Load And Throughput Quality Of Service Communications Essay

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WiMAX is a standards-based technology facilitating the provision of last mile wireless broadband access as an alternate to wired broadband similar to cable and DSL. WiMAX offers static, traveling, handy and, soon, mobile wireless broadband connectivity with no requirement for straight line-of-sight with a base station. For fixed and portable access applications, WiMAX systems can be anticipated to carry capacity of equal to 40 Mega bits per second for each channel, in a usual cell radius deployment of 3 to 10 Km. This is sufficient bandwidth to concurrently maintain 100 of businesses with T-1 speed connectivity and 1000 of homes with DSL speed connectivity. Mobile network deployments are estimated to give equal to 15 Mega bits per seconnd of capacity in a usual cell radius deployment of equal to 3 Km.

WiMAX is intended to become a "Next Big Thing" because regulatory agencies and standards groups around the globe accept this standard and it is built on open standards.

1.2 PROBLEM STATEMENT

The problem with WiMax is to provide thebetter Quality of service equivalent to that of the wired technology. Though the WiMax has built-in quality of service architecture but in actual deployment it does not provide the promised quality of service to end users over longer distances.

1.3 OBJECTIVES

The main objectives of this thesis are to give basic understanding of this emerging technology by describing its quality of service mechanisms that are used at the PHY and MAC layers, optimizing quality of service parameters (delay, load and throughput), which will help in improving the broadband WiMAX services.

1.4 METHODOLOGY

  • Detailed study of the related topics and literature.
  • Designing the different simulation based practical model scenarios of a WiMax network to simulate different QoS parameters (latency, jitter, delay, throughput etc) that may affect the QoS in wireless networks.
  • Model based solution will be proposed which will address these QoS parameters and the obtained results will optimize the performance of the network derived from different designed simulation models.

1.5 ORGANIZATION of the THESIS

This thesis consists of six chapters. Chapter 2 contains the detailed introduction of the standard IEEE 802.16 (WiMax), the different versions of the standard IEEE 802.16, its operation modes, and the protocol architecture of 802.16. Chapter 3 describes the usage models (fixed and portable), the features and the usage scenarios. The WiMAX standard addresses a broad variety of applications and usage scenarios which are assembled into 2 large categories, private networks and public networks. Chapter 4 describes the QoS mechanisms precise to 802.16. It explains how QoS is provisioned in 802.16. It also equates the QoS abilities of 802.16 with the rival technologies. Chapter 5 discusses the WiMAX network simulation scenarios developed in OPNET 14.5 simulation tool. Chapter 6 concludes the results obtained.

CHAPTER 2

LITERATURE REVIEW and INTRODUCTION to WIMAX (IEEE 802.16)

2.1 LITERATURE REVIEW

2.2 INTRODUCTION

The IEEE 802.16 Working Group on Broadband Wireless Access Standards was initiated in a meeting held by the National Institute of Standards and Technology (NIST). The working group has formed new standards for broadband wireless metropolitan area networks. The 1st edition of the IEEE 802.16 standard is 802.16-2001 agreed in December 2001 and in print in April 2002.

The IEEE 802.16 standard and its amendments describe the air interface specifications together with a common MAC layer and physical layers (PHY) dealing with the Data Link layer and Physical layer of Open Systems Interconnection (OSI) model [4]. The standards cover frequency ranges of 2-66 GHz, both licensed and license-exempt bands. The channel access method supports both Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD).

At the beginning, the intend of IEEE 802.16 media access control layer was to support PMP broadband wireless access (BWA) system. Later Mesh mode is added to support operating in mesh networks. It is anticipated to address high bit rate communications with various services. The IEEE Std 802.16 [5] defines three MAC sub-layers.

Service-specific Convergence Sub-layer (CS)

The Service-specific CS receives exterior network data via the convergence sub-layer SAP, transforms the data to media access control SDUs, and sends the media access control SDUs to the media access control service access point. The categorization of upper-layer protocol data units (PDUs) and sending those PDUs to the proper media access control service access point is the its key task. The standard specifies convergence sub-layer for ATM and packet-based protocols. The supported packet-based protocols are IEEE 802.3 standard/Ethernet, IEEE standard 802.1Q-1998 virtual local area network (VLAN), and Internet Protocol (IP), both IPv4 and IPv6[5].

MAC Common Part Sub-layer (CPS)

The MAC CPS receives MAC SDUs through the MAC SAP from the Service-specific CS. It provides method for access to the system, allocation of bandwidth, establishment and maintenance of connection. Currently, it supports PMP and Mesh modes[5].

Security Sub-layer

The authentication, secure key exchange, and encryption is provided by an encapsulation protocol and a key management protocol contained in Security Sub-layer. The PHY defines several specifications for different frequency ranges and applications[5].

2.3 BENEFITS

The following are some benefits given by the IEEE Std 802.16 and its amendments.

Large coverage

Compared to prior wireless technology, the IEEE Std 802.16 provides much larger area of coverage. In line-of-sight (LOS) environment, the coverage distance is up to 50 kilometers. In NLOS environments, a base station covers the radius of up to 8 kilometers[6]. The larger coverage needs few base stations, shorter installation time, and less cost.

High data capacity

It delivers up to 70 Mb/s in a single radio frequency (RF) channel. (This is theoretically maximum value under ideal circumstances. Practically it delivers less.) By using Orthogonal Frequency Division Multiplexing (OFDM) technique, the standard increases data capacity. Using multiplexing technique a broad channel is divided into several narrower channels with different frequencies. Therefore, we can simultaneously send several messages via a broad channel[6].

NLOS environment support

The IEEE Std 802.16a works in 2 to11 GHz frequency bands. Its low frequency and three new PHY specifications facilitate the IEEE Std 802.16a to support working in NLOS environments. The required PHY functionalities to operate under NLOS conditions are the advanced power management techniques, interference mitigation/coexistence, and multiple antennas[6].

Flexible channel sizes

The IEEE Std 802.16 allows an operator to use flexible channel sizes. It optimizes the use of spectrum. For example, an operator pays for 14 MHz spectrum and wishes to use all of granted spectrum. If the system can be deployed only with 6 MHz channels, they attain 2 channels and waste 2 MHz of spectrum. What they wish for is a system which can work on 7 MHz, 3.5 MHz, or 1.75 MHz channels[2].

