WiMax Security Issues, Mobility and Network Entities

WiMax supports Multiple-Input Multiple-Output (MIMO) schemes. In these schemes, both the Base Station and the Mobile Station establish a connection with more than one antenna.

In this way, the data rate can be increased proportionally to the minimum number of the antennae of each side. The MIMO technology improves the reception and allows for a better reach and rate of transmission. The IEEE 802.16 specification suggests and describes the use of four antennae on each side (4x4 MIMO link). The main drawback of this case, apart from the fact that more hardware concerning the antennae is physically needed, is that more advanced digital signal processing is applied, with increased processing power demanded. If the receiver has one antenna, then no better result than 1x1 is achieved. This is illustrated at Figure 5.

Figure 5: typical 2x2 MIMO WiMax channel in comparison to a 1x2 WiMax channel

Each part of a WiMax network contains several entities that form up the whole setup. The three most important ones are the Base Station, the Access Service Network Gateway and the Connectivity Service Network. The Base Station is mainly responsible for providing the air interface to the Mobile Station, but also for handoff triggering (when a Mobile Station changes from one cell to another), radio resource management, tunnel establishment, Quality of Service (QoS) policy enforcement (applies to services with higher sensitivity than the average and/or to end users with a premium subscription), traffic classification, Dynamic Host Control Protocol (DHCP) proxy, key management, session management, micromobility management functions and multicast group management. The Access Service Network Gateway acts as a layer 2 (OSI data link layer) traffic aggregation point within an Access Service Network and its main functions include intra-access service network location management and paging, radio resource management, admission control, caching of subscriber profiles and encryption keys, establishment and management of mobility tunnel with Base Stations, Quality of Service and policy enforcement and foreign agent functionality for mobile IP. The Connectivity Service Network provides connectivity to the Internet, other public and corporate networks, authenticates the connected devices, users and services. It also provides per user policy management of Quality of Service and security, manages the IP address allocation and supports roaming capabilities [13].

Figure 6: WiMax diverse usage towards full mobility

WiMax has four main user mobility scenarios:

·         nomadic, where the user is allowed to take a fixed subscriber station and reconnect from a different point of attachment

·         portable, where access is provided to a portable device  and the handover is best-effort and not guaranteed

·         simple mobility, where the user can move at speeds up to 60 kilometers per hour with brief interruptions of less than 1 second during handoff

·         full mobility, where the user can move at speeds up to 120 kilometers per hour and the handoff experiences latency less than 50 milliseconds and the packet loss is less than 1% [18]

WiMax supports nomadic and portable users (as in Figure 6) without efficiency issues, but simple and especially full mobility can severely affect the bitrate achieved at the side of the subscriber.

WiMax, as any other wireless technology, faces several security issues. The physical medium of the signal propagating is air, which is obviously spatially shared by all users. Therefore, WiMax has applied robust security, including support for privacy, device-user authentication, flexible key-management protocol, protection of control messages and support for fast handover. User data is encrypted by using Advanced Encryption Standard (AES) and Triple Data Encryption Standard (3DES) with an 128-bit or a 256-bit key, ensuring data privacy. WiMax authenticates subscriber stations and users with an authentication framework which is based on the Internet Engineering Task Force (IETF) Extensible Authentication Protocol (EAP) and supports a variety of credentials, like username-password, smart cards and digital certificates. WiMax uses the Privacy and Key Management Protocol Version 2 (PKMv2) for securely transferring keying material from the Base Station to the mobile station, by refreshing and reauthorizing the keys from time to time. The protection of control messages is secured by message digest schemes, such as AES-based Cipher-based Media Access Control (CMAC) or MD5-based (Message-Digest) Hash Message Authentication Code (HMAC). Fast handovers are achieved with the use of pre-authentication with a particular Base Station to facilitate accelerated reentry [13]. A 3-way handshake both optimizes this procedure and prevents any possible man-in-the-middle attacks.