03-09-2012, 11:36 AM
Cross-layer Design for Wireless Networks
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Abstract
As the cellular and PCS world collides with Wireless LANs and Internet-based packet data, new networking
approaches will support the integration of voice and data on the composite infrastructure of cellular
base stations and Ethernet-based wireless access points. This paper highlights some of the past
accomplishments and promising research avenues for an important topic in the creation of future wireless
networks. In this paper, we address the issue of cross-layer networking, where the physical and MAC layer
knowledge of the wireless medium is shared with higher layers, in order to provide efficient methods of
allocating network resources and applications over the Internet. In essence, future networks will need to
provide ”impedance matching” of the instantaneous radio channel conditions and capacity needs with the
traffic and congestion conditions found over the packet-based world of the Internet. Further, such matching
will need to be coordinated with a wide range of particular applications and user expectations, making
the topic of cross-layer networking an increasingly important one for the evolving wireless build-out.
Introduction
There continues to be rapid adoption of wireless technology, and when coupled with the explosive growth of
the Internet, it is clear that there will be increasing demand for wireless data services [1]. Traffic on future
wireless networks is expected to be a mix of real-time traffic such as voice, multimedia teleconferencing, and
games, and data-traffic such as WWW browsing, messaging and file transfers. All of these applications will
require widely varying and very diverse quality of service (QoS) guarantees for the different types of offered
traffic, and we are now in the early days of this eventual amalgamation. Various mechanisms have been
proposed and recently deployed to support data traffic over wireless media. This has ranged from Wireless
Local Area Networks (WLANs), mainly based on the IEEE 802.11b or HiperLAN standards, to wireless wide
area networks (WWANs) where data services are supported in the 2.5G and 3G system versions.
Network Service Types
The Internet today consists of thousands of access networks, which vary in scale from large wireline access
networks, such as campus wide area networks or Internet Service Providers, supporting tens of thousands of
users, to smaller wireless access networks supporting tens to hundreds of users. These access networks are
interconnected by core networks (such as AT&T, WorldCom, or Sprint backbone networks) which support
hundreds of millions of users. All of these networks primarily offer two types of services: guaranteed
service and best effort service.
In guaranteed service, the network provides some sort of service guarantee to individual users or groups
of users. These guarantees are often in the form of ensuring that the throughput for a group of users is greater
than some minimum value or that the delay experienced is smaller than some threshold. Often times, the
guarantee is offered in a statistical sense, e.g. providing a particular data rate x% of the time.
In best effort service, the network makes no promises. This service is typically used by elastic traffic.
Elastic traffic consists of traffic where users do not necessarily have any minimum requirements, but
would like to get as much data through to their respective destinations as quickly as possible. Individual user
data flows react to congestion in the network and adapt their transmission rate with the aim of minimizing
congestion. Email service is often provided on a best effort basis.
3 Differences between Wireline and Wireless Networks
Both wireless and wireline networks need to support the different network data service types described in the
previous section. The focus of this paper is on supporting such services over wireless networks, and the gains
that can be accrued by cross-layer techniques which do away with the firm boundary that currently exists
between the PHY and MAC layers, and the higher layers of the Network protocol stack. Before studying
mechanisms by which data services can be supported over wireless networks, we briefly describe the characteristics
of wireless systems. Based on limited frequency allocations and channel considerations, wireless networks have peculiarities which distinguish them from conventional wireline networks.
Data over Wireless in a Multi-user Context
We now discuss various issues related to supporting data services over cellular wireless networks. We illustrate
how channel-state dependent techniques (cross-layer methods) can lead to improved network throughput, and
survey some of the current related research.
Consider a conventional cellular system with a fixed base-station and a number of mobile users. Data
flows (packets) arrive from the wired Internet to the cell base station and are destined to the mobile users, with
the packets for each user being queued temporarily at the base station (a separate queue is maintained for each
user). The objective of the base station is to schedule these packets to various mobiles in a timely manner.