Mesh mode

The IEEE Std 802.16a-2003 provides full mesh networking capability with Mesh mode. In Mesh mode, a packet can be routed not only among the base station (BS) and subscriber stations (SSs) but also among two SSs. The Mesh mode operates with OFDM PHY layer and TDD.

Mobile user support

The IEEE Std 802.16e-2005 specifies a standard for mobile broadband wireless access at vehicle speeds.

Table 2.1 shows comparison of the IEEE 802.16 and other wireless technology standards.

Table 2.1: Comparison of wireless technologies

Characteristic

IEEE Std 802.15 (Bluetooth)

IEEE Std 802.11

(Wi-Fi)

IEEE Std 802.16

(WiMAX)

Application

WPAN

WLAN

WMAN

Maximum

Coverage area

100 m (Class 1)

10 m (Class 2)

1 m (Class 3)

105 m (Outdoors)

45 m (Office)

50 km (LOS)

8 km (NLOS)

Peak data rate

1 Mbps (Version 1.2)

3 Mps (Version 2 with

Enhanced data rate)

11 Mbps (802.11b)

54 Mbps (802.11a/g)

70 Mbps

Frequency band

2.4 - 2.485 GHz

2.4 GHz (802.11b/g)

5 GHz (802.11a)

2 - 66 GHz

2.4 APPLICATIONS

The IEEE 802.16 working group does not aim to replace the IEEE Std 802.11 with the IEEE Std 802.16, rather the IEEE Std 802.16 serves as a complement of the IEEE Std 802.11. The IEEE Std 802.16 is planned for WMANs, whereas the IEEE Std 802.11 is planned for WLANs. Therefore, the IEEE Std 802.16 enables users to access into wireless network on a larger scale. Because of its better coverage, faster speed, and larger number of supported users per base station, the IEEE Std 802.16 tends to be used extensively as backhaul and last-mile solutions for Wi-Fi networks. A wireless network consisting Wi-Fi access points could use the IEEE Std 802.16 solution as a backhaul to deliver signal to the core network. As a last-mile solution, an internet service provider (ISP) could install the IEEE 802.16 technology, instead of cables, DSL, and T1, to deliver services to subscribers. It gives advantages over the traditional wired technology in case that cabling is not feasible or too expensive, for example, implementing in rural areas or developing countries where lack wired network infrastructures[12].Figure 2.1 shows an example of the IEEE 802.16 network.

Two usage models of the IEEE Std 802.16. The IEEE Std 802.16 base station provides wireless access services to fixed and portable subscribers. The fixed subscribers could be residential subscribers or businesses in buildings. For residential subscribers, the standard provides broadband wireless access as a last-mile solution, in place of using cable modem. The IEEE 802.16 technology can be a substitute to DSL or leased line services for small and medium size businesses, as well as a substitute to T1/E1 level services for enterprises. Users in the buildings can join to the network via Ethernet/IEEE 802.3 or WLAN/IEEE 802.11 standards. For mobile users, the IEEE Std 802.16e which is an amendment to IEEE Std 802.16-2004 adds the capability to provide services. The users can directly attach to the network using devices, such as personal digital assistants (PDAs), smart phones, and mobile laptops.

2.5 OVERVIEW of WiMAX

The IEEE 802.16 standard is commercially known as WiMAX, which is an acronym for Worldwide Interoperability for Microwave Access. WiMAX is a wireless metropolitan-area network technology that provides interoperable broadband wireless connectivity to fixed, portable and nomadic users. It covers up to 50- kilometers of service area, permit users to obtain broadband connectivity with no need of direct line-of-sight to the base station, and provides total data rates up to 75 Mbps, enough bandwidth to simultaneously support hundreds of businesses and homes with a single base station[2].

WiMAX covers a broad spectrum of bandwidth from a few hundred megahertz to tens of GHz. This encompasses far more spectrum than any earlier standard. WiMAX is intended to become a "Next Big Thing" analogous to the impact of the Internet because WiMAX is built on open standards and is extensively accepted by regulatory agencies and standards groups around the globe. This universal level of approval means that products can be prepared that will work wherever comparable service is provided.

It has been developing by the WiMAX Forum working groups. The WiMAX Forum established in June 2001 and is a nonprofit organization. The intend of the organization, similar to Wi-Fi Alliance, is to make sure the compatibility and interoperability of products conforming to the IEEE Std 802.16 and European Telecommunications Standards Institute (ETSI) HiperMAN standards. WiMAX plays a role as a complement of the IEEE Std 802.16. The IEEE 802.16 working group defines the standard for working, whereas the WiMAX Forum coordinates companies to make sure that equipment from each company can well operate with one another.

The WiMAX Forum creates system profiles and defines test suite for conformance and interoperability. “WiMAX Forum CERTIFIEDâ„¢” text and logo will be used by the equipment that passes the test and will be certified. The trademarks are shown in Figure 2.2. The certification is not provided to components but to the equipment. The test is done by WiMAX laboratory. The first official certification laboratory chosen by the WiMAX Forum is Cetecom in Malaga, Spain[2].

However, if equipment conforms to the 802.16 and ETSI HiperMAN standards, it might not achieve WiMAX certification. To pass the test programme, it does not need only compliance with the standard but also interoperability with equipment from other companies. There are at present more than 300 companies joining the WiMAX Forum from different industries, such as network equipment manufacturers, service providers, and chip manufacturers.

2.6 VERSIONS of the IEEE 802.16 STANDARD

The IEEE 802.16 Working Group has been adopting a number of projects to enlarge the IEEE Std 802.16 functionality. This section provides the general idea of the IEEE 802.16 standard and its amendments as of January 2006[21].

2.6.1 IEEE Std 802.16-2001

The IEEE 802.16-2001 standard is first edition of the IEEE 802.16 standard. It was accepted on 23 rd December 2001 and in print in April 2002. MAC and PHY are defined by the standard for fixed PMP BWA systems. The MAC structure support multiple PHY specifications. A system can provide multiple services for its subscribers through PMP connections[21]. It operates in the 10-66 GHz bands. Because of its high frequencies, the standard can function only in LOS environments to decrease multipath distortion.

2.6.2 IEEE Std 802.16c-2002

The first revision to the IEEE Std 802.16-2001standard is IEEE standard 802.16c-2002. The revision specifies the complete system profiles for working in 10-66 GHz. It standardizes the wireless technology in more details. Namely, it defines collections of aspects and functions used in execution [21]. These defined sets boost the consistency and interoperability between equipment from different companies.

