A Global Network Using Wimax Computer Science Essay

Published: Last Edited:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

The IEEE 802.16 standard is initially designed to provide a flexible, cost-effective, standards-based last-mile broadband connectivity to fill in the broadband coverage gaps that are not currently served by the 'wired' solutions such as DSL, the advanced versions of the standard are aiming to create new forms of broadband services both with high speed and mobility.

WiMax is based on the IEEE 802.16 standard and it is a technology that supports the delivery of last-mile wireless broadband access as an alternative to cable and DSL, regarding reference [1]. WiMax will provide fixed, nomadic, portable and ultimately mobile wireless broadband connectivity without the need for line-of-sight with a base station. The design of a WiMax network is based on the following major principles:

Spectrum: to be deployed in both licensed and unlicensed spectra.

Topology: supports different RAN topologies.

Interworking: autonomous RAN architecture that enables faultless incorporation and interworking with WiFi, 3GPP and 3GPP2 networks and existing IP core networks (e.g. DSL, cable, 3G) using IP-based interfaces.

Mobility management: opportunity to expand the fixed access to mobility and broadband multimedia services delivery.

WiMax has defined two MAC classification profiles -the ATM and the IP. They have also defined two primary PHY system profiles:

IEEE 802.16 standard is designed to develop as a set of air interfaces standards for WMAN based on a universal MAC protocol but using the physical layer specifications, which dependent on the range of use and the related regulations. The IEEE 802.16 working group designed a flexible MAC layer and associated physical layer for 10-66 GHz.

Citing reference [2], the first use case is based on fixed IEEE 802.16 equipment. Independent of the used version, the technology can be employed in a fixed infrastructure as shown in Fig. 1. In this setting, the two potential cases are the deployment of point-to-point connections that can span tens of kilometres. The deployment of local loop replacements to ADSL, where the receiver is positioned inside each house or inside customer sites using omni or angular antennae is the second scenario - Fig. 2.

The second use of the standard provides mobility scheme for the cellular like data transmission based on its Mobile version - Mobile WiMax. The standard offers the physical and the MAC layer only to maintain the handover. It differentiates from the 3G family of standards that provide an entire architecture solution- including management of the subscribers and the whole network, while Mobile WiMax stops at layer 2. As a result from that, the handover is done between neighbouring base stations and is not provided for interdomain operation.

WiMax Network Model

In the beginning of development, WiMax is targeted to rural areas particularly when local loops are not available. Another positive side of this technology is that it is financially much more effective and efficient. Usually, the base stations are located in a mid point easy to connect to the Internet and the outside world by satellite links or any other connections; those base stations are linked to remote trans-receivers called CPEs. The CPEs are normally located on top of the roofs and there should be a second technology that focuses the traffic from the whole building to that CPE. Using the CPEs, there is no need of any other technology to interconnect the final user to the WiMax network. The usage of this technology is regulated, the power has to be in the range of few watts and frequency is usually located in the 3.5 GHz band. IEEE802.16 focuses its protocols on QoS and it is a genuine competitor to the IEEE 802.11 standard, despite the fact that WiMax is much more costly for deployment than the standard Wi-Fi.

Citing reference [5], WiMax is developing through regularity procedures; it is likely to extend the existing capabilities of broadband networks. As a result it is inevitable not to mention the cost and charges for such a network from financial and economic point of view:

Fixed cost of providing a network infrastructure.

Non-fixed cost of connection to the network- typically paid by the user in the form of connection cost.

Cost of increasing the network's capacity. Users who want to reschedule their transmission during peak times should not be charged for the growth of the network's capacity.

Incremental cost of sending an extra packet. This cost should be very small or equal to zero without congestion, in view of the fact that the bandwidth of a broadband network is in general a shared resource.

Social cost defined as the extra delay which occurs to other users by the transmission of data.

[2] As the WiMax belongs to the IEEE 802 group, then the bridging or layer-2 concepts should be mentioned. The addressing is based on MAC addresses and the base station is perceived as a bridge. In order to implement all layer 2 functionalities, the device used should be a bridge-not a router. For the identification for nodes addressing is used; as long as the node is recognized by the network, the address is replaced by use of circuits with circuit identifier.

The layers are divided into two main ones - MAC and physical layer. Several sub-layers are defined in the Mac layer -the first one is called the service-specific and is the variation of IEEE 802.16 to the available packet types: ATM, Ethernet, and IP (it is universal in the available products).

The MAC layer is based on connection-oriented principle and it is very similar to the ATM transport protocol, which connection uses a context that describes the mapping between the incoming flows and the underlying QoS. A station registers itself to the base station, negotiates the physical layer characteristics and then can communicate bidirectionally. A service flow defines the negotiated QoS for all matching packets (service-specific sub layer). The QoS can be changed dynamically and it supports extremely well data bursts. Everything is negotiated separately for uplink and downlink.

