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WIRELESSINTERNET—ISITHAPPENING? 841MN using local ACKs not relayed to the CN. This mech-
anism shortens the retransmit time (most retransmis-
sions occur only over the short wireless portion). How-
ever, should the FA fail, packets will be lost and neither
side of the connection will be aware that this happened.
This scheme violates the end-to-end semantics of TCP.
Snooping TCP (Balakrishnan, Seshan, & Katz, 1995;
Brewer et al., 1998)—This variation on TCP does not
violate the end-to-end semantics of TCP as I-TCP does.
It resides on an intermediate node (such as a FA in the
case of MIP) that is attached to the wireless link. It
buffers all data that are meant to be transmitted over
the wireless channel so that if a loss occurs, it can re-
transmit the buffered data immediately, thereby avoid-
ing the long end-to-end delay that otherwise would oc-
cur and cause the session to time-out. Although it does
not violate the end-to-end semantics, it is not as effi-
cient as I-TCP as it does not completely hide the wire-
less link from TCP; the retransmissions do incur a delay
that could impact the performance of the end-to-end
TCP session.
Mobile TCP (M-TCP) (Brown, & Singh, 1997)—It is similar
in operation to I-TCP. However, no data are buffered
or retransmitted by the proxy. The proxy only monitors
the link, and if it determines that a loss has occurred, it
will freeze the connection so that TCP does not go into
slow start. On the wireline side it will close down the
window, forcing the sender to go into persistent mode,
and on the wireless side it uses a fast recovery TCP that
does not use slow start. The end-to-end semantics are
not violated as the proxy does not retransmit any data;
it only interferes in the transmission by forcing certain
TCP behavior on the detection of packet loss.Other techniques include selective TCP (SACK)
(Mathis, Mahdavi, Floyd, & Romanow (1996), which re-
quires only the lost packets to be transmitted, and fast
recovery/fast transmit (Caceres & Iftode, 1995), which
does not allow TCP to go into slow start by sending three
duplicate acknowledgments. All of the solutions above
provide some features to prevent TCP from entering the
slow start mode due to packet loss on the wireless chan-
nel.IEEE 802.11b/g/a
Wireless LANs (WLANs) constitute an alternative tech-
nology to wireline LANs. They use radio signals to trans-
mit data. Wireless technologies offer exceptional flexibil-
ity and mobility, thus making WLANs very attractive for
many environments, including office, home, and public
places. IEEE 802.11 (Institute of Electrical and Electron-
ics Engineers, 1999)-type technologies are the common
standard used in WLANs. Like all IEEE 802 standards,
the 802.11 standards focuses on the lower two layers of
the International Standards Organization (ISO) reference
model, the physical layer and the data link layer.Operation of 802.11
802.11 Operating Modes.802.11 defines two pieces of
equipment, a wireless station, which is usually a PC
equipped with a wireless network interface card (NIC),and an access point (AP), which acts as a bridge between
the wireless and the wireline networks. An access point
usually consists of a radio, a wireline network interface
(e.g., 802.3), and bridging software conforming to the
802.1d bridging standard. The access point acts as the BS
for the wireless network, aggregating access for multiple
wireless stations onto the wireline network. Wireless sta-
tions can be 802.11 PC Card, PCI, or ISA NICs, or embed-
ded solutions in non-PC clients (such as an 802.11-based
telephone handset).
The 802.11 standard defines two modes:infrastructure
mode andad hocmode. In infrastructure mode, the wire-
less network consists of at least one access point con-
nected to the wireline network infrastructure and a set
of wireless stations. This configuration is called a basic
service set (BSS). An extended service set (ESS) is a set
of two or more BSSs forming a single subnetwork. Since
most corporate WLANs require access to the wireline LAN
for services (e.g., file servers, printers, and Internet links)
they will operate in infrastructure mode. Ad hoc mode
(also called peer-to-peer mode or an independent basic
service set, or IBSS) is simply a set of 802.11 wireless sta-
tions that communicate directly with one another without
using an access point or any connection to a wireline net-
work. This mode is useful for quickly and easily setting up
a wireless network where a wireless infrastructure does
not exist or is not required or where access to the wireline
network is barred.802.11 Physical Layer.The three physical layers origi-
nally defined in 802.11 included two spread-spectrum ra-
dio techniques and a diffuse infrared specification. The
radio-based standards operate within the 2.4-GHz in-
dustrial, scientific and medical (ISM) band. These fre-
quency bands are recognized by international regulatory
agencies, such as the FCC (USA), the ETSI (Europe),
and the MKK (Japan) for unlicensed radio operations.
Spread-spectrum techniques, in addition to satisfying reg-
ulatory requirements, increase reliability, boost through-
put, and allow many unrelated products to share the
spectrum without explicit cooperation and with minimal
interference. The original 802.11 wireless standard de-
fines data rates of 1 and 2 Mbps via radio waves using
frequency-hopping spread spectrum (FHSS) or direct se-
quence spread spectrum (DSSS).802.11b Enhancements to the PHY Layer.The key con-
tribution of the 802.11b addition to the wireless LAN stan-
dard was to standardize the physical layer support of two
new access speeds, i.e., 5.5 and 11 Mbps. To accomplish
this, DSSS was selected as the sole physical layer tech-
nique for the standard since frequency hopping cannot
support the higher speeds without violating current FCC
regulations. The implication is that 802.11b systems will
interoperate with 1- and 2-Mbps 802.11 DSSS systems,
but will not work with 1- and 2-Mbps 802.11 FHSS sys-
tems.802.11 Data Link Layer.The data link layer within
802.11 consists of two sublayers: logical link control
(LLC) and media access control (MAC). 802.11 uses the
same 802.2 LLC and 48-bit addressing as other 802