Internet DRAFT - draft-fodor-reqts-cellular-netwks
draft-fodor-reqts-cellular-netwks
INTERNET DRAFT G. Fodor
Document: draft-fodor-reqts-cellular-netwks-00.txt F. Persson
Expires: Aug 2002 L. Westberg
B. Williams
J. Wiorek
Ericsson
Feb 2002
NSIS QoS Signaling Requirements from a Multi-access Wireless
Perspective
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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ABSTRACT
We consider access technology agnostic applications that use IP level
QoS primitives to specify their QoS requirements. In future wireless
systems (beyond what is often referred to as 3G), where mobile nodes
may access an IP backbone over diverse access networks, the IP level
QoS primitives are not only used to specify the requested IP level
service, but also (through an access specific translation function)
to help the mobile node's wireless link manager to configure the
wireless bearer service. In such a scenario, the IP level QoS
primitives must meet special requirements in order to support the
spectrum efficient management of the wireless resources.
In this draft we discuss the main characteristics of wireless access
networks and derive the requirements on IP level QoS primitives from
a wireless, and especially from a cellular perspective.
1 Background and Motivation
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One of the key objectives in the evolution and standardization of
future wireless access technologies beyond 3G (such as the IMT-2000)
systems, is to offer QoS support for a variety of services, including
IP services, while considering the critical aspect of optimizing
spectrum efficiency. Applications that make use of such services
include voice over IP, streaming services over IP and other QoS
enabled IP applications that access services of a QoS enabled IP
network over wireless accesses. Such future wireless systems are
expected to offer services over multiple access technologies in such
a manner that applications need not be aware of the specific link
layer(s) that is (are) available at any given time. For instance, a
device may have a combination of different interfaces such as wired
and wireless LAN, and a cellular wireless access. From a user
perspective, it is a key requirement that the one application can
operate effectively over any of these accesses. For cellular wireless
accesses, operating effectively includes consideration of spectrum
efficiency (which affects cost/performance).
Clearly, such applications need to be able to specify their end-to-
end QoS requirements in an access agnostic manner. That is, we assume
that each access system can have its own translation function, that
takes the IP level QoS parameters as input and generates the access
specific parameters from those (see also [1] for further details on
the role of such translation functionality and [2] for a suitable set
of such IP level parameters). Thus, applications only need to
communicate their QoS requirements through IP layer primitives (an
API) rather than through an access specific primitives.
Using IP level primitives towards diverse accesses necessitates the
need to investigate the requirements on IP level QoS primitives from
a wireless and specifically from a cellular perspective. Cellular
systems are generally characterized by the fact that their wireless
resources are scarce and costly.
In an end-to-end path containing one or more wireless accesses, it is
expected that the wireless links (providing transport over the air
interface) will be the most critical ones (_bottlenecks_) for QoS
delivery. Thus, the end-to-end service will be mainly influenced by
the suitability of the provided wireless link characteristics.
One of the challenges in the design of QoS services and the
associated enabling protocols in the multi-access environment is that
they must be able to be supported effectively by a variety of link
layers and QoS mechanisms, including those used by wireless access
networks.
It is believed that platforms supporting multiple layer 2 (L2)
interface types will provide generic (non-interface specific) QoS
primitives to applications. Since IP based QoS primitives are
expected to be widely available to application developers on some of
these platforms (e.g. laptops), it is useful to ensure that spectrum
efficient radio services can be provided for applications requesting
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QoS through such QoS primitives. Spectrum efficiency is a key factor
in enabling cellular operators to deploy affordable services.
In this draft we focus on applications/hosts requesting QoS through
such primitives, and list the requirements on the IP level QoS
signaling protocol from the perspective of such applications/hosts.
The draft is organized as follows. Section 2 describes the cellular
architecture and the main functionalities of the wireless network
elements. Note that this model is generic in the sense that it can be
applicable to a variety of wireless technologies. Section 3 builds on
the discussion of Section 2 and lists these requirements.
