|
by
Kostas Pentikousis, VTT
Most IPJ readers are familiar with Wireless Local-Area Networks (WLANs; see, for
example, IPJ Volume 5, No. 1). Some may even be familiar with recent
developments in Wireless Metropolitan-Area
Networks (WMANs), such as WiMAX. Although nonproprietary WMAN
technologies are still in the standardization phase, the IEEE 802.11 family
of protocols has reached maturity and rendered inexpensive (and often free)
WLAN access increasingly popular. Both WLANs and WMANs
provide high-speed connectivity (in the order of tens of Mbps), but user
mobility is restricted. In fact, it is probably more appropriate to talk
about “portability” rather than “mobility” [1] when referring to WLANs and
WMANs.
Wireless wide-area networks (WWANs), on the other
hand, allow full user mobility but at data rates typically in the order of
tens of kbps. This will change to some extent when third-generation (3G) cellular networks
are fully deployed. Still, 3G deployment is slower than originally
anticipated, a development often attributed to the combination of high
spectrum license costs, the recent economic downturn, and high equipment
costs. As a result, both population and geographical coverage tend to be
uneven. For example, in Finland, a forerunner in wireless communications,
population coverage is well below the 35-percent level, and geographical
coverage is even smaller.
This article introduces several wireless network technologies,
perhaps not so widely known, which deserve attention when considering how to
provide mobile connectivity to field personnel, introduce machine-to-machine (M2M) communication,
or deploy applications that require always-on connectivity. The approach
taken in this article is a bit different from the one typically followed in
the literature: We focus more on higher-level issues, the information
that is essential for application developers, instead of
modulation, channel coding, and other lowlevel details. Unlike WLANs and
WMANs, none of the networks surveyed provide data rates in the order of tens
of Mbps. Nevertheless, successful applications can be
built even with stringent bandwidth limitations. For example, online gambling
and several gaming applications can be served by really “thin” networks (and
possibly “thick” clients).
The Global System for Mobile
Communications (GSM) specifies a cellular, wide-area,
circuit-switched, digital mobile phone network architecture [2].
Circuit-switched networks such as GSM and IS-95, commonly referred to as Code Division Multiple Access (CDMA) in the United
States, can provide wireless data connectivity,
cover a large area, and handle mobile host handovers efficiently [3]. Users
can transfer data over, say, GSM, by establishing a “dialup” connection [4].
Mobile hosts can roam, even at high speeds, and remain connected throughout.
Communication is full-duplex at a radio data rate of 9.6 kbps or 14.4 kbps in
GSM Phase 2+ [5]. User throughput is always smaller than the nominal radio
data rate.
While the user is connected using a wireless circuit-switched
network, phone calls cannot be initiated or received whether data is being
transferred or not. This is not much different from wire-line dialups over
basic telephone service. The difference is that a dialup over a Public Switched Telephone Network
(PSTN) takes up a resource, namely the wire-line local loop, which is
dedicated to a single user, whereas a dialup over a cellular network such as
GSM consumes a resource, the radio channel, which is shared among many users.
Because of the burstiness that data traffic
usually exhibits, circuit switching may lead to inefficient use of the
network capacity. Establishing a GSM dialup connection usually takes several
seconds, meaning that if the user has a small amount of data to send, a small
e-mail message, for example, the overall experience is
poor. Moreover, after the connection is established, the channel remains idle
between traffic bursts and the allocated bandwidth is wasted. Packet
switching is more efficient for bursty data transmission over a shared
medium [6].
Another variable that favors packet-switching over circuit-switching,
especially over slow wireless networks, is billing. Users of circuit-switched
networks are usually charged based on the duration of a connection regardless of the amount of
traffic transmitted or received. On the other hand, users of packet-switched
networks can be charged based solely on the amount of data transferred—not
how long they remain attached to the network. In short, introducing packet
switching to wireless networks can lead to
better use of network resources and attract more users as data transfers
become more economical.