2.6.3 IEEE standard 802.16a-2003

The second revision for the IEEE standard 802.16-2001 is IEEE standard 802.16a-2003. It supports working at the 2-11 GHz frequencies, both licensed and license-exempt bands. The standard improves PMP MAC and defines new PHY specifications. By MAC functionality, the standard introduces a number of features, such as a specific quality of service (QoS) to optimize data, video, and voice services, Automatic Retransmission Request (ARQ) to get better end-to-end performance. The MAC in addition defines optional Mesh mode which enables every node to directly connect to others through multipoint-to-multipoint air interface and as a result extends the coverage of communication. Because low frequencies have capability to penetrate barriers, the IEEE 802.16a standard can function in near LOS and NLOS environments enabled by the new PHY functionality[21].

2.6.4 IEEE Std 802.16-2004

The revision and consolidation of the IEEE standard 802.16-2001, the IEEE standard 802.16a-2003, and the IEEE standard 802.16c-2002 is IEEE standard 802.16-2004. It is intended for both licensed and un-licensed frequencies. In 10 to 66 GHz bands, it requires LOS environment, whereas, in frequencies below 11 GHz, it provides the facility to support NLOS environment. The MAC supports both PMP and Mesh modes[21].

2.6.5 IEEE standard 802.16f-2005

The revision of the IEEE standard 802.16-2004 is IEEE standard 802.16f-2005. It defines a management information base (MIB) for the media access control, PHY, and management procedures to make standard for network management of the IEEE 802.16 networks.

2.6.6 IEEE standard 802.16e-2005

The revision of the IEEE standard 802.16-2004 is IEEE standard 802.16e-2005. The extension defines the standard for mobile subscribers moving at vehicular speeds. It provides handover function among BSs. Although mobile functionalities are added into the amendment, fixed SSs can function with the IEEE Std 802.16e-2005 BSs. The band used in the amendment is limited to below 6 GHz licensed bands[21].

2.7 OPERATION MODES

The two modes of operation supported by IEEE 802.16 are, Point-to-Multipoint and Mesh mode.

2.7.1 PMP mode

Figure 2.3 shows a network working in PMP mode. An IEEE 802.16 link is handled through a BS and a sectorized antenna. Multiple sectors can be handled simultaneously. The traffic in PMP mode occurs only among the BS and the SSs. The signal could be sent to or from the BS. Downlink is the direction from the base station to the SSs. Conversely, the direction from a subscriber station to the base station is uplink. In a FDD scheme, uplink and downlink signal transmissions are simultaneous. In a TDD scheme, transmission time is divided into uplink and downlink periods. The downlink is usually broadcast. The SSs which have connections to an antenna sector by means of a given frequency channel receive the same signal transmission[12]. The uplink bandwidth to the BS is shared by the SSs. Depending on the class of service utilized, the IEEE Std 802.16 defines the bandwidth allocation and request mechanisms.

2.7.2 Mesh mode

The Mesh mode has been introduced in IEEE 802.16 in addition to PMP. It allows the IEEE 802.16 nodes to create mesh networks. In mesh network a communicating node connects to other nodes. If each node in a network directly connects to every other node, it is called a “fully meshed network”. Otherwise, it is a “partial meshed network”[11]. The mesh topology is shown in Figure 2.4.

Figure 2.5 shows a network working in Mesh mode. The traffic in Mesh mode can happen from a SS to another SS or other SSs. The Mesh mode allows signal to be routed through SSs, whereas the PMP mode allows signal to be routed only from the BS or to the BS. That means a node is not only a SS, but also a BS[11]. When a message is sent from the source, if a message does not arrive at its destination, the node forwards it to another neighbor. Selecting the next node depends on every node's routing table.

2.8 SUMMARY of IEEE 802.16

Table 2.2: Summary of IEEE 802.16 Standards

Standard

802.16

802.16-2004

802.16e

Completed

End 2001

802.16a: Jan 2003

802.16-2004: End 2004

Early 2006?

Spectrum

10 to 66 GHz

2.5 - 11 GHz

2.5 - 6 GHz

Channel

Line-of-sight

Non line-of sight

Non line-of-sight

Bit Rate

32 to134 Mbps

up to 75 Mbps

up to 15 Mbps

Modulation

QPSK,

16 QAM,

64 QAM

OFDM256, OFDMA, 64 QAM, 16 QAM, QPSK, BPSK

OFDM 256, OFDMA, 64 QAM, 16 QAM, QPSK, BPSK

Mobility

Fixed

Fixed and portable

Mobility, regional roaming

Channel BW

20, 25 or 28 MHz

1.25 to 20 MHz with up to16 logical sub-channels cchannels

1.25 to 20 MHz with up to

16 logical sub- channels

Typical Cell

Radius

2 to 5 Km

5 to 8 Km(max 50 Km)

2 to 5 Km

2.9 PROTOCOL LAYERS

802.16 IEEE standard consists of a protocol stack with well-defined interfaces. It works on two layers Common Medium Access Control Layer (MAC) of Data Link Layer and Physical Layer[5]. MAC layer consists of three sub-layers. Service Specific Convergence Sub-layer (MAC CS), the MAC Common Part Sub-layer (MAC CPS) and the privacy sub-layer.

The Service Specific Convergence Sub-layer is the sub-layer that permits CS to communicate with higher layers and converts higher-level data services to MAC layer service flows and connections. There are two types of CS sub-layer: ATM Convergence Sub-layer which is used for ATM networks and services, and packet Convergence Sub-layer which is used for packet services like Ethernet, Point to Point protocol (PPP), IPv4, IPv6, and virtual local area network (VLAN).

The MAC Common Part Sub-layer (MAC CPS) is the heart of the standard. This sub-layer defines the rules for connection management, bandwidth allocation and mechanism for system access. Also uplink scheduling, bandwidth request and grant, connection control, and automatic repeat request (ARQ) functions are defined. In the MAC layer the communication among the CS and the MAC CPS is done by MAC service Access Point (MAC SAP). Communication is very easy as only four basic actions can be used: creation of connection, modification of connection, deletion of connection and transport of data over the connection.

The sub-layer among MAC CPS and Physical Layer (PHY layer) is Privacy Sub-layer. It is the sub-layer that is accountable for the encryption and decryption of data that is incoming and leaving the PHY layer and is also used for authentication and secure key exchange.