2 Wireless Network Architecture and Characteristics
We consider an architecture where mobile nodes (MN) access an IP
network via a cellular access network. We naturally assume that the
IP layer is present in the mobile node such that the user may
establish an IP connection to other IP endpoints through the wireless
access and the IP network. In such a network, a typical scenario
includes the following elements:
o The mobile node (MN) is considered to include the physical device
connecting to the Wireless Access Network (WAN) and the IP level
resource manager and signaling entity that allows applications to
request QoS enabled IP bearer services. The IP level resource
manager in the MN can provide the wireless link (air interface)
manager with QoS related information through a technology specific
translator entity. The wireless link provides the transport
service over the air interface between the MN and the base station
(which is part of the radio network of Figure 1).
o The Wireless Access Network (WAN) that consists of base stations
(BS), base station controllers BSC, (also referred to as radio
network controllers, RNC) and possibly other nodes responsible for
mobility management, location management, etc. The WAN connects to
the external IP Network (e.g. The Internet) through (a) gateway
node(s) (WAN GW). It is important to note that the WAN appears to
the MN as a L2 network; and is designed and optimized for the
transmission of radio packets.
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A simplified protocol stack of such a scenario is shown by Figure 1.
+----+ +----+
|Appl|<----+ |Appl|<----+
| | V | | V
+----+ +------+ +----+ +------+
|TCP/| |IP QoS| |TCP/| |IP QoS|
|UDP | |Module| |UDP | |Module|
+----+-+------+ +-----+ +----+-+------+
| IP |<---------------->| IP |<-------->| IP |
+-------------+ ,---. +--+--+ +--+--+ ,---. +-------------+
| | | | | |IP|<>|IP| | | | | |
| Radio L2 |<------->|R2+--+ +--+L2|<-------->+ L2 +
| | | | | |T2|<>|T2| | | | | |
+-------------+ | | +--+--+ +--+--+ | | +-------------+
| Radio L1 | \ / |R1|T1|<>|T1|L1| \ / | L1 |
+-------------+ `-' +--+--+ +--+--+ `-' +-------------+
MN Radio RNC/ WAN IP Remote
(mobile Network BSC GW Net User
node) \ /
-----v-----
WAN
Figure 1 Simplified end-to-end scenario with wireless access (WAN)
The radio packet delivery service (associated with a specific set of
traffic and QoS characteristics) that is provided by the WAN is often
referred to as the radio access bearer service (RABS). For instance,
a streaming RABS may mean a radio packet delivery service provided by
the WAN that provides bounded delay and limited packet loss ratio.
Typically, the terminal equipment has the responsibility for
identifying the radio access bearers that it needs, and how it will
use them. Thus, it is responsible for initiating the radio bearers
between the MN and the WAN GW. Since the applications may use access
technology agnostic QoS primitives (ie the applications do not have a
direct interface towards the wireless resource manager), the MN must
make use of the service information signaled at the IP level in
determining the appropriate radio bearers to establish.
The WAN provides RAB services in order to support the layer 2
connection between the MN and the WAN GW. The characteristics of
these RAB services are dependent on the wireless mechanisms
(see [1] for a detailed discussion on such mechanisms), and can be
markedly different from bearer services in traditional wired networks.
It is clear that different RAB services (in terms of the provided
delay, bit error rate, etc.) can be provided, resulting in quite
different characteristics of QoS, service costs and service
behaviors. A consequence of this flexibility is that sufficient
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detail about the applications' traffic and service requirements must
be known in order to determine the appropriate parameter settings to
enable service optimization.
3 QoS Requirements Imposed by the Wireless Link
The wireless link provides the RAB service for IP packets over the
air interface. The main characteristics of the wireless link may be
summarized as follows:
C1: Its resources (frequency, time slots, power, CDMA code, _wireless
bandwidth_ etc.) are generally scarce (and therefore expensive).
Moreover, some of its resources may change in time due to mobility
or interference from other wireless users.