Two-way, packet-switched WWANs permit users to roam freely indoors and
outdoors, even at relatively high speeds [7]. Most WWANs employ a cellular
architecture to take advantage of frequency reuse and increase capacity while
covering a larger area. Furthermore, because the coverage area of a single
cell is generally large (cell diameters are typically in the order of dozens
of kilometers), mobile hosts do not have to go through frequent and lengthy
handovers. Hosts remain connected throughout after they attach to the
network, permitting users to receive and transmit data on demand without
having to dial up. The following sections survey some of the most widely
deployed packet-switched wireless data networks.
Mobitex is the first digital data-only WWAN developed by Ericsson and Swedish Telecom.
Not based on IP, Mobitex was introduced in Sweden in 1986 for emergency
communications [8]. It uses a cellular architecture with cell diameters of up
to 30 km. Each service area can operate 10–30 channels [9] and each base
station is usually allocated 1 to 4 channels. Each channel is composed of a
frequency pair: different frequencies are used for the uplink and the
downlink.
Communication between the base station and a single mobile host is,
nevertheless, effectively half-duplex. Although base
stations can transmit and receive simultaneously, mobile nodes are unable to
do so [10]. The Mobitex Maximum Transmission Unit (MTU) is 545 bytes,
with up to 512 bytes of user data. Although the system has undergone several
revisions, the raw transfer rate remains only 8 kbps. Effective user
throughputs range from 4 kbps (for 125-byte packets) to 4.6 kbps (for
512-byte packets) [11], and round-trip times can be up to 10 seconds.
Mobitex deals with network lapses using a store-and-forward procedure:
Packets destined for a mobile node outside the network coverage area are
stored while awaiting delivery. When the mobile node reconnects, the stored
packets are delivered. Mobitex uses a hierarchical routing architecture that prevents
local traffic from being injected into the backbone network. In other words,
packets destined for a node in the range of the same base station are
switched locally [8]. Besides supporting unicast addressing, Mobitex allows
hosts to send one packet to several recipients [10]. According to the Mobitex Association (www.mobitex.org),
the technology features “true push functionality,” whereby data can be pushed
to both a single mobile node and a predefined group of nodes, a feature that
can be very useful when trying to send an urgent message to field personnel.
And, because the mobile host does not have to keep querying for pending data,
network traffic can be kept to a minimum. All these features can also
significantly boost battery life.
According to the Yankee Group, despite the limited data rates, a variety of
applications have been developed based on Mobitex, including: burglar and
fire alarm systems; paging, interactive messaging, e-mail,
form-based applications, and access to databases;
telemetry; credit card authorizations; field service; and fleet management.
Virtually all of them require small and bursty transfers. Mobitex does not
lend itself to large file transfers, e-mail with large attachments, or video
transmission. In fact, file transfers of more than 20
KB used to be discouraged [8]. On the other hand, by using a slotted ALOHA
[12] variation for channel access, Mobitex can provide
message delivery delay guarantees and support
hundreds of users within the same cell. Parsa [13] calculated that Mobitex
can accommodate 2,000 users per channel, assuming two uplink and
two downlink messages per hour. Other
networks simply cannot provide tight delay bounds for such a large number of
users. For example, the Mobile Data
Magazine (No. 1, 2002) reported that a Korean operator launched
real-time stock trading and horse gambling mobile applications with great
commercial success, by guaranteeing delay
bounds notwithstanding the low data rates.
DataTAC (also known as ARDIS in the United States) was developed by Motorola
in the mid-1980s. DataTAC is also a non-IP based, widearea, data-only message-oriented network. A
single base station can cover an area exceeding 20 km in diameter [14]. Like
Mobitex, communication between the base station and a single DataTAC mobile
node is half-duplex, and mobile hosts have to compete to get access to transmit and
receive data.
Unlike Mobitex, DataTAC was designed to provide optimal in-building coverage,
and it uses a cellular architecture that does not take advantage of frequency
reuse. Instead, a single frequency is used, increasing the probability that a
packet transmission is successful (because the same
transmission can be picked up by more than one base
station), but at the expense of network capacity [8]. Bodsky notes that the
U.S. DataTAC operator formerly recommended refraining from transferring files
larger than 10 KB.
