loss.

Handoff issue becomes more highlighted in the unpredictability of the networking environment. Wireless technologies make the environment even more unpredictable. Coverage area of the Access Points dependent on the surrounding areas. Just like walls and other electronic stuffs and other barriers can affect the radio signals of Access Points, which effect rapid decaying of the Access Points signal.

One more issue is broadcasting multipath signals from different Access Points also meet delay because of signals traveling longer paths to the Access Points. Overlapping signals also cause of slow down the demodulation of the signal. So as stated above wireless technologies may suffer stronger with interference of other obstructions like using of same frequency channels or electronic disturbance of radio signals

Wireless network based handoff requirements

After overview above handoff challenges in layer 2 (Link Layer) or Layer 3 (Network Layer), one of the most important study area (especially in the literature review chapter) is network based handoff requirements. Over burden on the wireless network is one of the major challenges. If we see that regular handoffs generate too much traffic on the wireless network which cause extra burden on the wireless network. Less handoffs or intelligent handoffs give more resources to use in wireless network. Unnecessary handoffs especially in the link layer handoff (Layer 2 handoff) caused delay until it is sure that handoff is needed. Delay in handoff also degrades the signal and this cause the effect of QoS which is not acceptable for the Real Time Applications. Another major problem in the wireless network is several Mobile Nodes targeting for handoff on the same Access Point at the same time. Channel Overlapping in the wireless network is another big issue if Mobile Node is moving across the cell edges, this may mess about the other signals of other Mobile Nodes which are communicating the other cells. That means common issue in the wireless network is unbalanced traffic distribution

Handoff Latency

Handoff in Detail

As discussed above handoff is a most significant issue in Wireless LANs (802.11 standards). We have discussed some handoff challenges especially limitations in Wireless LANs. Now in the part of the literature review I would like to give overview of Layer 2 and Layer 3 based handoff also known as Link Layer and Network Layer handoff respectively.

Handoff Basic Algorithm

As per pervious handoff research work [7][8], I came to know that handoff Algorithm is known as relative signal strength with hysteresis and threshold used in[7][9] For example when Mobile Node moves from one Access Point to another Access Point then single to noise ratio (SNR) from first Access Point starts decreasing and on the other side SNR starts increasing so according the [7][9] we give signal to noise ratio values to Access Points at position x by S1(x) and S2(x)

Lucent[9] divided handoff initiation in two factors 1) handoff threshold Th and 2) hysteresis Hh. So it can be describe in this way that Mobile Node at any position x starts move from current Access Point (Ac) to another handoff (Ah) on the basis of following condition

S1(x)<Th

S2(x) - S(x) > Hh

It can be seen in fig (6) Mobile Node triggers Link Layer handoff at position X1 if Th = T1 and X2 if Th = T2. This basic handoff algorithm avoids the unnecessary handoffs if SNR is on a good strength and hysteresis position avoid the ping pong effect [7][8]

Steps involved for successful handoff

According to previous research [7][10] To complete successful handoff process following steps is involved

1)      Discovery Phase

2)      Layer 2 handoff

o       Authentication

o       Reassociation

3)      Layer3 handoff

o       Using care of address (CoA) to update binding information for heterogeneous networks

o       Using Mobile IP or IPv6

Discovery Phase

As discussed above basic handoff algorithm is based on SNR. When Mobile Node starts moving from the range of one Access Point to another Access Point, SNR goes decrease of current Access Point, that time Mobile Node require to register with new Access Point before dropping the connection from old Access Point. It goes on Discover Phase where Mobile Node trying to discover new access points. Discovery Phase also known as Scanning Phase. This process is divided into two methods, Passive Scanning or Active Scanning

Passive Scanning

Passive scanning involves waiting for a broadcasted beacon message, these beacon messages sent by Access Points as a frame regularly towards Mobile Nodes. Fig 7 can clear concept more clearly. By default beacon interval is configured to 100ms. IEEE 802.11 defined 11 channels and Mobile Node has to be scan all channels in Passive Scanning