PHY layer is the layer which is mainly adaptable to the requirements of the protocol. This means that the flexibility of the protocol permits the designers to make changes to it concerning modulation, an example is the addition of the 2-11 GHz band and the OFDM modulation in the 802.16a-2003 standard[5]. The PHY layer also supports different forward error corrections (FEC) like Reed-Solomon GF (256) with changeable block size and error correction capabilities and Block Turbo Codes. Layering of 802.16 protocol is shown in the figure 2.6.

From a security perspective the most significant layers in the MAC sub-layer are the MAC Common Part Sub-layer and Privacy Sub-layer. IEEE when refers to “the MAC” generally means the MAC CPS so from this point when MAC is referred, it implies the MAC CPS [3].

2.9.1 MAC layer

The nucleus of the standard is MAC sub-layer. It functions in the similar way to IEEE 802.11. There are several subscriber stations and one Base Station (BS). The BS is seen as the Access Points (AP's) in 802.11, while the both standards are entirely dissimilar in the way that they use the airwaves. MAC is designed to support point to multipoint technology[24]. On the contrary to 802.11 CSMA/CA scheme, 802.16 make use of UL and DL maps to guarantee crash free access. To share uplink, subscriber stations use Time Division Multiple Access (TDMA), whereas base station make use of TDM. UL and DL schedules are swapped in each frame by means of UL-MAP and DL-MAP messages[3][24].

2.9.1.1 MAC connections

IEEE 802.16 media access control is connection oriented. Each service is mapped to a connection, and each connection is identified through 16-bit connection identifier and might need constantly contracted bandwidth on demand [9][24]. Media access control layer links can be seen in a way similar to TCP links. Similar to TCP links, where a computer might have at the same time lots of separate active links in diverse ports, in MAC links the subscriber station might have several links to a base station for diverse services such as management of network or transport of user data (The management messages are carried by management links and where as other traffic, normally user data is carried by transport links. Though the main distinction is that in MAC links, each link might have dissimilar parameters for bandwidth, security and priority. Each link is recognized via its connection identifier that is allotted via the base station[24]. As links are unidirectional, so bi-directional link uses two CIDs. Three CIDs, When a subscriber station joins the network, are given to it each with different QoS necessities used by different management levels: Basic, Primary Management and Secondary Management links[24].

Basic link transmits brief, time-critical MAC and radio link control (RLC) messages. Primary Management link transports longer, more latency liberal messages like registration requests and privacy and key management messages- messages that are used for authentication and connection setup[24]. Secondary Management link will permit a particular protocol being run at a higher layer to transport standards-based management messages, for instance (DHCP), (TFTP), and Simple Network Management Protocol (SNMP). Additional connections are served by MAC for other reasons such as to transmit the UL and DL transmission schedules. A connection identifier might carry traffic for several different upper-layer sessions[24].

2.9.1.2 MAC message format

Base station MAC and subscriber station MAC swap messages known as Protocol Data Units. The messages comprise of three parts: the fixed length MAC header, the changeable-length Payload and the Cyclic Redundancy Code. There are two types of MAC header. The first is the Generic MAC header (GMH) used to transport about all the standard MAC Management messages. The second is the Bandwidth Request Header (BRH), it is sent alone, with no payload. Excepting Bandwidth Request PDUs (with no payload) MAC PDUs might hold either MAC management messages or convergence sub-layer data- MAC Service Data Unit (MSDU). Payload is optional also. CRC is optional too and is used only when SS request for it in its QoS parameters[24]. MSDUs are fragmented and packed within MPDUs which are then transmitted on air.

Transport CID in GMH Payload

GMH

MSDU ( may be packed, fragmented,

Carry an ARQ feedback payload, or

any combination therefore)

CRC

Transport CID

in GMH Payload

GMH

MAC management message

CRC

BRH

2.9.1.3 Generic MAC header format

Figure 2.8 shows the format of the GMH. HT stands for Header Type, this is used to specify whether the header is generic or bandwidth request. For generic header HT bit is set to zero. Encryption control (EC) bit specifies whether the payload is encrypted or not. If set to zero specifies that payload is not encrypted and set to one means payload is encrypted. For generic header it is set to one. The information regarding which management message is saved in the payload is contained in Type field. CRC indicator (CI) shows the existence of optional CRC at the ending of MPDU. CI=0 indicates no CRC is appended, CI=1 indicates CRC is appended to the PDU. Encryption key sequence field ensures that BS and SS are synchronized while making use of traffic encryption keys and Initializing Vectors (IV). This field is only significant when EC bit is set to1. 11 bit LEN field is used to specify the length in bytes of MAC PDU comprising header and CRC. Connection identifier (CID) 16 bit field is used to specify which connection the MPDU is servicing. Header check sequence (HCS) is an 8 bit field used to identify errors in the header field.

HT=0(1)

EC (1)

Type (6)

Rsv (1)

CI (1)

EKS

(2)

Rsv (1)

LEN

MSB (3)

LEN (LSB) (8)

CID (MSB) (8)

CID (LSB) (8)

HCS (8)

2.9.1.4 Bandwidth request PDU

A 6-Byte bandwidth request header is transmitted to ask for the changes for granted characteristics of a connection. Format of BRH is shown in figure 2.9. The HT bit is set to 1 to show that the header is a bandwidth request. The EC bit is set to 0. The 6-bit type field can acquire value 0 to show an incremental bandwidth request or a value of 1 to show a collective request. The bandwidth request (BR) field shows the number of uplink bytes of bandwidth being requested. The CID field shows the connection for which the bandwidth request is being made. The HCS field is used to identify the errors in the header field (first 5 bytes).

HT=1

EC=0

TYPE (6)

BR(MSB) (8)

BR(LSB) (8)

CID (MSB) (8)

CID (LSB) (8)

HCS (8)

2.9.2 Privacy sub-layer

The entire security of 802.16 lies in the privacy sub-layer. It provides access control and confidentiality of the data link. Following components are involved in the security of 802.16: Security Associations (SA), X.509 certificates, Privacy Key Management authorization protocol (PKM authorization), Privacy and Key Management (PKM).

Security Associations (SA)

Itmaintains the security state of every connection. 802.16 uses two SA, Data SA and Authorization SA. The data SA protects the communication among SSs and BS. When a new transport connection is created, SS begins a data SA with a create connectionrequest. Multiple CIDs may be served by a single data SA. When the SS joins the network, automatically a SA is assigned to it for the secondary management channel. Then each SS has either one SA for uplink and downlink transport connections together, or one SA for uplink transport connections and one for downlink transport connections. Also if there is a multicast group it requires a SA to share among group members.