Although neither Mobitex nor DataTAC provides native IP support, middleware
can take care of protocol translation and allow unmodified, off-the-shelf
applications to communicate. The maximum Data-TAC message size is 2048 bytes
[15], but the maximum over-the-air packet size depends on the link layer. For
rural areas the maximum radio data rate is 4.8 kbps, and the maximum
over-the-air packet size is 256 bytes. In metropolitan areas, the radio data
rate is 19.2 kbps and the maximum packet size is 512 bytes [16]; end-user
throughput does not exceed 10 kbps on average. Traditionally, DataTAC was
used for dispatching and law enforcement applications. The Worldwide Wireless Data Network Operators
Group
(www.datatac.com)
reports that DataTAC networks are also used for two-way messaging, wireless email, telemetry, access to corporate
databases, and package tracking by courier carriers.
Cellular Digital Packet Data
(CDPD) was designed by IBM and McCaw Cellular Communications in the early
1990s to take advantage of channels that do not carry voice traffic in the Advanced Mobile Phone Service (AMPS),
the first-generation analog cellular network [17]. Data channels are
allocated dynamically, sharing the network capacity with AMPS voice traffic,
which is quite different from Mobitex and DataTAC. This, for example, might
mean that data can be transmitted and received only when phone calls do not
consume all available capacity. One could argue that CDPD considers data
traffic less important than voice. However, the standard
allows network operators to specifically assign channels to data
traffic only. In theory, deployment can be more economical than it is for
other WWANs because CDPD takes advantage of existing AMPS infrastructure and
does not require licensing new spectrum. Original projections anticipated
that as CDPD gained popularity—and AMPS became obsolete—more CDPD dedicated
channels would be allocated. With time, CDPD would have taken over the
existing AMPS bandwidth, effectively becoming a data-only WWAN.
CDPD is based on a Carrier Sense Multiple
Access (CSMA) variant called Digital Sense Multiple Access [14] and transparently
provides IP services, constituting a great advantage. CDPD allows for an MTU
of 2048 bytes. However, one has to account for the TCP/User Datagram Protocol (UDP) and IP
headers that are used to encapsulate the application payload before sending
it over the CDPD network and also for the fact that CDPD user data is
transmitted in much smaller blocks. Although the CDPD raw data rate is 19.2
kbps, the effective throughput is in the order of 10 kbps and response times
have been reported to be in the order of 4 seconds [18].
The General Packet Radio Service
(GPRS) is overlaid on a GSM network in a fashion similar to the way CDPD is
embedded in AMPS: Voice and data traffic share the same bandwidth and network
infrastructure [14]. In other words, GPRS is an add-on to GSM networks, and
it requires certain hardware and software upgrades and introduces packet
switching to a circuit-switched architecture. GSM voice traffic is oblivious
to the presence of GPRS data traffic. Similar to CDPD, GPRS is designed to
appear as a regular IP subnetwork both to hosts attached over the air
interface and to hosts outside the GPRS network.
The GPRS standard was finalized by the European
Telecommunications Standards Institute (ETSI) in late 1997 as
part of GSM Phase 2+ [5]. It is regarded as a transitional technology toward
3G networks [19], and is commonly referred to as 2.5G. One of its main advantages
is that the same device can be used to transmit and receive data, and
initiate and accept phone calls. GPRS defines three classes with respect to
simultaneous usage of voice and data. Class A mobile hosts can
transmit and receive voice and data at the same time. Class B hosts can transmit
and receive either voice or data but not both simultaneously. Finally, class C hosts have the user
manually select if the host should be attached to the GSM (voice) or GPRS
(data) network. When compared to Mobitex, DataTAC, and CDPD, GRPS class A devices can have
simultaneous access to a packet-switched
and circuit-switched network. Of course, GSM-only devices do not have this
capability either, as mentioned earlier.