Active Scanning

It is inverse of Passive Scanning. In active scanning process Mobile Node broadcast Probe Reguest to Access Points and in the reply of Probe Request Access Point send Probe Response to Mobile Node. Fig 8 describes more briefly

Authentication Phase

After successful completion of discovery phase, Mobile Node attempts to authenticate with the discovered Access Point in Scanning Phase. In other it can be say that this phase allows Mobile Node to be part of Network. This part is precondition of association. In [11] IEEE 802.11 requires authentication as soon as Mobile Nodes enters in discovery phase. So now a days mostly vendors implemented their own preauthentication schemes and Inter Access Point Protocol (IAPP) based preauthentication schemes. In Preauthentication scheme Mobile Node authenticate with the Access Point straight away as it finish the scanning phase but on the other hand in IAPP based scheme, it trigger as soon as Mobile Node associate with first discovered Access Point in the Extended Service Set (ESS). Role of this protocol is send the authentication information to all Access Points with in the ESS. When required its already authenticated [11]

Reassociation Phase

Role of this phase starts after completion of authentication. In this phase it establish the communication link with Access Point and also exchange the beacon interval and transmission rates [7]. This phase consist of six step process in [11]

1.      Reassociation Request

2.      Reassociation Response

3.      Handoff Request

4.      Handoff Response

5.      Sending of Frames (It is optional step)

6.      Data Traffic

Handoff in different layers

Most challenging part of the handoff in Wireless LANs, handoff mechanisms are divided in two major network layers.

Link layer (Known as Layer 2 handoff)

Network Layer (Known as Layer 3 handoff)

Still sharing of layers (Link Layer and Network Layer) are common goal of researchers to overcome handoff issue. Just only link layer mechanism cannot chose the best access point without the knowledge of quality of the network layer connection. One more think Link layer mechanism are usually developed in the device deriver or the firmware of the network interface card. Network layer handoff mechanism is specially handle subnets issues and Mobile IP registration process. QoS management is also one of the parts of network layer handoff [4]

Layer 2 or Link Layer Handoff

Layer 2 handoff is basically based on signal strength of reachable Access Points. Key roles of the layer 2 handoff is received signal strength (RSS), signal to interface ratio (SIR) and bit error rate (BER) or bit error rate. In other words layer 2 handoff is based on strong signal level. Handoff occurs in low signal level always increase the packet drops or connection drop during handoff. So during handoff, availability of best RSS is a major task for Layer 2 handoff [4] [5]

Layer 2 Movement Detection

Movement detection is basically based on network related or mobile node. Movement detection occurs when mobile node realize the change of location. For network initiated handoff Mobile Node realized change of location, when Access Points sends the disassociation messages. In other case two ways of movement detection happens. In first Mobile Node realized change of location after a number of failed frames transmissions. Otherwise if Mobile Node does not detect beacon messages, it assumed that Mobile Node has moved.

Searching and Switching New Access Points

When mobile node realized the change of location it starts searching for new available Access Points to associate with. The 802.11 standard define two search mechanisms. Active Scanning and Passive Scanning. [6] Detail of Active Scanning and Passive scanning will describe later in the chapter. When Mobile Node finds an Access Point it begins switching phase. This phase involves authentication and reassociation. [5]

Layer 3 Handoff or Network Layer Handoff

A layer 3 handoff mechanism forms mobility binding of care of address. Layer 3 handoff is quite complex as compare to the Layer 2 handoff, that's mean difficulties and issues are more than Layer 2 handoff. Layer 2 handoff process can be complete without participating of Layer 3. But usage of Layer 3 handoff imitates when handoff required between two different networks or in two different subnets. For example if any organization two different networks are running (like 802.11 wifi and 802.16 wimax) or they are using different subnets e.g one subnet is 192.168.0.x other subnets is 172.168.0.x. In such kind of situation Layer 2 handoff is not sufficient. In other example Layer 3 handoff require when mobility required between 802.11 standards and in between cellular network. Fig [5] is a example of layer 3 network layer hanoff

