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rface (i.e., telephony services and programming interfaces); WAP Content Types. protocols and has been optimized for wireless communication networks. It includes data integrity checks, privacy on the WAP gateway-to-client leg and authentication. Wireless Datagram Protocol (WDP): WDP is transport layer protocol in WAP [159]. WDP supports connectionless reliable transport and bearer independence. WDP offers consistent services to the upper layer protocols of WAP and operates above the data capable bearer services supported by various air interfaces. Since WDP provides a common interface to upper-layer protocols, security, session and application layers are able to operate independently of the underlying wireless network. At the mobile terminal, the WDP protocol consists of the common WDP elements plus an adaptation layer that is specific for the adopted air interface bearer. The WDP specification lists the bearers that are supported and the techniques used to allow WAP protocols to operate over each of them [152]. The WDP protocol is based on UDP. UDP provides port-based addressing and IP provides Segmentation And Re-assembly (SAR) in a connectionless datagram service. When the IP protocol is available over the bearer service, the WDP datagram service offered for that bearer will be UDP. 9.3.1 Bearers for WAP on the air interface Let us refer to the Global System for Mobile communications (GSM) network, where the following bearer services can be adopted to support WAP traffic [118]: Protocols for High-Efficiency Wireless Networks - Part II 251 Unstructured Supplementary Services Data (USSD); circuit-switched Traffic CHannel (TCH); Short Message Service (SMS); General Packet Radio Service (GPRS), plain data traffic; Multimedia Messaging Service (MMS) over GPRS. Let us compare these different options. TCH has the disadvantage of a 30-40 s connection delay between the WAP client and the gateway, thus making it less suitable for mobile subscribers. Both SMS and USSD are inexpensive bearers for WAP data with respect to TCH, leaving the mobile device free for voice calls. SMS and USSD are transported by the same air interface channels. SMS is a store-and-forward service that relies on a Short Message Service Center (SMSC). Whereas, USSD is a connection-oriented (no store-and- forward) service, where the Home Location Register (HLR) of the GSM network receives/routes messages from/to the users. The SMS bearer is well suited for WAP push applications (available from WAP release 1.2), where the user is automatically notified each time an event occurs. USSD is particularly useful for supporting transactions over WAP. Finally, GPRS radio transmissions allow a high capacity (up to 170 kbit/s using all the slots of a GSM carrier with the CS-4 coding scheme) that is shared among mobile phones according to a packet switching scheme. Hence, GPRS can provide a powerful scheme for WAP contents delivery. 9.4 Tools and applications for WAP The WAP programming model is similar to the WWW programming one. This fact provides several benefits to the application developer community, including a proven architecture and the ability to leverage existing tools (e.g., Web servers, XML tools, etc). Optimizations and extensions have been made in order to match the characteristics of the wireless environment. Different WAP browsers can be found in reference [160]; they are useful tools for developing WAP-based services for mobile users. WAP allows customers to easily reply to incoming information on the phone by adopting new menus to access mobile services. Existing mobile operators have added WAP support to their offering, either by developing their own WAP interface or, more usually, partnering with one of the WAP gateway suppliers. WAP has also given new opportunities to allow the mobile distribution of existing information contents. For example, CNN and Nokia teamed up to offer CNN Mobile. Moreover, Reuters and Ericsson teamed up to provide Reuters Wireless Services. Protocols for High-Efficiency Wireless Networks - Part II252 Location-aware services; Web browsing; Remote LAN access; Corporate e-mail; Document sharing / collaborative working; Customer service; Remote monitoring such as meter reading; Job dispatch; Remote point of sale; File transfer; Home automation; Home banking and trading on line. Protocols for High-Efficiency Wireless Networks - Part II 253 New mobile applications that can be made available through a WAP interface include: Another group of important applications is based on the WAP push service that allows contents to be sent or “pushed” to devices by server- based applications via a push proxy. Push functionality is especially relevant for real-time applications that send notifications to their users, such as messaging, stock price and traffic update alerts. Without the push functionality, these applications would require the devices to poll application servers for new information or status. In cellular networks such polling activities would cause an inefficient and wasteful use of the resources. WAP push functionality provides control over the lifetime of pushed messages, store-and-forward capabilities at the push proxy and control over the bearer choice for delivery. Interesting WAP applications are made possible by the creation of dynamic WAP pages by means of the following different options: Microsoft ASP; Java and Servlets or Java Server Pages (JSPs) for generating WAP decks; Subscriber Identity Module (SIM) - Toolkit: the use of SIMs or smart cards in wireless devices is already widespread. Windows CE: this is a multitasking, multithreaded operating system from Microsoft designed for including or embedding mobile and other space-constrained devices. JavaPhoneTM: Sun Microsystems is developing PersonalJavaTM and a JavaPhoneTM Application Programming Interface (API), which is embedded in a JavaTM virtual machine on the handset. Thus, cellular phones can download extra features and functions from the Internet. 