MAC Layer

The MAC layer specifies the operation in point to multipoint and mesh. Point to multipoint is the standard operation, which means that a base station is sending data to other stations (CPEs). Dissimilarity between point to multipoint and mesh operation exists in the usage of a different factor to distinguish neighbours in the mesh scenario. This is called link ID and is 8 bits wide, whereas a station or a base station identifies itself by the MAC address (48 bits unique ID). The MAC ID is used at the beginning for identification and authentication, but afterwards it is replaced by a connection ID (16 bits wide).

Scheduling and QoS

QoS is an important part of the standard. An IEEE 802.16 connection is associated to a specific QoS. Management messages dynamic service addition and dynamic service change are used for that reason. Four classes of services are supported:

Joining an IEEE 802.16 network and initialization

WiMax is very complex and before any successful connection, many parameters need to be settled. Network entry and initialization are the two most frequent terms used for registering and initialization of a new station. It is defined for a PMP and mesh, with some exceptional added extras for the mobile version.

The physical layer is a inheritance of several developments that are called: SC, SCa, OFDM, orthogonal frequency division multiple access and Mobility addendum. It operates in large band of 40 GHz, which directs many possibilities. The scheduling of connections is described in time, frequency and space domains. This means that it uses time division duplex to share uplink and downlink, but also frequency division duplex, OFDMA and directive antennae that restrict their beams to a specific end point. The space domain is also managed through multi-antenna techniques.

The physical layer is very complex because it can simply combine all the mechanisms mentioned above into one transmission event. It is also based on the MAC signalling layer, the MAP messages.

OFDM physical layer

The OFDM is used for low-frequency operation (below 11GHz) in a non-line-of-sight scenario. It is based on orthogonal symbols and uses inverse Fourier transform. A symbol time structure is shown in Fig. 7- the concatenation of a useful symbol time Tb for a given data to be sent and the last Tg, which is the period used to collect multipath measurements to maintain orthogonality between signals. This collection period is used by the transmitter and is kept always fixed. The role of the receiver at the beginning is to guess the duration among a set of values and once it is found, the receiver is said to be synchronized in the time domain.



Fig. 7 Symbol time structure for OFDM

The OFDM symbol is based on subscribers in the frequency domain. The number of subscribers is based on the Fourier matrix size. There are three kinds of subscribers:

Data subscribers

Pilot subscribers used for measurements

Null subscribers to keep bands and synchronize

The IEEE 802.16 OFDM is described by four primitives:

The bandwidth of the channel is multiple of 2.0, 2.75, 1.75 or 1.5 MHz

The number of sub channels is equal to 200 and FFT size is 256

The sampling factor that gives the distance between sub channels and hence the duration of a symbol, equal to 8/7 in general

The ratio between the CP and the useful time, ¼, 1/8, 1/16 or 1/32


This is the last physical layer specified in the IEEE 802.16, which is based on OFDM operating in the lower bands below 11 GHz and in non-line-of-sight conditions or mobile conditions.

OFDMA is similar to OFDM taking into account the symbols and procedures. Subscribers are grouped together, and each group is or may be allocated to an exact connection. This means that a group of subscribers called a sub channel may be allocated in the downlink to a receiver and in uplink it could be reserved to a given station.

The primitives explained in the OFDMA physical layer are the same:


Number of subscribers, FFT is 2048 instead of 256

Sampling factor

G ratio for the CP (ratio of CP time to useful time)

The Effect of Handovers

According to reference [6] handover or handoff occurs when the quality of the channel that is used degrades, and therefore the call is switched to a newly allocated channel. If the new channel belongs to the same base station, then this is called an intra-cell handover; respectively, if the new channel belongs to a different base station, it is called an inter-cell handover. Usually intra-cell handovers occur when the channel quality degrades due to interference or when the channel allocation algorithm decides that a channel reallocation will help increase the system's performance and capacity. Inter-cell handovers happen primarily because the mobile device moves outside the cell area; for this reason, the signal strength corrupts, requiring a handover to a closer base station.

Handovers have a significant effect on the performance of channel allocation algorithms. At high traffic loads, the majority of forced call terminations are as a result of the lack of channels available for handover rather than to interference. This is a major problem in microcellular systems, where the rate of handovers is considerably higher than that in normal cellular systems.

There are a number of solutions to reduce the performance penalty caused by handovers. One of them is to reserve some channels solely for handovers, generally referred to as cut-off priority or guard channel schemes. However, this solution reduces the maximum amount of carried traffic or system capacity and hence yields increased new call blocking.

