Network Working Group I. Johansson Internet-Draft Ericsson AB Intended status: Informational Oct 18, 2009 Expires: April 21, 2010 Congestion in wireless access, input to ConEx BoF draft-johansson-wireless-congestion-properties-00 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 21, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This document outlines the congestion properties for wireless access types such as HSPA and LTE defined in 3GPP. The target audience is people interested in the Congestion Exposure work but the document Johansson Expires April 21, 2010 [Page 1] Internet-Draft Congestion in wireless access Oct 2009 may also be of interest for others who wish to get a basic knowledge about 3GPP wireless access and its properties. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions, Definitions and Acronyms . . . . . . . . . . . . . 3 3. Wireless access, an outline . . . . . . . . . . . . . . . . . . 3 4. Congestion in wireless access . . . . . . . . . . . . . . . . . 4 5. Implications for the ConEx WG . . . . . . . . . . . . . . . . . 5 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6 Johansson Expires April 21, 2010 [Page 2] Internet-Draft Congestion in wireless access Oct 2009 1. Introduction This document describes in brief the characteristics of wireless shared channel access such as LTE and HSPA defined in 3GPP. The motivation for this document is the emerging interest in Congestion Exposure as a component that can be used to control IP traffic behavior. The document should not be taken as an argument for or against the Congestion Exposure work, rather the intention is to give some input in order to determine what extra requirements wireless access may impose on the protocol design. Wireless access technologies are becoming an important part of the IP communication infrastructure and therefore it is important that the Congestion Exposure WG address the requirements of wireless access as well. The description applies mainly to best effort type of access. Certain services like VoIP may receive special treatment such as QoS, which is outside the scope of this document. 2. Conventions, Definitions and Acronyms This document focuses on cellular wireless access technologies standardized in 3GPP, e.g. HSPA and LTE, therefore an explanation of some of the "buzzwords" related to this technology is in order. o NodeB = base station = wireless access point (it is called eNB in LTE - evolved NB) o UE = User Equipment = Wireless endpoint o Uplink = Direction UE to NodeB o Downlink = Direction NodeB to UE o BLER = Block error rate o LTE = Long Term Evolution o HSPA = High Speed Packet Access 3. Wireless access, an outline Wireless shared channel access has the property that many UE's within a limited geographical area (a.k.a cell) shares the same radio channel in the communication between UE and NodeB. The transmission Johansson Expires April 21, 2010 [Page 3] Internet-Draft Congestion in wireless access Oct 2009 power in the downlink is limited, a typical value may be 20W but this varies depending on setup, the transmit power in the uplink is up to 0.2W. When data (TCP, UDP, whatever) is to be transmitted to a UE (downlink transmission) the NodeB first schedules the data for transmission. The design of the scheduler is up to the implementer of the NodeB and not specified by the standard to allow for product differentiation. Given the total load, the data may be transmitted immediately (low load) or with some delay (high load). The data may be split into several radio-frames, this may happen because an IP packet is too large to fit in a single radio frame or because the radio conditions require that more error correction is applied in order to ensure sufficiently reliable transmission. A radio-frame is an entity that carries data (IP packet or part of IP packet) over the radio interface. A radio frame is typically 1ms duration and the number of useful payload bits in a radio-frame varies with radio conditions. Transmission in the uplink is similar to what is mentioned above with the addition that a UE that needs to transmit data in the uplink must ask the NodeB for permission to send the data. Once the UE receives a scheduling grant it starts to send the data at the time instant indicated in the grant. If the data did not fit into one radio-frame the data is split and transmitted in pieces, more grants may have to be requested. Also as in the case with downlink transmission multiple retransmissions may occur. The term "sufficiently reliable transmission" deserves some explanation, the goal is not to strive for 100% successful transmission in the first transmission attempt, instead a target of e.g. 10% BLER is set and if necessary the information from this transmission attempt and a subsequent retransmission can be combined (soft-combining) to retrieve the data. A number of retransmit attempts can occur and this gives rise to a jitter that typically increases with load and distance from UE to closest NodeB. Given that many details around the implementation of actual schedulers are proprietary it is to be expected that two different brands of base station equipment may give different jitter characteristics for the same load condition. 4. Congestion in wireless access The laws of physics spiced with a grain of information theory gives that the available throughput for a UE depends on a number of Johansson Expires April 21, 2010 [Page 4] Internet-Draft Congestion in wireless access Oct 2009 factors: o Distance to closest NodeB, the line of sight attenuation is ~10dB for each doubling of the distance at 2GHz. However line of sight assumptions seldom applies to wireless deployments. o Interference or total traffic load, this typically varies as a function of time (busy-hour issue). o Shadowing, line of sight to NodeB is obstructed o Multipath fading, reflections in e.g walls and roof-tops interfere with the direct path radiowave from NodeB to UE Radio planning and the use of multiple neighboring cells in combination with handover mechanisms typically ensure that the available throughput does not vary too much. Still the available throughput for a given UE will vary depending on the conditions given above and there are factors such as shadowing that are difficult to alleviate. The available throughput can, because of the factors given above, change rapidly. A user of a hand-held device may move into a more loaded cell, he or she may step into an elevator or drive with the car into a tunnel. As the channel is shared the end result may be that a few UEs with very poor radio conditions may consume a lot of the available transmission resources (transmission power) in order to achieve successful transmission. The consequence can be that what is experienced as isolated congestion or poor throughput for the individual UE can cause noticeable problems for many non-congested UE's. To put it real short one can say that in wireless access scenarios bitrate is not equal to consumed resources. In short it is quite straightforward to realize that transmission over wireless links such as the ones described above gives both more rapid changes in congestion experience as well as more a dynamic throughput experience. This may have impact on algorithms and protocols specified in the Congestion Exposure WG as the dynamic congestion behavior may impose requirements on possible policing algorithms which in turn may impose additional requirements on the underlying protocol design envisioned in the Congestion Exposure WG 5. Implications for the ConEx WG Wireless access is becoming more and more widely used, and as it is likely a bottleneck that must be considered, it is important that the work of the ConeEx WG takes the particular properties described in Johansson Expires April 21, 2010 [Page 5] Internet-Draft Congestion in wireless access Oct 2009 this document into account when protocols and algorithms for congestion exposure is designed. 6. IANA Considerations N/A 7. Security Considerations N/A 8. Acknowledgements The author would like to thank Sara Landstroem for comments and suggestions. Author's Address Ingemar Johansson Ericsson AB Laboratoriegrand 11 SE-971 28 Lulea SWEDEN Phone: +46 73 0783289 Email: ingemar.s.johansson@ericsson.com Johansson Expires April 21, 2010 [Page 6]