254 Protocols for High-Efficiency Wireless Networks - Part II XSL Transformation (XSLT) for generating WAP pages adapted for displays of different characteristics and sizes. Alternative approaches to the use of WAP for mobile applications could be as follows: SIM Toolkit and Windows CE are present days technologies as well as WAP. SIM Toolkit implies the definition of a set of services “embedded” on the SIM that allow contacting several service provides through the mobile phone network. The Windows CE solution is based on an operating system developed for mobile devices, supporting different applications. Finally, JavaPhoneTM will be the most sophisticated option for the development of device-independent applications. Within ETSI and 3GPP, activities are in progress for the definition of new architectures providing mobile information services. Accordingly, a new standard, called Mobile station application Execution Environment (MExE), has been defined [161]. MexE is a VHE technology, according to the description given in Chapter 3 (Section 3.6) in Part I. In particular, in order to insure the portability of a variety of applications, across a broad spectrum of multi-vendor mobile terminals, a dynamic and open architecture has been conceived in MExE for both the Mobile Station (MS) and the SIM, i.e., a common set of APIs and development tools. MExE is based on the idea to specify a terminal-independent execution environment on the client device (i.e., MS and SIM) for non-standardized applications and to implement a mechanism that allows the negotiation of supported capabilities (taking into account available bandwidth, display size, MExE classmark 1: it is based on WAP, requires limited input and output facilities (e.g., as simple as a 3 lines by 15 characters display and a numeric keypad) on the client side and is designed to provide quick and cheap information access even over narrow and slow data connections. MExE classmark 2: it is based on PersonalJavaTM, provides and utilizes a run-time system requiring more processing, storage, display and network resources, but allows powerful applications and more flexible MMIs. MExE classmark 2 also includes the support for MExE classmark 1 applications (via the WML browser). Protocols for High-Efficiency Wireless Networks - Part II 255 processor speed, memory, MMI). The key concept of the MExE service environment to make mobile-aware applications (i.e., aware of MS capabilities, network bearer characteristics and user preferences) is the introduction of MExE classmarks that have been standardized as follows: This page intentionally left blank Chapter 10: The mobile Internet Recent years have seen a strong development of wireless and mobile devices, such as palmtops, personal communicators and Personal Digital Assistants (PDA), characterized by increasing processing capabilities and memory storage. Such devices give the possibility of accessing the network, sending and receiving e-mails and browsing the Web while on the move. The wish to connect to the Internet and maintain communications anytime and anywhere has led to the need of the mobile Internet. Today, support of Internet services in a mobile environment is an emerging requirement. The issues to be faced in order to support the wireless and mobile Internet are related to different protocol layers: Network layer: the Internet Protocol (IP) needs modifications in order to manage the routing to/from a mobile node; Transport layer: the Transmission Control Protocol (TCP) should be refined in order to work efficiently on error-prone wireless links. 10.1 IP and mobility The TCP/IP suite was originally designed to work with wired networks. One basic problem with mobile Internet is related to the routing mechanism for delivering packets to mobile stations. As a matter of fact, IP addresses are defined according to a topological relation with the connected nodes, assuming that any node has always the same point of attachment to the Internet. According to the original IP addressing scheme, when a computer moves to a new point of attachment, it should be assigned a new IP configuration (i.e., IP address, netmask and default router) in order to be visible in the Internet. In the scenario depicted in Fig. 47, datagrams addressed to the laptop in subnet B will be always routed through link B; if this node moves to subnet C, it will not receive datagrams anymore, because packets will still be routed to link B. 258 Protocols for High-Efficiency Wireless Networks - Part II Mobile IP [162] was introduced by IETF with the