The authorization SA is shared among a BS and a SS. The Authorization Key (AK) should be treated by BS and SS as a secret. BS uses the authorization SAs to configure the data SAs on the SS.

The X.509 Certificates

The X.509 Certificates are used to recognize the communicating parties. It comprises of the following fields:

  • X.509 certificate format version
  • Certificate serial number.
  • Certificate issuer's signature algorithm Public Key.
  • Cryptography Standard 1-that is, RSA encryption with SHA1 hashing.
  • Certificate issuer.
  • Certificate validity period.
  • Certificate subject or certificate holder's identity, (station's MAC address).
  • Subject's public key or certificate holder's public key.
  • Signature algorithm identifier
  • Issuer's signature

The standard operates with two certificate types: manufacturer certificates and SS certificates. The manufacturer of an 802.16 device is identified by Manufacturer certificates. It could either be issued by a third party or be self-signed. Single SS is identified by SS certificates and subject field contains MAC address of the SS. SS certificates are usually issued and signed by manufacturers. BS verifies the SS certificate by using manufacturer's public key.

The PKM Authorization Protocol

This protocol makes the BS to recognize SS. There are three steps in authorization protocol: two messages are sent from SS to BS and after that one message is sent from BS to SS.

  • Step 1: A message is sent by SS to BS, that includes a X.509 certificate recognizing SS's manufacturer. BS uses this message so as to make a decision if the particular SS is a trusted device.
  • Step 2: A second message is sent by SS without waiting for an answer from BS. The SS's X.509 certificate and its public key, the SSs security capabilities and its SAID (unique SA identifier) are contained in this second message. BS make use of X.509 certificates to know if the SS is authorized, and to reconstruct the replying message BS make use of SS's public key.
  • Step 3: Third message is sent by BS when if it determines that SS is authorized, this message initiates a SA among BS and SS. An authorization key (AK) is sent to SS that is encrypted with the public key of the SS. SS then gains the authorization to access the WMAN channel if AK is used properly. 802.16 design supposes that only BS and SS share the AK-AK must never be disclosed to another entity.

Privacy and Key Management Protocol

This protocol establishes a data SA among BS and SS. This is accomplished, via two or three messages sent between BS and SS.

  • Step 1: This step is non-compulsory and this message is sent only if BS wishes to force rekeying of a data SA or to make a new SA. BS after computing HMAC (1) (hash function-based message authentication code) permits SS to recognize falsification.
  • Step 2: SS requests SA parameters by sending a message to BS. SS must make use of the SAID from the authorization protocol SAID list. SS sends a separate second message for each data SA. SS computes HMAC (2) too to permit BS to recognize falsification. BS can strongly authenticate SS by HMAC (2).
  • Step 3: BS sends message 3 when if it finds that HMAC (2) is valid and SAID is certainly one of the SS's SA. Message 3 includes the old TEK (temporal encryption key) that is used to reiterate the active SA, the new TEK is used after the expiration of the existing TEK.

2.9.3 Physical layer

Initially IEEE 802.16 standard supported lots of physical medium interfaces. In 2001 only single carrier modulation was supported by the standard 802.16 but now Orthogonal Frequency Division Multiplexing (OFDM) is also supported, which is a multi-carrier modulation scheme. There are two OFDM-based modes in 802.16 standard: OFDM and OFDMA. Depending on distance and noise, these technologies permit sub-carriers to be modulated (QPSK, 16-QAM, 64-QAM) adaptively. The scalability options in OFDMA offer higher efficiency in bandwidth utilization. Also in the begging standard only used 10-66 GHz bands, but now 2 to11 Gega Hertz and 10 to 66 Gega Hertz bands are used also. This has to do with the modularity of the standard, that permits modifications to happen in the Physical layer with very few variations to the rest of the protocol [24].

In 10 to 66 Gega Hertz 802.16 PHY layer specification, line-of-sight transmission was an essential. Single-carrier modulation was chosen for that reason. The air interface in that case, is known as “Wireless MAN-SC”. Base station sends out TDM signal and SSs assigned with a time slot serially. Uplink although, is carried by TDMA [24]. Afterward, another profile was adopted, based on the demand for burst design. This one permits both TDD, in which channel is shared by uplink and downlink but transmission is not simultaneous, also FDD, where different channels are used for uplink and downlink, sometimes simultaneously.

The 2 to11 Gega Hertz 802.16 PHY layer specification addresses both the licensed and un-licensed bands. The requirement of None Line of Sight operation motivated this design. The two air interface specifications up to now are [10] Wireless MAN-OFDM which make use of OFDM with 256-point transform. It is accessed through TDMA. This is compulsory interface for un-licensed bands [24]. Wireless MAN-OFDMA which make use of OFDMA with a 2048-point transform. In this system, to offer multiple access, a subset of the various carriers to individual receivers is addressed.

2.9.3.1 TDD frame format

On air transmission time is divided into frames by 802.16 PHY. Every frame is of a fixed length, and is first divided into the Downlink Sub-frame and after that the Uplink Sub-frame. Two types of transmission duplexing are there: Time Division Duplexing and Frequency Division Duplexing. In TDD, the entire DL sub-frame is sent by BS, that starts with downlink-MAP and uplink-MAP which explain the contents of the downlink and the uplink[24] (DL-MAP and UL-MAP are the directories of slot locations)

Preamble

DL- MAP

UL- MAP

Traffic

Burst

Scheme

a

Traffic

Burst

Scheme

B

Traffic

Burst

Scheme

C

Tx/Rx Gap

Ranging contention

Contention

BW Request contention

Contention

SS a Uplink

Burst

SS b Uplink

Burst

SS c Uplink

Burst

Empty

Tx/Rx Gap

Downlink Sub-frame Adaptive Uplink Sub-frame

Figure. 2.10: TDD frame format [10]

During the downlink, BS informs SS when it is planned to move transmission burst scheme. During UL, every SS is informed by the schedule that when it will be permitted exclusive use of the transmission spectrum[24]. In FDD, while DL-MAP and UL-MAP still schedule the transmissions, UL and DL transmissions occur simultaneously, on dissimilar frequencies.