GSM uses a combination of Frequency
Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) for channel
allocation, as explained in detail in [5]. In short, each frequency channel
carries eight TDMA channels. Each of these channels is essentially a time slot in
a TDMA frame. Thus, any GSM frequency channel can carry up to eight
circuit-switched connections with each slot reserved for a single connection
(read voice call). In GPRS,
each slot is treated as a shared resource and any mobile host can use it to
transmit or receive data. In addition, a mobile host can be allocated more
than one of the eight available slots in the same TDMA frame. In other words,
GPRS can multiplex different traffic sources in one channel and allocate
several channels to the same traffic source.
GPRS defines four different channel coding schemes [20], namely CS1, CS2,
CS3, and CS4, with radio data rates 8.8 kbps, 13.3 kbps, 15.6 kbps, and 21.4
kbps, respectively. CS1 is the most “conservative” (includes more error
correction bits) and is used for signaling packets and when poor channel
conditions prevail. CS4 is the most “optimistic” (includes minimal error
correction bits), and, assuming excellent channel
conditions, allows operators to advertise a maximum radio data rate of 171.2
kbps per 200-kHz frequency channel (or TDMA frame).
In practice, CS4 is rarely used because it can lead to frequent retransmissions of lost packets
and overall network underperformance. CS3 is commonly used, providing 124.8
kbps per frequency channel. Because a mobile host can be allocated multiple
slots, user throughputs can range between 40 and 60 kbps. Mobile hosts
typically use an MTU of 1500 bytes.
Communication between the base station and any given mobile host is
full-duplex but can be asymmetric;
that is, the downlink and uplink capacities need not be the same. The GSM Association has defined 12
multislot classes for GPRS. Each class is associated with a maximum
number of uplink and downlink slots that can be allocated to a single mobile
host. The slot allocation is usually written as M + N,
where M is the maximum number
of downlink slots and N is
the maximum number of uplink slots. For example, class 1 is “1 + 1” (one
downlink slot plus one uplink slot); class 2 is “2 + 1”; . . . ;
and class 12 is “4 + 4” (four downlink and four uplink
slots). In addition, each multislot class has an active slot
constraint: A mobile host cannot use more than K active slots simultaneously.
Given the number of slots and the channel coding scheme, one can calculate
the peak rate. For example, for a class 12 device the sum of
the physical downlink and uplink rates cannot exceed 124.8 kbps, if CS3 is
used. However, the active slot constraint limits this rate even further. In
the case of a class 12 mobile node, K = 5,
that is, only “4 + 1”, “3 + 2”, “2 + 3”, or “1 + 4” slots can be used
simultaneously. See www.gsmworld.com
Enhanced Data for GSM Evolution
(EDGE), also known as Enhanced GPRS, builds on the changes introduced by GPRS
to GSM. EDGE essentially increases the radio data rates by
using a more efficient modulation scheme [21], namely 8-Phase Shift Keying (8-PSK) instead of
the Gaussian Minimum Shift
Keying
(GMSK) used by both GSM and GPRS. EDGE defines nine modulation coding schemes
named MCS1 to MCS9. MCS1 to MCS4 use GMSK with radio data rates similar to
the four GPRS coding schemes. The real throughput improvements come from MCS6
(29.6 kbps per slot) through MCS9 (59.2 kbps per slot). The data rate usually
associated with EDGE is a (shared) 384 kbps. This
corresponds to using MCS7 for all 8 TDMA slots. Higher data rates are
theoretically possible (up to 473 kbps
using MCS9) but are not commonly deployed.
EDGE improves not only on the high end of data rates but also on the low end
[22]. First, the greater diversity of coding schemes permits an EDGE network
to choose the most appropriate one depending on channel conditions. Changing
coding schemes is dynamic. Second, EDGE supports packet resegmentation: Packets that failed to be
transmitted successfully can be
resegmented and retransmitted using a more “conservative” coding scheme.
