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research less than 10 years ago, the Internet was now a natural part of work as well as spare time.
The Internet Protocols in Wireless
The advent of the mobile Internet in the late 1990s created content that people could previously only access at certain fixed locations but now was potentially available everywhere. While the thought of reusing as much as possible of the existing Internet infrastructure and protocols was beneficial in many ways, it also created some problems. The protocols and content were created with user models in mind that were not always applicable to mobile users. Although many of us will use our laptop to access the same content wirelessly as from our desktop PC, the majority of the mobile Internet use is from much smaller handsets and with slower links. Therefore, the mobile Internet application developer needs to be aware of how the Internet protocols behave in a wireless environment.
The OSI Model for the Internet
After using TCP/IP for a while, people realized the importance of a flexible protocol stack. In the mid-1970s, the Open Systems Interconnect (OSI) model emerged. This model divides communication into layers. Each layer fulfills certain tasks and makes different combinations for different applications. While the OSI model might be too basic for advanced developers, we will quickly refresh your memory. Figure 6.1 shows the generic OSI model and how we can interpret it for the Internet.
The different layers of the Internet model are as follows:
Network interface layer. This layer is where the actual bits are transported and the hardware addresses (such as
Ethernet) for the physical host computers are specified. The network interface layer formats packets and sends them via the underlying network. For the mobile Internet, this layer includes the air interface.
Internet layer. This layer is equivalent to the network layer in the OSI model and primarily includes IP. IP addresses make it possible to locate the destination host and to send packets to it without having to be on the same subnet. A Domain Name Server (DNS) makes it possible to translate an easy-to-remember address, such as www.ibm.com, to the IP address that specifies where the IP packets should be sent.
Figure 6.1 The generic OSI model compared to the Internet model.
Transport layer. Now that a packet has reached the host computer, it needs to know which application we want. A port number of 80 specifies that we want to talk to the Web server (HTTP), and 21 specifies a File Transfer Protocol (FTP) server, and so on. Another important feature of the transport protocols is the capability to deliver the packets reliably, in the right order, and at the appropriate speed. TCP performs all of these tasks while User Datagram Protocol (UDP) just delivers the packets as they come (without caring about how many get through). This feature is good for real-time applications, however.
Application layer. The user usually is not faced with any parts of the protocol stack except for the application layer protocols. Here, we find FTP, HTTP, and other protocols that format the content and deliver it. The application layer corresponds to all three layers at the top of the OSI model: the session layer, the presentation layer, and the application layer.
Internet Protocol (IP)
IP transports packets to the desired destination host on the network. IP is a connectionless protocol and is not aware of any sessions. Every packet is routed independently, and different parts of the same transmission might take a different route. Along the way a packet might be lost, corrupted, duplicated, or delivered out of sequence (in other words, the first packet that is sent might not be the first one that is received). If the underlying network is not capable of transmitting packets as large as those that higher layers try to get IP to send, IP will fragment the packets in order to fit the network. For example, if someone tries to send a 2,300-byte packet over an Ethernet network that only can handle packet sizes of 1,500 bytes or fewer, the 2,300 bytes will be segmented into two IP packets. Incidentally, IP sends packets by using the best-effort principle, and whatever gets lost or received out of sequence is the responsibility of the higher-layer protocols.
IP is definitely the future, and we will see mobile networks comprising an everincreasing degree of pure IP. For a protocol that was specified in the late 1970s, IP has proved to be a versatile and scalable part of the TCP/IP protocol suite. The main challenge with IP today is that the currently used version, IP version 4 (IPv4), has too limited an amount of available addresses. Just with the growth of the fixed Internet, the addresses are predicted to run out rather soon. The lack of IP addresses is already today a limiting factor for the growth of some applications. With the advent of the mobile Internet (and its predicted 600 million users in 2004), it will be tough to find any free addresses. Also, today the distribution of IP addresses is not really fair (with one university getting more IP addresses than the entire Republic of China).