Downlink Sub-frame

In downlink sub-frame, first there is a preamble. The function of this is to facilitate the SS to synchronize to the frame, because it may have lost synchronization towards the end of the preceding frame. Following this preamble there is DL-MAP and UL-MAP. The access to the downlink information is defined by the DL-MAP. It tells every SS about timings when changes in modulation and coding will take place. The subscriber station listens to all the data it is able to until it reaches data for itself. The access to the uplink channel is defined by the UL-MAP. In this, a burst profile and a time for every SS are specified to transmit their data. The BS transmits its TDM data following this DL-MAP and UL-MAP. The robustness of the transmitted data is decreased in terms of the burst profile. The reason for this is so that a SS does not drop synchronization.

Uplink Sub-frame

In Uplink sub-frame first there are ranging contention opportunities. This is to permit any SS, which have not previously registered to send a ranging request message. The function of this message for a SS, which is not registered, is to determine network delay and to discuss a burst profile or power change. Since this is the first communication this SS is having with the BS, there are no pre-defined uplink opportunities. All SS's, with no such uplink opportunities, transmit their range request in this initial ranging contention opportunity time. There is BW Request Contention following this ranging contention opportunity, which is used to send BW request message, and following this the SSs transmit their data through TDMA, in the time slot given to them by the UL-MAP.

2.10 ESTABLISHING a CONNECTION

Upon installation, in order to detect an operating channel, SS begins to scan its frequency. After detecting the channel SS may be programmed to register with a certain BS it wants to join; SS tries to synchronize to the downlink transmission. Then SS will wait for the periodically broadcast downlink channel descriptor (DCD) and uplink channel descriptor (UCD) messages to learn the modulation and the FEC schemes used on the carrier.

After learning what parameters it must use for initial ranging transmission, SS will scan the UL-MAP to get an opportunity to perform the ranging since it is very essential to tell SS about when to send the ranging, if various SS would try to join the network at the same time, that would have an extremely bad effect on the network efficiency. SS will attempt to send the request (RNG-REQ) with minimum power, and if it doesn't find any reply, then will try with increasingly higher transmission power. BS replies to SS (RNG-RSP) with a timing advance and a power adjustment. Furthermore BS uses this message to tell SS about the Basic and the Primary Management CIDs [10].

After determining the timing advances of the SS transmissions, SS and BS keep on exchanging RNG-REQ and RNG-RSP until a satisfactory radio link is established. The basic system capabilities can be negotiated by using the management messages over the management connections.

The next step to enter the network is SS authentication and Registration. PKM protocol is used for this reason. Certificates and RSA public key methods are used by PKM to authenticate SS to a BS. Once the SS is successfully authenticated, it will register with the network. After that it will set up the secondary Management connection and it will be assigned to its Secondary Management CID. To receive standard based secondary messages for different services this CID then is used for the SS. Dynamic Host Configuration Protocol (DHCP) is one of those. Through DHCP then, SS is attained with an IP address and also is given the address of the Trivial File Transfer Protocol (TFTP) server from which SS can ask for a configuration file. The network time of day through Internet Time Protocol is the last part of the SS initialization.

CHAPTER 3

WiMAX as a METRO-ACCESS DEPLOYMENT & its POTENTIAL FEATURES

3.1 USAGE MODELS

WiMAX being a global certification addresses interoperability across IEEE 802.16 standards-based products. The two usage models addressed by the IEEE 802.16 standard with precise revisions are:

  • Fixed
  • Portable

3.1.1 Fixed

The IEEE standard 802.16-2004 (that modifies and reinstates IEEE 802.16a and 802.16REVd versions) is intended for static-access usage models. This standard might be known as “fixed wireless” since at the subscriber's site it uses a mounted antenna. Alike to a satellite TV dish, the antenna is set up on a roof or mast. The indoor installation is also addressed by the IEEE 802.16-2004, but it is not as durable in as outdoor installations. The wireless solution for static broadband Internet access is the standard 802.16-2004, providing an interoperable, last mile carrier-class solution. For static access, the Intel WiMAX solution works in licensed 2.5-GHz, 3.5-GHz and un-licensed 5.8-GHz bands. The cable modem, digital subscriber lines of any type (xDSL), transmit/exchange (Tx/Ex) circuits and optical carrier level (OC-x) circuits [11] can be substituted with this wireless technology [25].

3.1.2 Portable

The standard IEEE 802.16e is a modification to the 802.16- 2004 base specification. The mobile market with adding up portability and the facility for mobile users with IEEE 802.16e adapters to get connected directly to the WiMAX network, is its target. OFDMA is used by the 802.16e standard, that is analogous to OFDM in that carriers are divided into many sub-carriers. OFDMA, though, is a step advance where many sub-carriers are grouped into sub-channels. The entire sub-channels contained in carrier space may be used by a single user or SS for transmission, or several users may transmit with every one making use of a fraction of the total number of sub-channels at the same time. The standard IEEE 802.16-2004 makes better last-mile delivery in numerous key features:

  • Multi-path interference
  • Delay spread
  • Robustness

The multi-path interference and delay spread enhance performance in situations When there is no straight LOS path among the BS and the SS. The MAC management messages contained in 802.16 specification permit the BS to inquire the SS, but a definite amount of time delay is involved there. TDD is used by WiMAX products functioning in un-licensed bands, products functioning in licensed bands will use either TDD or FDD [25].

3.2 RANGE and SCALABILITY

The standard 802.16-2004 is dependent on a grant-request access protocol that doesn't permit data collisions, as is in contention-based access used in 802.11 and, consequently, the available bandwidth is utilized more effectively. The BS synchronizes entire communication [25]. Further characteristics of the standard 802.16-2004 comprise:

Improved user connectivity

The 802.16-2004 standard maintains more users connected due to its flexible channel widths and adaptive modulation. Since it uses channels narrower instead of fixed 20-MHz channels used in 802.11, lower data-rate subscribers can be served by the 802.16-2004 standard without misusing bandwidth. When subscribers come across noisy conditions or low signal strength, they might be dropped but the adaptive modulation scheme keeps them connected.

Higher quality of service

This standard also facilitates WISPs to guarantee QoS for customers who need it and to tailor service levels to meet different customer requirements. For example, the 802.16-2004 standard can promise high bandwidth to business customers or low latency for voice and video applications, whereas residential Internet surfers can only be provided with best-effort and lower-cost service.

Full support for WMAN service

In contrast to last-mile implementations based on the 802.11g standard, 802.16-2004 is capable of supporting more users and deliver faster data rates at longer distances.