Table 1 summarizes the main high-level features for the WWANs surveyed.
|
|
Transmit/Receive
|
Radio Data Rate
|
User Throughput
|
MTU
|
|
Mobitex
|
Half duplex
|
8.0 kbps
|
<4.6 kbps
|
512 B
|
|
DataTAC
|
Half duplex
|
19.2 kbps
|
<10 kbps
|
2048 B*
|
|
CDPD
|
Full duplex
|
19.2 kbps
|
<10 kbps
|
2048 B
|
|
GPRS
|
Full duplex
|
<171 kbps
|
40–60 kbps
|
1500 B
|
|
EDGE
|
Full duplex
|
<473 kbps
|
50–60 kbps
|
1500 B
|
* Typically 512 B
Among the WWANs presented, Mobitex and GPRS can be singled out as the most
widely deployed; they also have enjoyed significant gains in the number of
users and traffic volume in recent years. The popularity of enterprise wireless e-mail (due in part to
the success of the Research in Motion BlackBerry devices)
allowed Mobitex and DataTAC operators to revive their business models briefly.
Worldwide, however, GSM dwarfs all other technologies: There are more than 1
billion GSM subscribers compared to the 1 million Mobitex users. DataTAC
enjoys an even smaller user base. Even if a small percentage of GSM subscribers
use GPRS and EDGE, the potential market for wireless applications is
tremendous. On the other hand, subscribers who do not take advantage of GPRS
or EDGE do use the inexpensive, (two-way) Short
Message Service (SMS), which is built
in GSM. Two-way messaging was available for
many years but was certainly popularized by less affluent and younger
GSM users in the late 1990s. SMS is now commonplace, and in many countries it
is more popular than e-mail. Dedicated data-only networks such as Mobitex have
to look elsewhere for their niche.
For some, Mobitex, let alone DataTAC and CDPD, is virtually moribund. In the
United States, for example, Cingular sold its Mobitex network and is
investing heavily on GRPS and EDGE. DataTAC and CDPD are phased out by service
providers in the United States in favor of newer technologies. Low-speed
packet radio is considered lackluster and is not popular with younger crowds.
After all, narrowband WWANs had their chance and failed to attract large
numbers of subscribers. Recent pricing trends, too, reveal a heavy operator
push in favor of GRPS and EDGE. In Finland, for example, 100 MB over GPRS
costs less than 18 euros (approximately $24). Compare
that to the $30–50 that 1 MB of traffic costs over Mobitex. Service and product
popularity create economies of scale that cannot be ignored.
Nonetheless, open standards, an explicit focus on business applications with Quality-of-Service (QoS) guarantees in
service response times, and narrowband M2M communication may well keep Mobitex
going for years to come. Besides, bundling Mobitex with a wireless network that features
fast and inexpensive connectivity, for example, WLAN or Bluetooth, might be
promising: Large downloads and software updates can be done over the
high-speed wireless network and critical
messages can always reach the user through the
WWAN.
Bundling several functions in a single handheld device is, after all, a major
trend in the industry. Vendors scramble to integrate Personal Information Managers (PIMs),
voice and data communications, as well as entertainment features (digital
camera, games, or digital music players) in a single product. This is quite
different from earlier mobile devices, which tended to be either
single-purpose or tied to a particular set of applications. Even the
BlackBerry devices still work, to some extent, in a closed architecture.
Enterprise e-mail systems need to be supported by and integrated with
BlackBerry servers in order to be accessible over the WWAN.
Yet, one of the main objectives in 2.5G and 3G is to allow mobile users to
use standard Internet protocols on a mobile radio network at significantly
higher bit rates than other systems. In particular, GPRS was designed with
certain office applications in mind and can support consumer and enterprise
mobile communications alike, without being tied to any given platform or
application servers. I expect that functionality bundling and 2.5G and 3G
WWANs will allow for more open systems and will expedite the transformation
of WWAN operators from integrated application providers to wireless ISPs.
[1] Charles Perkins, Mobile IP Design
Principles and Practices, ISBN 0201634694, Addison-Wesley, 1998.
[2] Joachim Tisal, GSM Cellular Radio
Telephony, ISBN 0471968269, John Wiley & Sons, 1998.
