Wireless Application Protocol Wireless LAN T-110.300/301
Background for WAP • Handheld devices have – – – –
Weak CPU Little memory Limits on electrical power Limited user I/O
• Most of the time communications are over a cellular phone radio link – Narrowband – High latency – Typically burst errors (packet loss is not due to congestion)
Wireless Application Protocol • An industry-wide standard to develop applications for wireless environments • Ericsson, Motorola, Nokia... • Based on WWW technologies and philosophies • See http://www.wapforum.org/ • Several parts (version 1.0): – – – – –
WAE - Wireless application environment WAP - Wireless application protocol WSP - Wireless session protocol WTP - Wireless transport protocol WTA* - Wireless telephony application...
Goals • To design a protocol family suitable for building interactive applications for devices with – limited capabilities: – battery power – memory – screen size – limited bandwidth – possibly long latencies – different user interfaces
• Global and international usability • Security and access control
The WAP 1.0 Protocol Stack Wireless Markup Language (WML)
Other Services and Applications
Common Services Session Layer (WSP) Security Layer Transport Layer (WTP) Bearer Systems (GSM data, SMS, GPRS)
The Transport Protocol (WAP 1.0) • Connection oriented WTP/C – Optimized for low bandwidth wireless bearers – Efficient for request-reply interaction – Reliability
• Datagram oriented WTP/D – Datagrams are moved transparently on the bearer network
• Port based abstract interface to upper layers. • Low memory and CPU speed requirements.
The Session Protocols • Mechanisms for exchanging typed data between WAP applications in a secure manner • In WAP 1.0 Architecture the specification is coupled with the Security Layer • Optimized for asymmetric client–server functionality. Includes the server initiated “push” model. Application Layer Session Layer Security Layer Transport Layer Bearers
WAP architecture binary WML WSP/WTP GSM data
WAP handset
WML HTTP TCP/IP
WAP gateway, WML to binary format conversion
HTTP server, .WML pages
• WML is a markup Language modeled after HTML
WML Design • Providing WML services to a handset is not as easy as making web pages – The limited available memory of the first handsets limited the amount of pages the handset can cache – The pages should take full advantage of the available screen space – Customization is required for different handsets
• The amount of WAP servers is currently growing (2004), due to WAP over GPRS • HTML -> WML conversion is also possible, but often not very efficient
Wireless Application Protocol 2.0 • A new specification that improves on the earlier specifications • Interoperability with GPRS and 3G standards • Interoperability with TCP/IP protocols – The lower WAP layers can be replaced by TCP and HTTP
• More support for device characteristics • XHTML support
WAP 2.0 Layers WAP Device
Web Server
WAE
WAP Gateway
WAE
WSP
WSP
HTTP
HTTP
WTP
WTP
TLS (SSL)
TLS (SSL)
WTLS
WTLS
TCP
TCP
WDP
WDP
IP
IP
Bearer
Bearer
Bearer
Bearer
• •
•
WAE is not a protocol but a service environment Note that the WAP stack provides services that correspond to TCP/IP services, but both the structure and the services are different WAP 2.0 stack is not compatible with WAP 1.0
WAP 2.0 Protocols • Wireless Session Protocol (WSP) – HTTP 1.1 like functionality – Connection oriented and connectionless sessions
• Wireless Transport Protocol (WTP) – Lightweight transaction oriented protocol – Re-transmissions and acknowledgements
• Wireless Transport Layer Security (WTLS) – Privacy, data integrity, authentication – Similar to TLS 1.0 (SSL) – With added datagram support
• Wireless Datagram Protocol – General datagram service
WAP 2.0 Over TCP WAP Device
Web Server
WAE
WAE
HTTP
HTTP
TLS (SSL)
WAP proxy
TLS (SSL)
TCP*
TCP*
TCP
TCP
IP
IP
IP
IP
Wireless
Wireless Wired
• Transport Layer Security is optional • TCP* is the Wireless Profiled TCP • The WAP proxy is optional
Wired
Wireless Profiled TCP •
TCP design assumes that packet loss is due to congestion – This design assumption affects the protocol implementation, not the PDUs
•
IETF working group Performance Implications of Link Characteristics (PILC) has studied TCP over other than traditional networks – TCP implementations can be optimized for certain types of networks without affecting the interoperability – E.g. larger initial window and MTU, selective acknowledgment, path MTU discovery
•
WP-TCP takes advantage of PILC work – The TCP implementation in the WAP device can be changed – The server side TCP implementation can not be changed, but a WP-TCP proxy can be added to the data path
WAP Proxy • WAP 2.0 can use HTTP directly – The proxy is no longer required
• The proxy can perform useful tasks – Feature enhancements – Location, privacy, presence – Wireless Profiled TCP – Content translation – WML to binary format – WAP push – Content compression
Backwards Support • WAP 1.x protocol stack can co-exist independently with WAP 2.0 stack – Protocols should not be mixed between 1.x and 2.x WAP Device Wireless Application Environment WAP 1.x Stack
WAP 2.x Stack
Wireless Application Environment • •
A service environment Microbrowser – Wireless Markup Language (WML) – + WMLscript – XHTMLMP – Extensible Hypertext Markup Language Mobile Profile – Replaces WML (Wireless Markup Language) in WAP 2.0