purpose to support mobile devices while dynamically changing their access points to the Internet. The mobility concept can be categorized in two classes [163]: 10.1.1 Mobile IP Both ends of a TCP session (connection) need to keep the same IP address for the whole life of the session. This address, assigned for an extended period of time to a mobile node, is called home address and it remains unchanged regardless of where the node is attached to the Internet. As explained before, the IP address needs to be changed when a network node moves to a new place in the network. This new address, called care-of-address, is associated to the mobile node while it is away Macro-mobility: this term relates to movements of a mobile nod e among different IP domains or different wireless access networks; mobility management is held by a macro-mobility scheme, named Mobile IP. Micro-mobility: it relates to movements carried out among different micro-cells within the same IP domain. Mobile IP is not appropriate to support fast, seamless handoffs between cells and a micro-mobility scheme is needed for managing micro-mobility. from home and it is used for routing purposes. Mobile IP solves the IP mobility problem by means of a routing approach, managing a dynamic association between a care-of-address to a home address, called a binding. According to this mechanism, Mobile IP is an extension to IP protocol, allowing a mobile node to use two different IP addresses, a static one (home address) for its identification and a dynamic one (care-of- address) for routing. In such a way the node can continue receiving datagrams, independently of its location. The Mobile IP Working Group has developed routing support to permit IP nodes (routers and hosts) using either IPv4 or IPv6 to seamlessly roam among IP sub-networks. It allows macro-mobility management independent of radio access technology and provides seamless roaming among heterogeneous wireless networks (i.e., GPRS, UMTS and wireless LAN). Transparency above the IP layer is supported, including the maintenance of active TCP connections and UDP port bindings. The cellular and wireless industry is considering using Mobile IP as a technique for IP mobility for wireless data. 10.1.2 Micro-mobility and the Cellular IP approach Even if Mobile IP provides a simple and scalable mobility scheme, it is not appropriate for high mobility and seamless handoffs. In fact, it envisages that every time a node migrates, a local address must be obtained and communicated to a distant location directory, called home agent. This updating procedure, together with route optimization, introduces delays and data transfer disruption while the correspondent node obtains the new binding. The effect of these delays grows with the frequency of handoffs. Moreover, when host mobility becomes ubiquitous and cell size smaller, the traffic load generated by the update messages can have a drastic effect on the Internet and on the home agent as well, being proportional to the number of mobile hosts. Cellular IP [164],[165] is one of the most attracting schemes for managing micro-mobility. It is aimed at optimizing handoffs in a restricted geographical area, rather than supporting global mobility. Fig. 48 depicts a possible scenario in which local and wide area Protocols for High-Efficiency Wireless Networks - Part II 259 260 Protocols for High-Efficiency Wireless Networks - Part II mobility are separated: Mobile IP manages global mobility, Cellular IP manages migrations at the local level (i.e., within the wireless access network). According to this general scenario, handoffs within the access network are locally handled. Hence, handoffs can be faster and the impact on active data sessions is limited. Cellular IP defines a wireless access network architecture and protocol for managing micro-mobility. It is based on cellular technology principles for mobility management, passive connectivity (i.e., paging) and handoff support. It operates at the network layer, substituting the IP routing mechanism in the wireless access network, without modifying the packet format and the IP forwarding mechanism. The Cellular IP node embeds different functions, such as: wireless access point, IP packet routing and cellular control functionality, traditionally found in MSC and BSC. The nodes implement Cellular IP integrated routing and location management and are built on regular IP forwarding engine. Protocols for High-Efficiency Wireless Networks - Part II 261 A gateway connects the Cellular IP network to the Internet. Its IP address is used by mobile hosts attached to the network as their Mobile IP care-of address (see Fig. 49). Uplink routing (i.e., from MN to gateway) is performed on a hop-by- hop basis. Nodes on the route cache the path taken by uplink packets. After MN data transmissions (see Fig. 49), the routing cache in BS2 includes a mapping (MN, a), indicating that MN is reachable through interface “a” (see the path labeled with “b” in Fig. 49). Cache entries are used to route downlink packets (i.e., from gateway to MN) on the reverse path. Cache is refreshed also by route-update packets (empty IP packets) that are periodically sent to the gateway by MNs that are not regularly transmitting data. In this way the downlink routing state (soft- state