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Monday, July 4, 2011

WiBro: The First Mobile WiMAX System


WiBro, an abbreviation of wireless broadband, refers to the 2.3-GHz frequencybased Mobile WiMAX system developed and deployed in Korea. The WiBro system has selected a collection of features out of the mandatory and many optional specifications adopted in the IEEE 802.16e standards to form its unique profile.
In 2002, the Ministry of Information and Communications (MIC) in Korea reallocated the 100-MHz frequency band at 2.3-GHz spectrum for portable Internet services (or WiBro services), instead of fixed wireless local loop (WLL) services. The 100-MHz band was divided into three triplets of 9-MHz band each and some guard bands, with each triplet consisting of three 9-MHz bands. In 2004, MIC and the Telecommunication Technology Association (TTA), the Korean domestic standardization body, issued the basic requirements on WiBro. In 2005, the MIC issued WiBro licenses to three operators including KT (formerly, Korea Telecom) and SK Telecom. In 2005, Samsung Electronics developed the world’s first commercial Mobile WiMAX system based on the 2.3-GHz WiBro profile and KT deployed the WiBro network based on the 27-MHz band in the middle (i.e., B-band). In June 2006, KT made a large-scale trial of the 2.3-GHz WiBro network deployed in the Seoul metropolitan area, and, in April 2007, KT started full commercial WiBro services in the Seoul metropolitan area and its vicinity for the first time in the world.

As is the case of Mobile WiMAX, WiBro is designed to be an IP-mode mobile network, in contrast to the existing cellular mobile networks such as WCDMA/HSDPA or cdma2000/1x-EVDO, which are circuit-mode-based with packet-mode hybridization. Since WiMAX adopts an all-IP network structure tailored for Internet service provision, the network structure is simple, inexpensive to construct, and adequate for providing a diverse set of services.

The illustration below shows the architecture of the WiBro network. The WiBro network is composed of MSs, an access service network (ASN), and a connectivity service network (CSN). The ASN contains RASs (or BSs) and ACR (or ASN-G/W), which are managed by Mobile WiMAX system manager (WSM). Located in the user side of the ASN are the MSs.
As such, the WiBro network architecture is much simpler than that of existing mobile communication networks. CSN is composed of various servers, namely, an authentication, authorization, and accounting (AAA) server, a home agent (HA), a dynamic host configuration protocol (DHCP) server, a domain name service (DNS) server, and a policy and
charging rules function (PCRF) server. ASN is connected with CSN via a router or a switch.

ACR is the central system of the WiBro network, which connects the CSN and RAS. It enables multiple RASs to interwork with CSN and IP networks and sends and receives traffic between the external network and MS. Basically, it performs the routing function for transferring data between the RAS and the Internet, and the control function for controlling the WiBro users, services, and mobility. It corresponds to the combined functions of the circuit-mode-based BSC and the packet data serving node (PDSN) in the existing cellular mobile network. The functions of the ACR include handover control, IP routing and mobility management, user service profile information provision, billing service provision to billing server, and security function. The ACR performs the packet classification and packet header suppression (PHS) functions in support of the convergence sublayer (CS). It performs the header compression function, and supports robust header compression (ROHC). In addition, it performs the paging and location register functions for the MSs in idle mode. For authentication, the ACR performs the authentication and key distribution functions by interworking with the AAA server, with the RAS performing the key receiver function to receive the security key from the key distributor. ACR interworks with the AAA server of the CSN for charging services too. It also interworks with the HA of CSN for mobile IP (MIP) services, and supports both proxy MIP (PMIP) and client MIP (CMIP).

WSM provides the management environment for network operators to operate and maintain the ACR and RAS. The WSM performs ACR and RAS architecture management, fault management, statistics management, configuration management, command line interface, and software version management. Operators can inquire the status of the system, sectors, frequency assignments (FAs), and repeaters to the WSM. The WSM classifies and selects the commands that can be executed in the ACR and RAS.

RAS is an entry system of the WiBro network, which connects ACR and MS. It receives subscriber data via wireless path, passes the data to the ACR in the upstream, and distributes the data received from the ACR to user terminals in the downstream. The RAS performs various functions, including transmission and reception of physical layer signals with MSs, modulation/demodulation, coding, packet scheduling for QoS assurance, allocation of radio resources, service flow management, ARQ processing, and ranging function. In addition, the RAS controls the connection for packet calls and handover. RAS performs service flow management (SFM) function to create/ change/release connections for each service flow (SF). An admission control function is required while creating/changing the connections. In support of the SFM function of the RAS, the ACR performs an SF authentication (SFA) function to obtain QoS information from the policy function (PF) and applies it when creating the SF, and also performs an SF identification (SFID) management function to create/ change/release SFID and map the SF according to the packet classification. In relation to handover, the RAS performs a handover control function to determine the initiation of the corresponding handover procedure. It checks the neighbor RAS list and relays the handover signaling message to the right target RAS system. At that time, the ACR and RAS conduct the context function to exchange the context information between the target RAS system and the serving RAS system. For radio resource management, the RAS performs radio resource control (RRC) and radio resource agent (RRA) functions to collect/manage the radio resource information from MSs and the RAS itself.
MS is the end-user device of the WiBro network that performs the input/output function and various other functions to process the information, to access IP-based WiBro networks, and to perform the various functions required for the terminating network element. The functions of MS include wireless access to the ASN, IP-based call services, the support of IP mobility, the authentication and security of MS and subscribers, and the reception of multicast services.

CSN servers include the basic network servers for service provision and various application servers for providing application services. The network servers include the HA for management of home address, the AAA server for security and accounting functions, the DNS server for conversion of IP addresses and system names, the DHCP server for dynamic allocation of IP, the PCRF server for managing the service policy and for sending QoS setting and accounting rule information. The application servers include the servers for push-to-talk (PTT), instant messaging (IM), multimedia messaging system (MMS), location-based service (LBS), games, and other services. The HA accesses other networks and enables MIP users to access the Internet. The HA interworks with the ACR that performs a foreign agent (FA) function in mobile IPv4 environment and interworks with MS to exchange data in mobile IPv6 environment. The AAA server interfaces with the ACR and carries out subscriber authentication, authorization, and accounting functions. It interfaces with the ACR via the DIAMETER protocol and provides extensible authentication protocol (EAP) certification. The DNS server manages the domain names. It interprets the domain or host names to the IP addresses in the form of binary digits. The DHCP server manages the setup and the IP addresses of MSs. It performs the management and allocation of the IP addresses and other setup information for MSs. When external DHCP server does not exist, since the ACR includes the DHCP server and relay agent functions, the ACR performs the DHCP server function. The PCRF server manages the service policy and sends both QoS setting information for each user session and accounting rule information to the ACR.

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