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Wireless Telephony Application (WTA) – Integrates WAP and the telephone part of the handset
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Several content formats and services – vCard and vCalendar – SyncML data synchronization service
New WAP 2.0 services • WAP Push – A push type extension that can send a message from the WAP proxy to the WAP device – Used by the MMS service
• User Agent Profile – Based on the Composite Capabilities / Preference Profiles work by the W3C – Sends servers and proxies information about the terminal capabilities
• Wireless Identity Module WIM – SIM like model and hardware device for providing cryptographic services
WAP 2.0 Future • • • •
WML or XHTML? WAP stack or TCP/IP? Something else? Currently nobody knows
• What is in the standard is not what is getting implemented – Not all services will be available
i-Mode •
Introduced by NTT DoCoMo in Japan 1999 – A closed system developed for one operator
• • •
IP datagram transmission over existing PDC telephone system A scaled down version of HTML, cHTML language to create pages A winning business model – Simple services for a monthly low fee, ~ 1-2 € – A large content producer base, of which a few successes – Customer need for a "national language Internet" – Traditional Internet not very popular – User friendly handsets
Wireless LAN •
After wired LANs, e.g. Ethernet, became popular, wireless options were requested – Easier network installation – Laptops imply the need for limited mobility – Full mobility, like the cellular systems provide, is usually not as important
•
Currently the dominant standard is the IEEE 802.11 family – An Ethernet-like local radio network with a range of ~60 m and data rates of 1 - 54 Mbps – The 802.11b (2-4 Mbps) and 802.11g (~20 Mbps) standards are currently significant
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WLAN business – Hotspots (APs) in restaurants, cafes etc. – Free, or charging can be based on credit card numbers or telco SIMs (connection to HLR/AuC in GSM/3G network)
802.11b • A.k.a. Wi-Fi (a marketing name of Wi-Fi alliance, which certifies products) • Operates at the 2.4 GHz band – License free in most countries – Exact spectrum depends on national allocation, equipment needs to know in which country it operates – Also 100mW in Europe, 1W in USA
• Maximum bit rate 11 Mbps – 1, 2 or 5 Mbps in practice
• Now being superseded by IEEE 802.11g – 20+ Mbps, up to 54 Mbps – in the 2.4 GHz band.
Modes •
Ad-Hoc – No structure – All nodes connect point to point – No relaying – Called Independent Basic Service Set
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Access Point mode – – – – –
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Infrastructure base station All traffic from a node to the access point A single AP is called Basic Service Set The AP is configured to operate on a certain channel The stations scan the channels looking for the AP
Both access points and stations in ad-hoc mode are identified by SSID (Service Set Identification) – Commonly referred to as "network name"
The Hidden Node Problem •
CSMA/CA – Carrier Sense Multiple Access With Collision Avoidance – The 802.11 WLAN family is a lot like the Ethernet (802.3) – The Collision Detection in the Ethernet CSMA/CD has been replaced with Collision Avoidance
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Two stations can see the AP, but are located too far to sense each other's transmissions – On the co-axial Ethernet all stations can sense each other
• • • •
If the stations transmit simultaneously, the AP can not resolve the colliding data Thus a station sends first a Request To Send (RTS) to the AP The AP replies with a Clear To Send (CTS) Also, the data is acknowleged (ACK) whe received
Handovers •
If several APs provide access to a same IP subnet, the node may select a new access point while keeping its IP address – In WLAN terminology this is called "roaming" – Compare to cellular roaming, which means that a mobile station is visiting a different network – Called Extended Service Set (ESS) – Must be activated at the APs
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Access point re-selection is usually activated by the stations software, when it detects that the AP signal is weak – The station scans the channels for a stronger AP with same SSID
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Current WLAN technologies do not support true mobility
WLAN Security • •
Radio networks are by default unsecure Simple access can be limited based on the station's MAC address – This can be circumvented by attackers
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Originally a Wired Equivalent Privacy (WEP) protocol was supposed to provide security – A shared secret of 40 or 128 bits between the stations and access point – Several major problems have been found in the WEP
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802.11i is a new mechanism, – Wi-Fi Protected Access (WPA) is a marketable subset of this
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Many prefer to use IPSec, SSH, SSL and other higher level security mechanisms
Other Wireless Networks • Bluetooth – A Personal Area Network (PAN) – Range of ~10 m, connects personal devices – Data rate of ~ 1/2 Mbps
• IEEE 802.16 family – – – –
Broadband Wireless Access (wireless T1) Range of ~ 20-50 km WiMAX is the industry advocacy group for these Might be of interest to for example the rural areas