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The i-mode Network and Delivery Protocols

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This excerpt provides a broad overview of the planning that went into building the NTT DoCoMo i-mode network and presents the server architecture and data-delivery protocols that are used. If you plan to build an i-mode Web site, here are the unique i-mode factors you should take into account.
This chapter is from the book

This chapter is from the book

In This Chapter:

  • Building the NTT DoCoMo Innovative i-mode Network

  • Delivery Protocols

  • Content Servers and Service Providers

  • Case Study: Tokyo-Mitsubishi TD Waterhouse i-mode Site

This chapter provides a broad overview of the planning that went into building the NTT DoCoMo i-mode network and presents the server architecture and data-delivery protocols that are used. This chapter assumes that you plan to build an i-mode Web site, and it considers the unique i-mode factors you should take into account. Although this chapter relies heavily on the single (NTT DoCoMo) i-mode network built so far, keep in mind that the technical details of future i-mode services might vary greatly, depending on an individual operator's implementations.

Building the NTT DoCoMo Innovative i-mode Network

The innovation behind i-mode lies chiefly not in the service's technology, but rather in the business model, which is based on always-on packet billing; serving as the billing agent; and providing a platform for third-party mobile service providers (MSPs) to access subscribers and earn revenues.

Although i-mode was formally launched in February 1999, the service's antecedents go back to 1994 and are linked to such unlikely factors as narrow-minded domestic regulatory policies, pressure from the U.S. government to open up the cellular industry, and corporate desire to reestablish Japanese telecommunications expertise as a significant force overseas and in the Internet space.

Personal Digital Cellular Voice Network

The NTT DoCoMo Personal Digital Cellular (PDC) voice network, developed in Japan, is a variation of the TDMA standard. PDC offers high spectral efficiency and provides six half-rate or three full-rate channels in a 25kHz frequency band. PDC transmits voice conversations as digital data at 5.6Kbps or 9.6Kbps and features several advanced services, such as caller ID, prepaid calling, personal numbers, and wireless VPN (virtual private network). A wireless VPN allows subscribers in different locations to communicate on a closed conference call. Typical cell sizes range from 1.5–10 kilometers, indicating that PDC requires much more powerful base stations and handsets than PHS, for example, whose cell sizes are typically about 100 meters.

The PDC network operates in the 800MHz and 1500MHz bands, although most traffic is carried in the 800MHz band, where the company has two 29MHz band allocations. In the United States, AT&T Wireless (DoCoMo's U.S. i-mode partner) operates with an average bandwidth of 32.9MHz (it varies by region). As mentioned in Chapter 2, "Handset and Service Features of i-mode," i-mode data traffic is carried on a packet-switched overlay, established by co-locating additional packet-handling hardware at each of the cell base stations. Nothing is particularly special about the PDC network that makes i-mode possible. Services similar to i-mode could be provided over any network that is capable of handling packet data.


In 1993, when the NTT DoCoMo's PDC system was launched, DoCoMo billed approximately JPY95 per minute for voice calls (that's about $0.789 cents per minute!), and basic monthly subscription plans started at JPY8,000 ($66.0068) per month.

Genesis of i-mode

In 1994, Japan's cellular sector was moribund. Government regulations mandated high tariffs and restricted cell phone ownership (the handsets could only be rented); in the 15-year period from the 1979 start of commercial cellular services (the world's first), only 2.1 million subscribers had signed up. Furthermore, Japan was reeling from the collapse of the asset-inflated bubble economy, and the tech sector was in a funk. After setting the world standard throughout the 1970s and 1980s in autos, consumer electronics, and manufacturing technology, Japan had entirely missed the digital revolution. The American Wintel duopoly (Windows and Intel) was unassailably in the lead, and Japan couldn't compete in the open-standards–based, hyper-innovative Internet space.

Ironically, it was America that helped spark Japan's mobile telecom revolution. In 1994, the U.S. government negotiated the Cellular Telephone Agreement with Japan, leading to the removal of handset ownership restrictions and greatly reduced tariffs, which led to a tenfold increase in cellular services by 1996, according to the American Chamber of Commerce in Japan.

Engineering Pride

Meanwhile, NTT DoCoMo's engineering pride and unwillingness to use European or U.S. cellular systems ensured that Japan was (and still is) the only country in the world using the PDC TDMA-based standard. This situation led inevitably to NTT paying a significant premium to infrastructure and equipment manufacturers (which have included NEC, Ericsson, Fujitsu, and Motorola), who have little incentive to compete on price. At the same time, the number of Japanese mobile subscribers started to rise rapidly. By December 1996, subscribers totalled 18 million, and this number more than doubled to 38.9 million in December 1998.