To choose which network at the time of handoff in network layer is a complex decision. For example excellence of network layer connection to correspondence nodes and to the home agent, handoff mechanism at the time of handoff, duration that the care of address (CoA) can be used, type of user application and utilization of bandwidth is also included

Layer 3 Movement Detection

In layer 3 Mobile Node hears router advertisement. In network layer Mobile Node realize the movement when Mobile Node stops to receiving certain number of router advertisement. After completion of Layer 2 handoff and after established the connection, Mobile Node starts to receive once again router advertisement. Missing router advertisement also indicates some thing set to prohibit causes like collisions [5]

Layer 3 handoff implementation

After completion of Layer 2 handoff and layer 3 based movement detection process, system executes a Duplicate Address Detection (DAD) process to find out the distinctiveness of a local address for a newly assigned link. The new care of address (CoA) assigned on the based information of router advertisement. After assigned the care of address (CoA) Mobile Node can update its Home Agent to keep tracking of Mobile Node current location, this process completed with coordination of binding process. Details of Layer 3 handoff with help of Mobile IP (MIP) will describe later in the chapter [5] [6]

Mobile IP

Mobile is actually Internet Engineering Task Force (IETF) standard protocol. This is designed for especially for mobility between different networks. For example It allows the communication and mobility between GSM network and IEEE 802.11 standard and mixture of other network. It allows transparent routing of IP packets to Mobile Nodes. It consist of Mobile Node (MN) is recognized by its home address (HA) and it is associated with care of address (CoA) to find out its current location. Mobile IP consist of two mobility agents home agent (HA ) and foreign agent (FA) Mobile IP is a best choice for Layer 3 (Link Layer) mobility and handoff.

Why chose Layer 2 (Link Layer)

This paper is not doing any research on Layer 3 (Network Layer) mobility, because Layer 3 handoff is just required where mobility requires between two different networks or mobility requires between different subnets. Implementation of Layer 3 handoff is not suitable for the small business, airports, hotels, cafes. Significant work has been done on Mobile IP

Handoff Delay

Probe Delay

Authentication Delay

Reassociation Delay

References

[1] Frank Ohrtman, 2004 Voice over 802.11 Publisher: Artech House

Telecommunications Library

[2] Shepard, Steven, 2005 Voice Over IP Crash Course Published by McGraw-Hill Osborne

[3] Jim Geier, 2007 Deploying Voice over Wireless LANs Published by Cisco Press

[4] Johni Bask, May 2003 Heterogeneous Handoff Algorithm for Mobile IPv4 without FA

[5] Siksik, M.; Alnuweiri, H.; Zahir, S., 2005 A detailed characterization of the handover process using mobile IPv6 in 802.11 networks submitted to IEEE Pacific Rim Conference

[6] IEEE. Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. IEEE Standard 802.11, 1999.

[7] Minho Shin, Arunesh Mishra, William A. Arbaugh Year of Publication: 2004 Improving the Latency of 802.11 Hand-offs using Neighbor Graphs Source: International Conference On Mobile Systems, Applications And Services. The ACM Digital Library. ISBN:1-58113-793-1

[8] G. P. Pollini, Mar 1996 Trends in Handover Design, IEEE Communications Magazine

[9] Lucent Technologies Inc Dec 1998 Roaming with WaveLAN/IEEE 802.11, Tech. Rep. WaveLan Technical Bulletin 021/A

[10] A. Mishra, M. Shin, and W. A. Arbaugh, Apr 2003 An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process, ACM Computer Communications Review

[11] Li, Chung-Sheng, Tseng, Yung-Chih, Chao, Han-Chieh  April 2007 A Neighbor Caching Mechanism for Handoff in IEEE 802.11 Wireless Networks IEEE Digital Object Identifier: 10.1109/MUE.2007.32, ISBN: 0-7695-2777-9

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