route) can be maintained. Handoffs are initiated by MNs on the basis of measurements of the BS signal strengths. While moving from BS3 to BS4 (see Fig. 49) during an active data session, the MN detects the stronger BS4 signal, tunes its radio to the channel used by BS4 and transmits a route-update packet (dotted line with “b” label in Fig. 49) that is cached by BSs along the path. BS2 adds to its routing cache the new mapping (MN, b), thus keeping a double entry related to MN (the old and the new route). Since the old mapping will be cleared only after the routing-cache timeout extinguishes, before this timeout both routes will coexist and packets addressed to MN will be delivered through both interfaces/path “a” and “b”. In the case that an MN does not receive packets for the active-state- timeout, it enters an idle state, letting its soft-state routing cache mappings time out. The following paging mechanism, derived from cellular telephony, is adopted by Cellular IP to reach idle hosts. Paging-update packets (i.e., empty IP datagrams) are periodically sent by the MN to the gateway in order to update the paging cache that is optionally maintained in Cellular IP nodes. When a node finds no valid routing cache mapping for an idle destination MN, paging occurs and IP packets are routed according to paging cache mappings (a node with no paging cache forward packets to all its interfaces except the source one). The paging cache mechanism allows avoiding broadcast search procedures. Unlike in other solutions (e.g., HAWAII [166]), Cellular IP limits the use of explicit signaling messages and exploits IP datagrams for exchanging information on the position of mobile hosts. Moreover, it requires a simple configuration in the access network allowing easy employment and administration. A 3G.IP group has been created to promote a common IP based wireless system for 3G mobile communication systems and to favor the standardization of an all IP-based wireless network architecture in 3GPP Releases 5 and 6 [167]. 262 Protocols for High-Efficiency Wireless Networks - Part II Protocols for High-Efficiency Wireless Networks - Part II 10.2 Wireless TCP Most popular Internet applications, such as SMTP (e-mail), HTTP (WWW surfing) and FTP (file transfer), use the reliable services provided by TCP, a transport layer protocol in the Internet. The performance perceived by users mainly depend on the good behavior of TCP. Hence, studying its performance dynamics becomes a crucial part for the design of mobile networks that adopt the TCP/IP protocol suite. TCP has been defined for traditional wired networks, characterized by low error rates and high bandwidth. The protocol interprets a packet loss in the network as an indication of network congestion (i.e., packet loss is due to the discard operated by a congested buffer encountered in the route), thus invoking congestion control and avoidance algorithms [168]. Such assumption is not correct over lossy links, such as wireless and satellite links, since packet losses are due to errors rather than to network congestion. Wireless links are characterized by low bandwidth, high latency, high bit error rates and temporary disconnections. In this environment the throughput at the TCP level may considerably degrade, thus affecting the behavior of applications. Wireless networks share common characteristics, however, three main categories can be considered as different environments for data communications: 263 Wireless Local Area Networks (WLAN), with short links and high bandwidth; Wireless Wide Area Networks (W-WAN), often referred to as Long Thin Networks (LTN), where “long” indicates high latency and “thin” stays for low bandwidth; Satellite networks, often referred to as Long Fat Networks (LFN), where “fat” indicates high bandwidth. The differences between them rely on the Delay-Bandwidth Product (DBF), that defines the capacity of a network path, that is the number of data segments that TCP should maintain “in flight” (i.e., sent but not yet acknowledged) in the channel in order to use efficiently the available resources. Delay refers to the Round Trip Time (RTT), while bandwidth refers to the capacity of the bottleneck in the network path. Assuming for WLANs (of the IEEE 802.11 type) RTT = 3 ms and a bandwidth of 1.5 Mbit/s, we obtain BDP = 4.5 Kbits. Instead, a 3G cellular system (W-WAN) can offer a maximum bandwidth of 2 Mbit/s and RTT = 200 ms, thus resulting in DBP = 50 Kbytes. This value is higher than the standard dimension of a TCP buffer (8 Kbytes) adopted by most TCP implementations; W-WANs will behave inefficiently unless buffer dimension is incremented. Finally, a link between two earth stations through a satellite GEO link presents a more critical situation for channel efficiency and TCP performance [169]: assuming RTT = 500 ms and a bandwidth of 36 Mbit/s, the result is DBP = 18 Mbits. 