Robust carrier-class operation

The standard was intended for carrier-class operation. As more users join, they have to share the cumulative bandwidth and their individual throughput reduces linearly. The reduction, however, is not as dramatic as experienced under 802.11. This ability is termed “efficient multiple access.”

3.3 FLEXIBLE CHANNEL BANDWIDTH

When subscriber moves by walking or driving in a car, the distance among a subscriber and the base station (or AP) increases, then it becomes more difficult for that subscriber to transmit successfully at a given power level back to the base station. For power-sensitive devices such as laptop computers or handheld devices, if the channel bandwidth is broad, then it's not possible for them to transmit to the base station over long distances. To make possible transmission over longer ranges and to different types of subscriber devices, 802.16 has elastic channel bandwidths between 1.5 and 20 MHz. In addition, this flexibility of channel bandwidth is also important for cell planning, particularly in the licensed spectrum. For scalability, an operator with 14 MHz of accessible spectrum, may have multiple sectors (transmit/receive pairs) on the same base station by dividing that spectrum into four sectors of 3.5 MHz. With a dedicated antenna, each sector can provide users with more throughput over longer ranges as compared to an omni-directional antenna.

3.4 SMART ANTENNA SUPPORT

Smart antennas enhance the spectral density (the number of bits communicated over a given channel in a given time) and also boost the signal-to-noise ratio for both Wi-Fi and WiMAX solutions. Because of performance and technology, several adaptive smart antenna types supported by the 802.16-2004 standard include[21]:

Receive spatial diversity antennas

Involves more than one antenna receiving the signal. To function efficiently, the antennas have to be placed no less than half a wavelength apart. Note that wavelength can be found by taking the inverse of the frequency. Maintaining this smallest distance makes sure that the antennas are incoherent, that is, additive/subtractive effects of signals arriving by means of multiple paths impact on them differently.

Simple diversity antennas

The signal strength of the multiple (two or more) antennas attached is detected and that antenna is switched into the receiver. The possibility of getting a strong signal will be higher if selection is from more incoherent antennas.

Beam-steering antennas

The antenna array pattern is shaped to produce high gains in the useful signal direction or notches that reject interference. High antenna gain enhances the signal, noise and rate. The interference is attenuated out of the main beam with directional patterns. If multi-path components arrive with a enough angular separation, selective fading can be alleviated.

Beam-forming antennas

Permit the area to be divided around a base station into sectors, allowing additional frequency reuse between sectors. The number of sectors can be as few as four to as many as 24. Base stations which cleverly manage sectors have been used for a long time in mobile-service base stations.

3.5 BNEFITS and CHALLENGES

The WiMAX basis benefits are:

  • Built-in QoS
  • High performance
  • Standards-based
  • Smart antenna support [25]

For WiMAX the most important challenge is that it's a fresh technology with emerging support.

3.6 FEATURES

The WiMAX is designed with various aims in mind. Below is the summary of potential features of WiMAX:

Flexible Architecture

Numerous system architectures are supported by WiMAX, consisting P2P, P2MP, and omnipresent coverage. The time slot for every SS are scheduled through WiMAX MAC to support Point-to-Multipoint and ubiquitous service. When there is only one SS in the network, the WiMAX BS uses Point-to-Point service to communicate with the SS. In P2P configuration to cover longer distances a BS might use a narrower beam antenna.

High Security

WiMAX encrypts the links among the base station and the subscriber station to provide privacy (reluctant to eavesdropping) and safety across the broadband wireless link to the users. With security operators can be protected reluctant to stealing of service.

WiMAX QOS

WiMAX is dynamically optimized for carrying mix of traffic. WiMaX supports four service types: UGS,rtPS,nrtPS, BE Services.

Quick Deployment

WiMAX needs little or no external plant construction as compared with the deployment of wired solutions, For example, digging to support the trenching of cables is not needed. Operators do not have to submit further applications to the Government that have acquired licenses to use one of the licensed bands, or that wish to use one of the unlicensed bands. Once the installation of antenna and equipment is complete and they are powered, WiMAX is ready for service. In most cases, deployment of WiMAX network can be deployed with in hours, as compared to other solutions that take months for deployment.

Multi-Level Service

The QoS provisioning is generally based on the Service Level Agreement (SLA) among the service provider and the end-user.

Interoperability

WiMAX is based on international, vendor-neutral standards, which facilitate end-users to travel and use their SS at different locations, or with different service providers. Interoperability protects the initial investment of an operator as it can choose equipment from diverse equipment vendors, and it will prolong to bring the costs of equipment down as a result of mass adoption.

Portability

Like current cellular systems, once the WiMAX SS is powered up, it recognizes itself, finds out the characteristics of the link with the BS, provided that the SS is registered in the system database, and then its transmission characteristics are negotiated by it accordingly.

Mobility

To support mobility, key features have been added in IEEE 802.16e amendment. The OFDM and OFDMA physical layers have been improved to support devices and services in a mobile environment. The standard has hereditary OFDM's better NLOS (Non-Line Of Sight) performance and multipath-resistant operation, so it is highly appropriate for the mobile environment.

Cost-effective

The costs of the standard will drive down considerably due to mass adoption of the standard, and the use of low-cost, mass-produced chipsets, and the consequential competitive pricing will present significant cost savings for service providers and end-users.

Wider Coverage

Multiple modulation levels are supported by WiMAX including BPSK, QPSK, 16-QAM, and 64-QAM. WiMAX systems cover large geographic area when outfitted with a high-power amplifier and functioning with a low-level modulation (BPSK or QPSK) and when the path among the BS and the SS is unhindered.

High Capacity

End-users can be provided significant bandwidth by making use of higher modulation (64-QAM) and channel bandwidth from (1.25MHZ to 20 MHz).

3.7 USAGE SCENARIOS

WiMAX technology will modernize the way we communicate. The people who are highly mobile are provided freedom and will stay connected with voice, data and video services. The WiMAX standard addresses a large range of applications and usage scenarios which are grouped into two broad categories, private networks and public networks.

3.7.1 Private networks

Single organization, institution or business uses private networks to provide dedicated communication links for the secure and reliable transfer of voice, data and video. High priority is generally given to quick and easy deployment, and configurations are typically Point-to-Point or Point-to-Multipoint[21].