C2: It imposes delay on the bits that are transmitted over it. This
delay is typically orders of magnitude longer than that caused by
fixed (e.g. optical) transmission links. Part of this delay is
lower bounded by physical characteristics of radio wave
propagation and is not possible to control by resource management
techniques.
C3: It causes bit-errors typically with orders of magnitude higher
probability than that caused by fixed (e.g. optical) transmission
links. It is possible to reduce the bit error probability
associated with the wireless link to a _reasonably low_ level at
the expense of consuming more wireless resources (e.g. power).
Because of these main characteristics, the resources of the wireless
link in most wireless technologies (and in most cellular systems) are
carefully managed (as opposed to over-provisioning). More precisely,
the objective of the wireless link resource management is twofold:
O1. To provide the required QoS for the entity (e.g. an IP
application running on a mobile node) that requests the wireless
transport service over the air interface. In the case of an IP
application, this transport service means the transport of the
bits of the IP packets. The quality of this wireless transport
service is typically characterized by quantifying C2 and C3 above.
O2. To make as efficient use of the wireless resources as possible
(because of C1 above).
In order to meet these two objectives, wireless resource management
protocols typically manage resources dynamically at a low level in
the sense that they control the use of the wireless resources at a
time scale on the order of the radio frames, which can be much
shorter than the packet length. For instance, if the quality of a
wireless link degrades because of increasing interference, the
resource management technique may increase the transmission power
over the wireless link.
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From these two objectives (O1-O2) it follows that the radio resource
management requires information about the required QoS and the
characteristics of the traffic that is transported over the wireless
link. These two pieces of information are required to exercise
admission control and to allocate the necessary resources.
In a scenario, where the wireless link provides the transport service
for applications running over IP it is a challenging task to provide
these two pieces of information, because the application may not be
aware that it runs over a wireless link. Therefore, in such a
scenario, the QoS and resource reservation mechanisms at the IP layer
(e.g. resource reservation and/or signaling protocols, and the IP
level bearer service, such as the set of integrated services) need to
provide support for the resource management of the wireless link
(i.e. the R2/R1 layers below IP in Figure 1).
From this perspective, the basic requirements on an IP level
signaling protocol and an IP level bearer service include support for
the following:
R1: Fine grained QoS and traffic characterization per user IP-flow
There must be sufficient information available at the IP layer[1],
(including parameters that characterize the required QoS and the
actual traffic), which (through the access specific translation
function) allows the wireless resource management to derive the
required QoS parameters and obtain as precise information about
the offered traffic as possible. These bearer parameters need to
support diverse wireless access technologies. To this end [2]
proposes an extension of the controlled load integrated service
that meets this requirement.
R2: QoS differentiation within a user IP-flow and/or within a user
IP-packet
Since the wireless resource management often operates on the bit-
level, the wireless resource management can improve the provided
QoS and the resource efficiency if it has knowledge about the
structure of the IP packet payload. For instance, for some voice
applications (codecs) some bits in the IP payload may be more
important for the human ear than other bits, in which case bit-
specific bit error rate is provided for these different voice
sample bits within the packet. This technique is often referred to
as unequal error protection (UEP) [3].
Another technique that may be used over the wireless link uses
different drop levels for packets within the same user flow. This
technique is similar to the unequal error protection but operates
at the packet level. It can for instance allow a voice application
(codec) in the mobile node to drop packets before sending over the
wireless link and thereby to save wireless resources.
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R3: Bi-directional asymmetric communication
It is expected that the up-link (i.e. from mobile node toward the
network) and the down-link (i.e. from the network towards the
mobile node) can be asymmetric in terms of the required resources
(e.g. allocated bandwidth). In such a case the wireless link must
be configured accordingly in the respective directions, such that
the necessary QoS and resource efficiency objectives (O1 and O2
above) are met in both directions. Therefore, there has to be
support for asymmetric bearers that allow bi-directional
communication between two mobile nodes (or between a mobile node
and a server) over an IP network.