Ray Tsuchiyama, Japan country manager for Tegic, a developer of intelligent text input systems for cell phones, says, "In 1997, frightening scenarios were aired about the collapse of the PDC network by 2001. Too many subscribers were making calls simultaneously, which meant that available 'time slots' fell short of demand. To expand capacity in the mid-1990s, NTT DoCoMo had to spend over $6 billion on infrastructure. Even after that buildup, by the late 1990s, NTT had more consumer complaints about voice quality and dropped calls. Advertisements by [competitors] stressed [their] better voice quality."

As a result, DoCoMo faced the unsavory prospect that it would have to borrow even further to expand its decade-old PDC infrastructure while its market share fell even further to competitors that were fielding modern, reliable CDMA networks. "The entire network would be obsolete before the debts were paid off," says Tsuchiyama.

The answer was to launch data-based services that would help boost subscriber demand and usage while relieving the network of some of the voice burden. The goal was to buy time in which to amortize the existing infrastructure investment while postponing the need to build an entirely new network (that would have to wait for the third generation). Of course, pride again played a role. If, as many observers agreed, Japan had lost the first rounds of the PC-based technology and Internet revolution, maybe telecommunications—and, specifically, high-speed cellular networks integrated with Net access—would be the arena in which Japan could stage a comeback. If that comeback happened to be led by a resurgent NTT, or NTT DoCoMo, so much the better. The i-mode system was launched on February 22, 1999, and the rest is history.

The i-mode Business Solution

The key importance of i-mode, however, is not the relatively unsophisticated technology used to build the platform. It is instead i-mode's solution as a business model, which features always-on packet billing, serving as the billing agent, and providing a platform for third-party MSPs to access subscribers and earn revenue. i-mode is the kernel around which a growing number of application providers, software developers, and service providers are coalescing, attracted by the phenomenal subscriber numbers. This situation is in turn helping i-mode morph into a media platform on which a multitude of subscribers and businesses are being connected for e-commerce, consumer and corporate services, and the provision of entertainment, personal networking, and social interaction.

i-mode: Leading 3G Networks

With the adoption of the global-standard W-CDMA protocols for third-generation (3G) networks in Europe, the United States, and Japan, NTT DoCoMo will have a significant lead over any other cellular carrier in building, operating, and monetizing a wireless information service. As John Ratliff wrote in a September 2000 research paper submitted to the Telecommunications Policy Research Conference:

"Since the emergence of e-commerce, the challenge has been to develop a business model that actually results in profitability. Most Internet portals and other service providers have attempted to make money through advertising, or through marketing products online. DoCoMo... focuses on increasing network revenues like airtime fees and commissions for collecting fees for value-added services. In other words, DoCoMo has generated revenue directly out of its core competencies as a mobile telecommunications carrier: building wireless networks and growing their subscriber bases. DoCoMo has successfully rolled out a mobile Internet service, gaining valuable technical and business experience, all in a low-band[width] environment. This has placed DoCoMo in an excellent position to provide leadership in rolling out similar services in the coming broadband 3G networks." (Source: DoCoMo as National Champion: The i-mode, W-CDMA, and NTT role as Japan's pilot organization in global telecommunications, by John Ratliff, at http://www.tprc.org/abstracts00/docomopap.pdf)

Whether i-mode will allow Japan's high-tech sector to gain dominance or even a share in the open-standards–based international Internet business space is still an open question. As you'll see in the next chapter, the DoCoMo lead in 3G has recently been called into question. i-mode represents the best chance yet for the Japanese to regain global technology leadership.

The i-mode Gateway

NTT DoCoMo refers to its i-mode gateway, somewhat ironically, as GRIMM, for Gateway Service Representative Internet Market Mobile Access Exchange. GRIMM serves as the relay point between the packet transmission overlay portion of the PDC voice network and the Internet.


DoCoMo as National Champion

NTT DoCoMo is viewed within the Japanese government, bureaucracy, and senior corporate circles as the leading hope if Japan is to have any success in the Internet space. In a February, 2002, TIME magazine article, Japanese Diet member Hiroyuki Arai was quoted as saying: "Our mobile-phone technology is ahead of the rest of the world for now. DoCoMo is our flagbearer. If the company takes its time getting into the global arena, we will lose our lead to American or other foreign companies. Without that kind of commitment from Japanese companies, our economy will never recover."

Gateway Architecture

The overall organization of GRIMM, at the time of the i-mode launch in February 1999, is shown in Figure 3.1.