3.7.1.1 Cellular backhaul

The cellular services market is becoming progressively competitive. For cellular operators to stay in the business, they must look for ways to lower operating costs. Backhaul costs for cellular operators represent a considerable portion of their recurring costs. WiMAX can provide Point-to-Point links of up to 30 miles (50 km), with data rates capable of supporting multiple E1/T1s. WiMAX equipment can thus be used by cellular operators for backhauling Base Station traffic to their Network Operation and Switching Centers, as shown in figure 3.3..

The built-in QoS attribute of WiMAX is highly suitable for cellular traffic which is a mix of voice and data. Backhaul facilities leased from local telephone companies can be too expensive, and installing a fiber solution, can be both costly and prolonged. For providing cellular backhaul, wired solutions are not often cost-effective in rural or suburban areas, and most versions of DSL and cable technology cannot present the requisite bandwidth, particularly for backhauling 3G networks.

3.7.1.2 Wireless service provider backhaul

WiMAX equipment is used by wireless Service Providers (WSPs) for backhauling traffic from BS in their access networks, as shown in figure 3.4.

WiFi, WiMAX or some proprietary wireless access technology might be used as an access network. As WSPs normally proffer voice, data and video, the WiMAX built-in QoS attribute will assist in prioritizing and optimizing the backhauled traffic. The working expenditure are raised for hiring backhaul services from the local telephone company, and deploying a fiber network can be incredibly expensive and entails considerable long time. Moreover, in rustic and suburban areas, fiber, DSL and cables are not money-making, and the capacity necessary for these networks might not be offered by DSL and cable technologies.

3.7.1.3 Banking networks

The branches and ATM sites of large banks carrying voice, data and video traffic can be connected to their regional office through a private WiMAX network as shown in figure 3.5. These banks require high safety and bandwidth to manage the traffic as they are usually spread over a large area.

To defend against undesired interruption and management of sensitive banking traffic WiMAX data encryption offers superb link security. Due to large coverage and high capacity, a huge no: of diversely situated branch offices and ATM sites can be linked to the bank's regional office.

3.7.1.4 Education networks

School board office can connect the schools with their office by making use of Private WiMAX network as shown in figure 3.6.

Using QoS, the complete range of communication necessities, comprising telephony voice, operating data (for example records of students), email, Internet and intranet access (data), and distance education (video) can be delivered by the education networks based on WiMAX, among the school board office and all of the schools in the range, and among the schools themselves [26].

3.7.2 Public networks

In public network, resources are accessed and shared by both businesses and private individuals. In Public networks the users position is neither predictable nor static so they usually need a cost-effective means of providing ever-present coverage. Voice and data communication are the major applications of public networks, while video communication is becoming more and more popular. As several users share the network, security is a critical requirement. These concerns are addressed by data encryption and built-in VLAN support. Quite a few public network usage scenarios are discussed below [21].

3.7.2.1 Wireless service provider access network

WiMAX networks are used by Wireless Service Providers to offer connectivity to both housing (voice, data and video) and business (mainly voice and Internet) [26] clients, as shown in figure 3.7.

3.7.2.2 Rural connectivity

WiMAX networks are used by Service providers to bring service to underserved markets in rural areas and the suburban outer edges of cities, as shown in figure 3.8.

In many developing countries and underserved areas of developed countries, the delivery of rural connectivity is critical, where little or no infrastructure is available. Rural connectivity brings much-needed voice telephony and Internet service. Because of the extended coverage of WiMAX solution, it is a greatly more cost-effective solution as compared to wired technology in areas with lower population densities. WiMAX solutions can be installed rapidly, providing communication links to these underserved areas, providing a more safe environment, and helping to get better their local economies.

CHAPTER 4

QUALITY of SERVICE in IEEE 802.16

4.1 QUALITY of SERVICE (QoS)

QoS refers to the network's capability to offer improved service to elected network traffic over several technologies. The aim of QoS technologies is to prioritize (comprising committed bandwidth to manage jitter and delay) various real-time and interactive traffic, while ensuring that in doing so the other traffic doesn't stop working [25]. In licensed-exempt bands deployment is generally open to everyone, so they can be subject to QoS issues. However, there are progresses in the associated standards and technologies, that help alleviate harms with licensed-exempt bands, for example multi-path interference [14].

On the Internet and in other networks, the idea behind QoS is to calculate, enhance, and, to some degree, assure the transmission rates, error rates, and other characteristics in advance. QoS is of particular concern for the nonstop transmission of high-bandwidth. The wireless technology's capability to efficiently bring high value services for example voice and video is determined by Quality of Service. Latency, jitter, packet loss, load and throughput are the chief detractors to good QoS.

WiMAX presents lower delay across the wireless extent. There are many vendors having products with delay less than 10ms from BS to customer premises equipment and vice versa. Keeping that in mind, delay have to be calculated end-to-end. VoIP, for instance, is extremely vulnerable to delay. If delay go beyond 150ms for example, the conversation quality starts draging. At or beyond 200ms, conversation might be meaningless for several viewers.

The figure 4.1 shows that delay on the network's wireless section is smallest compared to that on network's wired section.

4.2 IEEE 802.16 QoS MECHANISMS at PHY LAYER

The 802.16 standard incorporates numerous QoS mechanisms at the PHY layer, for example Time Division Duplex (TDD), Frequency Division Duplex (FDD) and Orthogonal Frequency Division Multiplexing (OFDM). All can assist in providing QoS. TDD can dynamically distribute uplink and downlink bandwidth, based on their requirements[19]. For instance, more bandwidth can be assigned to uplink traffic when it is more, and when there is less uplink traffic bandwidth can be taken away. This is illustrated in Figure 4.2. Every 802.16 TDD frame is one downlink sub-frame and one uplink sub-frame, separated by a guard slot. 802.16 adaptively assigns the number of slots for each, based on their bandwidth requirements.

Guard Slot

In FDD, transmission occurs on different sub-bands, and therefore there is less chance of interfere among stations. This permits for even more bandwidth distribution flexibility. And, OFDM offers better spectral efficiency, and alleviates interference with its tighter beam width and its distribution of data across different frequencies.

There are a couple of QoS attributes that are specific to OFDM. Forward Error Correction (FEC) builds redundancy into the transmission by replicating a few of the information bits, so bits that are lost or in error can be repaired at the receiving end. with no FEC, error correction would involve entire frames to be retransmitted, resulting in latency and poor QoS. The other way OFDM assists with QoS is with interleaving[19]. As OFDM make use of multiple sub-carriers, a fraction of each information bit can be car

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