R4: Local (as opposed to end-to-end) resource reservation
The cellular network may provide access to a variety of IP
networks, including the ones that provide best-effort
differentiated or integrated services. Therefore, in some cases it
is important that the resource reservation should apply within the
access network only rather than end-to-end. Therefore, the QoS
solution should provide support for non end-to-end resource
reservations.
R5: Multi-cast
Multi-casting is an efficient way to save wireless link resources.
A prime example on this is a scenario where multiple mobile nodes
access a streaming server. In such a situation the wireless
gateway node may broadcast the streaming data to the participating
mobile nodes rather than using dedicated communication channels.
Multi-casting (as opposed _multi-unicasting_) allows some of the
wireless link resources to be re-used between the mobile nodes.
Therefore, there needs to be a way to create and modify multi-cast
trees of mobile nodes and wireless nodes.
R6: Local Control
In a typical wireless scenario where the mobile node (client)
requires a service from a server (such as in a streaming or WWW
services), it is the mobile node that is aware of the wireless
link conditions including the available wireless bandwidth. Also,
in such scenarios the wireless node is typically the charged party
for both the up-link and down-link service. Therefore, it is
important that the wireless node has some mechanism that allows it
to control the bandwidth of the traffic that is transmitted over
the wireless link for both directions. This control of the QoS
service from the local access allows the wireless node to make
efficient use of the wireless resources, and at the same time to
control network charges.
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R7: Rate Adaptivity
Because the resources of the wireless link may fluctuate in time
(see C1), the utilization of the wireless resources may be
increased by allowing the mobile (receiver) node to dynamically
control the sending rate of IP packets. In particular, the
application running on the mobile node needs to be able to
increase/decrease the sending rate of the streaming server
depending on the available wireless bandwidth. This requirement is
especially important in a situation where the mobile node executes
a hand-over from a high bit-rate access network to a low bit-rate
access network, where the available bit-rates may be different by
several orders of magnitude.
Therefore, in this sense, there has to be support for adaptivity
that allows the wireless resource management to make best use of
the (currently) available wireless resources.
4 Conclusions
This draft considered applications that use IP level QoS primitives
to communicate their QoS requests towards diverse access network
types. These types of scenarios are expected to be important in
future wireless systems, where users on the move access services of
an IP backbone network over multiple accesses, including wireless and
wired ones. A key requirement on such systems is that applications
should be able to request QoS in an access agnostic manner. This is
important because the one user application must be able to work over
different interface types (eg high speed lan, dial-up modem, wireless
access). Therefore, we have throughout assumed that applications use
IP level QoS primitives to express their QoS requirements and
different link layers use a link layer specific translation function
to derive the access specific QoS parameters.
We have also argued that in the case of wireless and especially
cellular accesses, the end-to-end QoS is mainly influenced by the
appropriate configuration of the wireless link. The wireless link
configuration determines the resource efficiency and thereby it has a
direct impact on the cost of the services that are offered by
cellular operators.
The set of requirements for an access agnostic IP level QoS
primitives which shall control the QoS signaling at both the IP and
access layer which is suitable for a cellular network is thus driven
by the QoS requirements of the wireless link. These requirements
include the support for:
R1: fine granularity QoS parameters per user IP flow
R2: QoS differentiation within a user flow and within a user packet
R3: bi-directional asymmetric communication
R4: local (as opposed to end-to-end) reservations
R5: multi-casting
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R6: local control
R7: rate adaptivity.
References
[1] G. Fodor, F. Persson, B. Williams, _Application of Integrated
Services over Wireless Accesses_ draft-fodor-intserv-wireless-issues-
01.txt, work in progress, January 2002.
[2] G. Fodor, F. Persson, B. Williams, _Proposal on New Service
Parameters (Wireless Hints)_ in the Controlled Load Integrated
Service_, draft-fodor-intserv-wireless-params-01.txt, work in
progress, January 2002.
[3] Sjoberg, J, Westerlund, M, Lakaniemi, A, Xie, Q, _RTP Payload
Format and file storage format for AMR and AMR-WB audio_
draft-ietf-avt-rtp-amr-11.txt, work in progress
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