The Internet Encyclopedia (Volume 3)

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WIRELESSINTERNET—ISITHAPPENING? 839

and SMS include much enhanced speed, “always-on”

connection, and spectrum efficiency.

Although the underlying GSM networks uses a circuit-

switched architecture, GPRS uses packet-switching tech-

nology to achieve “always-on” connectivity and higher

spectrum efficiency. All eight timeslots in a frame can be

used in GPRS to achieve the peak rate of 171.2 Kbps. Un-

like in CSD, the timeslots are not set aside to a user for

the duration of the connection. Instead, radio resources

are used only when data are actually being transmitted.

Thus, the same channel can be shared among many users

concurrently. This efficient sharing of the limited band-

width allows the network operator to maximize use of the

limited radio resources, and lower the cost for the mobile

users.

Since the existing GSM MSCs are based on circuit-

switching technology, they cannot handle the operation

of packet-switched GPRS connections. Thus, two new

network components called serving GPRS support node

(SGSN) and gateway GPRS support node (GGSN) have

been added to the GSM architecture. The SGSN behaves

much like the MSC, but for GPRS traffic. It is responsi-

ble for delivering packets to the mobile users in its service

area, and also handles queries to HLR for roaming opera-

tion. The GGSN is the interface to the external networks,

such as the Internet. It maintains address and routing in-

formation necessary to tunnel data packets to appropriate

SGSNs and onto the MSs. This makes it possible to use

existing IP applications over the GPRS network. This and

other advantages of GPRS already mentioned may help

initiate the long-awaited proliferation of wireless Web ac-

cess.

HDR

HDR is based on CDMA technology and achieves a peak

data rate of 2.4 Mbps. Instead of sharing the channel with

voice data as is done in GPRS, in HDR the entire fre-

quency channel is allocated to data traffic. By decoupling

the data service from voice service, the overhead required

to meet strict latency requirements of voice calls no longer

degrades the system’s ability to handle packet data effi-

ciently.

Large improvement in spectrum efficiency is achieved

by measuring the signal-to-noise-plus-interference ratio

between the BS and the MS, and adapting the modulation

scheme and the forward error correction to achieve the

optimal efficiency for the given channel condition. Since

only a single user is served at any given time (in units

of 1.67-ms packets), there is no degradation in capacity

due to inference among the MSs. In addition, by taking

advantage of varying and more relaxed latency require-

ments of data traffic, further increase in throughput can

be achieved (Bender et al., 2000).

The network architecture of HDR has been designed

with the Internet in mind. Selection of the point-to-

point protocol (PPP) (Simpson, 1994) and the PPP mul-

tilink protocol (MP) (Sklower, Lloyd, McGregor, Carr, &

Coradetti, 1996)were based on the need to support IP traf-

fic with different QoS requirements while utilizing low

overhead. In addition, the radio link protocol (RLP) has

been designed to achieve the level of data fidelity, i.e.,

bit error rate, which PPP and IP experience in wireline

networks. This is important since many upper layer pro-

tocols, such as transmission control protocol (TCP), had

been designed and optimized for the conditions observed

in wireline networks. Such network architecture and en-

hanced data rates are expected to improve and enhance

the usability of wireless data services.

WIRELESS INTERNET—IS IT

HAPPENING?

Mobile IP (MIP)

Mobility is not a feature that was incorporated into IP

when it was conceived several decades ago. Mobile IP con-

sists of the necessary extensions needed to support mobil-

ity in the Internet (Perkins, 1997). IPv4, currently the most

prevalent IP version on the Internet, has no provisions for

mobility. Each host computer on the Internet has a unique

address in the hierarchical IP addressing space. Each ad-

dress consists of a network prefix and a host number. The

network prefix of the address determines the location (i.e.,

campus network) of the host computer. Routers do not

contain the address of each individual host in their rout-

ing tables; instead, the network prefix of each address is

used to forward packets to the next hop en route to the

destination network. To reach a host, you must know its

IP address; if a host moves, all traffic addressed to it will

be sent to its home location following the rules of net-

work prefix routing (see Dynamic Routing for more de-

tails). If a host is assigned an address at its current new

location, the sender must be made aware of it so that the

appropriate destination address can be used in the data

packets. As the host moves, its IP address will change to

reflect its new location. This means that the sender must

be informed of every change in location/address to main-

tain the connection and data flow. Packets in transit will

be lost unless some provision is made to have them fol-

low the host to the next location. For TCP connections

the problem is further exacerbated as the IP address of

the host is used in the TCP connection for session iden-

tification purposes. Thus, if the host moves and changes

its IP address, the connection identifier will no longer be

valid, thereby causing the session to be terminated. It is

for these reasons that MIP was conceived. For true mo-

bility, the whereabouts and mobility of a host should not

affect its ability to be reached by any sender, and in ad-

dition, the sender should not have to be responsible for

tracking a remote host as it moves about the Internet.

When designing MIP, it was obvious that it had to be

Compatible with the existing installed base of IP and the

layers below it;

Transparent to the layers above IP;

Scalable and efficient, and capable of supporting large

numbers of hosts and not impede the functionality of

the Internet in anyway; and

Secure, and the forwarding of connection control infor-

mation must be authenticated to prevent traffic from

being diverted to other destinations.

The design of MIPv4 accomplished the above four require-

ments. By all means, it is not the most optimal solution.