10.2.1 Mechanisms for improving wireless TCP performance on error-prone channels When dealing with wireless links, two problems arise: one is due to the characteristics of the link and the second is due to the mobility of the receiver. Mobility can cause temporary disconnections due to handoffs or to black holes in the coverage area. When disconnections are too long, the sender could give up and close the TCP session. The following three different approaches are possible for improving TCP over wireless links: 264 Protocols for High-Efficiency Wireless Networks - Part II End-to-end schemes: they work at the transport layer, usually implementing the solution at the TCP sender; Split-connection schemes: it splits up the TCP connection by two, a wired connection (between the sender and the base station) and a wireless one (between the base station and the mobile terminal); Link layer schemes: these solutions do not directly affect TCP, since they are implemented in the link layer. We limit the following study to a general level, without presenting the detailed description of the specific protocols. Moreover, we assume that the reader has a general background on TCP [170]-[172]. Protocols for High-Efficiency Wireless Networks - Part II 265 10.2.2 End-to-end approach TCP grants reliability of data delivery by sending acknowledgments (ACK) from destination to source on an end-to-end basis. Optimization techniques at the transport layer are based on modifications to TCP only at the end points of a connection (see Fig. 50). This approach does not alter the semantics of TCP sessions and it acts in a way to use more efficiently wireless links. Moreover, it should not affect the standard mechanisms for congestion control, like slow start and congestion avoidance. An end-to-end scheme is the Explicit Loss Notification (ELN) [173] that adds an ELN option to TCP ACKs. After a packet loss in the wireless link, the future cumulative ACKs related to the lost packets are marked in order to signal that a non-congestion loss has occurred. Hence, the sender does not invoke any congestion control technique. This method changes TCP and does not solve the problem of temporary disconnections. 10.2.3 Split-connection approach These solutions are based on the assumption that wired and wireless links have different characteristics and hence it is necessary to manage them separately (see Fig. 51). The neuralgic point of this approach is the Performance Enhancing Proxy (PEP), an intermediate node that allows to realize the two TCP connections and to exchange packets between them. The main advantage of this scheme is that congestion losses (in the wired links) and error losses (in the wireless links) can be separately treated and an appropriate wireless link specific protocol can be adopted for a better performance. However, TCP semantics is violated, since the fixed sender receives “false” ACKs, before data has successfully reached its final destination. 10.2.4 Link layer approach The general idea of this approach is shown in Fig. 52. A link layer scheme is the Snoop protocol [174]. According to Snoop, PEP maintains a cache of TCP packets sent from the source and not yet acknowledged by the mobile. When Snoop detects a packet loss in the 266 Protocols for High-Efficiency Wireless Networks - Part II Protocols for High-Efficiency Wireless Networks - Part II 267 wireless link (either through a duplicate ACK or through a local timeout), it locally retransmits the packet. In this way, the Snoop protocol hides temporary degradations and occasional disconnections to the sender that does not invoke congestion control mechanisms. The main disadvantage is the strong relation between the link layer that performs local retransmissions and the TCP layer. In fact, there is the possibility of having both the sender and the base station re- transmitting the same packet, especially in case of losses due to congestion. 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Protocols for High-Efficiency Wireless Networks 281 [168] [169] [170] [171] [172] [173] [174] This page intentionally left blank Access scheme, 1, 50, 108, 125, 128, 219, 227 Adaptation layer, 58 Aloha, 65, 129, 155, 217, 228 Astrolink, 112 Burstiness, 130, 136, 137, 139, 143, 147 CDMA, 1, 8, 47, 68, 110, 242 Cdma2000, 11, 49 Cellular IP, 259 Complex scrambling, 72 Congestion avoidance, 142, 265 Cyberstar, 112 Direct sequence, 8, 50 DiffServ, 62, 128, EDGE,29 E-GPRS, 29 EPA, 227 FDMA, 2, 25, 107, 129 FOMA, 51 Frequency hopping, 8 FSK, 9 Globalstar, 94, 103, 108 GPRS, 21, 22, 25 GSM, 3, 17, 20 HAPS, 114, 123, 219 HIPERACCESS, 121 HIPERLAN/2, 121 HIPERLINK, 121 Intelsat, 92 Leaky bucket, 132 LMDS, 121 LRD, 143, 145 MAC, 1, 26, 42, 65, 124, 127, 151, 176, 183, 187, 206, 217 Micro-mobility IP, 258 Macro-mobility IP, 258 MMDS, 120 MMPP, 142, 238 MMS, 83 Book index OVSF, 70, 81, 194 OFDM, 47, 118 PDP context, 38, 63, 155 Policer, 129, 132, 242 Polling, 156, 205, 242 Power control, 11, 14, 71, 166 PN code, 8 PRMA, 129, 217, PSK, 8, 11 QPSK, 8, 75, 79, 114 Radio block, 26, 30, 151, 153, 160 Resource reuse, 4 Round robin, 131, 162, 234 RRM, 127, 151, 165, 175, 205, 217, 227 SAP, 175, 176 Scheduling, 1, 130, 185, 187, 211, 234 Self-similarity, 143 SIP, 63 Soft-handoff, 15, 110 Spreading, 8 Skystation, 114 SkyBridge, 94, 111 SMS, 18, 21, 252 TBF, 30, 152, 154, 160 TDMA, 1, 2, 4, 25, 49, 81, 107, 129, 217 Teledesic, 94, 113 Token bucket, 132, 207 Tunnelling, 23, 40, 41 UMTS, 48, 52, 55, 66, 68, 82, 83, 104, 169, 175 UPC, 127, 128, 132 USSD, 21, 251 UWB, 118 VHE, 83 WAP, 139, 245 WATM, 124, 125, 134, 240 WLAN, 93, 116, 121, 243, 263 WCDMA, 47, 48, 50, 68, 69, 106, 114, 165 WildBlue, 113 Protocols for High-Efficiency Wireless Networks284