Figure 3.1 The i-mode gateway as initially deployed by NTT DoCoMo in 1999.

The gateway's major functions are summarized in this list:

  • Serve as the gateway that relays packet data from the i-mode Mobile Station (cell phone) to public and private Web servers

  • Provide mail storage for i-mode mail (SMTP mail), pending download to the Mobile Station

  • Manage Mobile Station client-management services, including managing subscriptions to for-fee i-mode sites and IP packet management

  • Track and collect IP packet usage data for individual Mobile Stations, thus enabling per-packet billing

Note that the discussion in this section refers to the logical architecture of the gateway and not to the physical layout. Since i-mode's launch, the physical organization of GRIMM has undergone a number of upgrades because of i-mode's tremendous use. The Tokyo server center has been decentralized, and subcenters exist in Yokohama and Haneda as well as in several other locations. NTT DoCoMo is understandably reluctant to provide firm details for security reasons.

The forward link of the GRIMM gateway is connected to plural Message Packet Gateway (M-PGW) Modules, which provide the links to the cellular network, and the rearward link communicates with the Itemization Center (for billing), the Customer Center (for subscriptions), a Maintenance Terminal, and public and private Web servers. Internally, GRIMM comprises functionally separate server modules that cooperate to provide specific i-mode services. This functional separation permits load sharing and is scalable according to customer demand and growth (each module can comprise multiple servers operating in parallel). These functional modules are described in Table 3.1.

Table 3.1—Gateway Functional Server Modules1

Server Module

What It Does

Database Mobile Access Exchange (D-MAX)

Allows the collection and analysis of marketing data based on i-mode site traffic, service use and other metrics.

Interface Mobile Access Exchange (I-MAX)

Connects to M-PGW terminals and to other server modules and performs load balancing using a round-robin method.

Mail Mobile Access Exchange (M-MAX)

Transmits SMTP mail to and from the Mobile Station (MS) and stores the mail before it's downloaded to the MS.

Name Mobil Access Exchange (N-MAX)

Manages mail account usernames selected by subscribers, who can freely change mail addresses on a first-come, first-served basis; also stores subscriber PIN codes used for mail and site access.

User Mobile Access Exchange (U-MAX)

Serves as a master database of i-mode subscriber information, including passwords; updated from Customer Center once every 24 hours.

Web Mobile Access Exchange (W-MAX)

Acts as the local content server for i-mode; provides the default i-mode menu displayed on the MS, stores individual subscribers' My Menu pages, and caters for regional specialization.

1Source: NTT DoCoMo Technical Journal, Vol. 1, No. 1. October 1999. Some server modules omitted for clarity.

Push and Pull Delivery

The i-mode gateway provides two types of content delivery: push and pull. Pull delivery responds to requests from the MS for Web site content, mail, and personalized My Menu registrations. Push delivery is intended to serve mobile service providers (MSPs) that want to provide information and content download to subscribers. Subscribers can register to receive free or for-fee text messages on a push basis. The gateway can also push administrative information and other notices to subscribers.

Typical Transaction: Site Registration

The MS transmits a request via an M-PGW to I-MAX. I-MAX relays the request to W-MAX (which stores subscriber My Menu pages), N-MAX (to access subscriber PIN code), and U-MAX (to look up subscriber info). After verifying that the subscriber's information is valid, I-MAX queries the MS for the PIN code (which the subscriber is prompted to enter). I-MAX, W-MAX, and N-MAX then cooperate to verify the PIN and, if it's valid, allow the registration. The three servers then send a registration notification to the MS.

The New Gateway

In May 2001, Tokyo-based technology companies NTT Data Corp. and NEC Corp. announced that their proposal to NTT DoCoMo to construct a new i-mode Gateway Server system had been accepted. The new gateway, a large-scale system, specifically designed for high capacity and future expansion, is based on Unix servers and other yet-to-be-announced open standard products (see Figure 3.2). NEC confirmed that, although no firm plans are in place yet, the contract would open up the possibility of NEC and NTT Data cooperating in the future to build i-mode gateways for operators other than DoCoMo.

The new server center can also be operated on an outsource basis by the two contractors, relieving DoCoMo of the burden of managing every aspect of the i-mode system. No date for the new gateway's launch was announced, although a late-2002 timeframe seems reasonable.

Figure 3.2 The proposed new i-mode gateway will be built jointly by NEC and NTT Data and will provide i-mode services to both the existing PDC network and the new DoCoMo 3G W-CDMA network. Copyright© NEC Corporation

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