Algorithms that give higher priority to requests for handovers than to new calls are called Handover prioritization schemes. Guard channel schemes are therefore a type of handover prioritization arrangement. Another type of handover prioritization is constituted by handover queuing schemes. Normally, when an allocation request for handoff is rejected, the call is forcibly terminated. By allowing handover allocation requests to be queued temporarily, the forced termination probability can be reduced. The simplest handover queuing schemes use a First-In First-Out (FIFO) queuing regime. A non-pre-emptive priority handover queuing scheme in which handover requests in the queue that are the most urgent ones are served first.

A further alternative to help reduce the probability of handover failure is to allow allocation requests for new calls to be queued. New call allocation requests can be queued more readily than handovers because they are less sensitive to delay. Handover queuing reduces the forced termination probability owing to handover failures but increase the new call blocking probability. New call queuing reduces the new call blocking probability and also increases the carried teletraffic. This is because the new calls are not immediately blocked but queued, and in most cases they receive an allocation later.

Hard Handover

According to reference [7], during hard handover the MS communicates with only just one BS in each time. Connection with the old BS is broken before the new connection is established. Handover is executed after the signal strength from neighbour's cell is exceeding the signal strength from the current cell. This situation is shown in Fig.8. The thick line at the boarder of the cells presents the place where the hard handover is realized.

Fig. 8 Hard handover realization

Macro Diversity Handover

When MDHO is supported by MS and by BS, the "Diversity Set" is maintained by MS and BS. Diversity set is a list of the BS's, which are involved in the handover procedure. Diversity set is defined for each of MS's in network. MS communicates with all BS's in the diversity set (Fig. 9). For downlink in MDHO, two or more BS's transmit data to MS such that diversity combining can be performed at the MS. For uplink in MDHO, MS transmission is received by multiple BS's where selection diversity of the received information is performed. The BS, which can receive communication among MS's and other BS's, but the level of signal strength is not sufficient is noted as "Neighbour BS"

Fig. 9 Macro Diversity Handover

Fast Base Station Switching

In FBSS, the MS and BS diversity set is maintained similar as in MDHO. MS continuously monitors the base stations in the diversity set and defines an "Anchor BS". Anchor BS is only one base station of the diversity set that MS communicates with for all uplink and downlink traffic including management messages (Fig. 10). This is the BS where MS is registered, synchronized, performs ranging and there is monitored downlink channel for control information. The anchor BS can be changed from frame to frame depending on BS selection method, which means each frame can be sent via different BS in diversity set.

Fig. 10 Fast Base Station Switching



Design simulation of a network using WiMax and understand the principles behind the WiMax systems

Since this WiMax network simulation design is used for academic purposes, the size of the network will be based on up to 10 base stations and will focus on the services listed below. Perhaps, one kind of such network topology may be used for the purposes of university campus

Analysis of the Handovers occurring while using mobile WiMax

Analysis of the performance of the network and the related services that are provided

Service Class


QOS Specifications

Unsolicited Grant Service


-Jitter tolerance

-Maximum latency tolerance

-Maximum sustained rate

Real-time Packet Services

Streaming Audio/Video

-Traffic priority

-Maximum latency tolerance

-Maximum reserved rate

-Maximum sustained rate

Extended real time Packet Services

VoIP (VoIP with Activity Detection)

-Traffic priority

-Jitter tolerance

-Maximum latency tolerance

-Maximum reserved rate

-Maximum sustained rate

Non-real time Packet Services


-Traffic priority

-Maximum reserved rate

-Maximum sustained rate

Best Effort

Data transfer, web browsing

-Traffic priority

-Maximum sustained rate

Table 1. WiMax Quality of Service Classes [8]


For the successful completion of the project, the following software will be used: Opnet IT Guru [9]. IT Guru automates analysis and planning of multi-technology, multi-vendor enterprise networks, supporting organizations to accurately plan for growth, change, technology migration, and new application deployment. It supports key initiatives such as:

Unified communications including VoIP, IP telephony, multimedia conferencing, and tele-presence

VPNs and firewalls

Data centre migration and consolidation

Disaster recovery and business contingency planning

Transitioning to IPv6

Exclusive OPNET algorithms automatically optimize the network, in terms of capacity and configuration, to meet traffic growth projections, and achieve performance and survivability.

Enterprise network planners and operations engineers obtain significant investment from IT Guru's ability to rapidly roll out new applications, meet performance and availability requirements, and maximize existing network investments.


Literature survey was done concentrating the topic on how the system has been developed and improved in the last couple of years

What are the practical usages of those systems

How it is implemented in the mobile industry

What is the future of the WiMax systems, either fixed or mobile