[3] Tero Ojanpera and Ramjee Prasad, WCDMA: Towards IP Mobility and Mobile Internet, ISBN B000066OB4,
Artech House, 2001.
[4] R. Ludwig, B. Rathonyi, A. Konrad, K. Oden, and A. Joseph, “Multi-layer
tracing of TCP over a Reliable Wireless Link,” presented at
ACM SIGMETRICS 1999.
[5] C. Bettstetter, H.-J. Vogel, and J. Eberspacher, “GSM phase 2+ General
Packet Radio Service GPRS: Architecture, Protocols, and Air Interface,” IEEE Communications Surveys & Tutorials,
Vol. 2(3), pp. 2–14, 1999.
[6] Larry L. Peterson and Bruce S. Davie, Computer
Networks: A Systems Approach, 3rd ed., ISBN 155860832X,
Morgan-Kaufmann, 2003.
[7] Rudi Bekkers and Jan Smits, Mobile
Telecommunications: Standards, Regulation, and Applications, ISBN
0890068062, Artech House, 1999.
[8] Ira Brodsky, Wireless: The Revolution in
Personal Telecommunications, ISBN 089006717, Artech House, 1995.
[9] Nathan J. Muller, Wireless Data Networking, ISBN 0890067538,
Artech House, 1995.
[10] A. K. Salkintzis and C. Chamzas, “Mobile Packet Data Technology: An
insight into Mobitex Architecture,” IEEE
Personal Communications, Vol. 4(1), pp. 10–18, 1997.
[11] M. S. Taylor, M. Banan, W. Waung, and M. Taylor, Internetwork Mobility: The CDPD Approach,
ISBN 0132096935, Prentice-Hall, 1996.
[12] Andrew S. Tanenbaum, Computer
Networks, 4th ed., ISBN 0130661023, Pearson Education, 2003.
[13] K. Parsa, “The Mobitex packet-switched Radio Data System,” presented at
IEEE PIMRC ’92, 1992.
[14] Sami Tabbane, Handbook of Mobile
Radio Networks, ISBN 1580530095, Artech House, 2000.
[15] J. Rodriguez, W. Schollenberger, M. Anzib, and B. Widyarso, Mobile Computing: The eNetwork Wireless Solution, ISBN 0738412856, IBM
Redbooks, 1999.
[16] Research in Motion, “Developer’s guide for BlackBerry and RIM Wireless Handhelds—Radio API
(DataTAC) Version 2.1,” 2001.
[17] John Agosta and Travis Russel, CDPD: Cellular Digital Packet Data Standards and
Technology, ISBN 0070006008, McGraw-Hill, 1996.
[18] P. Sinha, N. Venkitaraman, T. Nandagopal, R. Sivakumar, and V.
Bharghavan, “A Wireless Transmission Control Protocol
for CDPD,” presented at IEEE WCNC ’99, 1999.
[19] A. K. Salkintzis, “A survey of mobile data networks,” IEEE Communications Surveys & Tutorials,
Vol. 2(3), pp. 2–18, 1999.
[20] L. F. Chang, “Wireless Internet—Networking
Aspect,” in Wireless Communication
Technologies, New Multimedia Systems, N. Morinaga, R. Kuhno, and S. Sampei,
Eds., Kluwer Academic Publishers, 2000, pp. 215–244.
[21] Behrouz A. Forouzan, Data
Communications and Networking, 2nd ed. Update, ISBN 0072822945,
McGraw-Hill, 2002.
[22] Alexander J. Huber and Josef F. Huber, UMTS and Mobile Computing, ISBN B000089CJ3, Artech House,
2002.
KOSTAS PENTIKOUSIS, PhD, studied computer science at Aristotle University of
Thessaloniki and Stony Brook University. He is an
ERCIM Fellow at VTT, The Technical Research Center of Finland, and currently
resides in Oulu, Finland. For more about his research and publications visit:
www.cs.stonybrook.edu/~kostas.
The best way to reach him is via skype. E-mail: kostas@cs.sunysb.edu
|
|