第二章宽带业务特征与服务质量保证
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12005-11-09
牛志升◎清华大学1《宽带信息网络》
第二章
宽带业务特征与服务质量保证
牛志升
清华大学电子工程系
niuzhs@tsinghua.edu.cn信息与电子工程专业研究生学位课
2005-11-09牛志升◎清华大学2几个基本问题
为什么电话网采用了层次化的电路交换技
术？
为什么Internet采用了非层次化的分组交换
技术？
下一代网络能够简单地延续目前Internet的
结构吗？在哪些方面应该重新设计？
22005-11-09牛志升◎清华大学3目录
2.1 几种有代表性的宽带业务及其特征描述
¾分组语音与视频会议
¾音频与图像流
¾音频与视频点播
¾WWW
2.2 宽带业务性能需求（服务质量保证）
¾为什么需要服务质量保证？
¾服务质量(QoS)的定义与描述
¾服务质量保证机制概述
2005-11-09牛志升◎清华大学4宽带业务：按业务类型
用户信息
¾语音/音频(Voice and Audio)
¾静止/活动图像(Image and Video)
¾数据(Data)
网管/控制信息
¾信令(Signaling)信息
¾控制(Control)信息
¾网管(OAM)信息
32005-11-09牛志升◎清华大学5宽带业务：按QoS要求
实时业务
¾Delay-sensitive, but generally loss-tolerable
¾Voice and video, and real-time data
非实时业务
¾Delay-insensitive, but generally loss-sensitive
¾Non-real-time data
2005-11-09牛志升◎清华大学6宽带业务：按业务模式
恒定速率业务
(CBR: Constant Bit Rate)
¾Voice, MPEG1, Circuit Emulation, ……
变化速率业务
(VBR: Variable Bit Rate)
¾Video, MPEG2, Data, ……
可变速率业务
(ABR: Available Bit Rate)
¾Data, Virtual LAN, ……
无固定速率业务
(UBR: Unspecific Bit Rate)
¾Data, ……
42005-11-09牛志升◎清华大学7典型宽带业务特征（1）
音频与视频流(Audio or Video Streams)
¾One way transmission (e.g., MP3 or WebTV)
¾Absorb the delay fluctuations by buffering at receiver
¾Delay jitter is much more serious, but fixed delay is
unimportant
¾Transmission rate ranges from 8Kbps to 30Kbps
(audio) or from 40Kbps to 80Kbps (video)
¾Some transmission errors are acceptable (e.g.,
BER<10-4for uncompressed signal or BER<10-6for
compressed signal)
2005-11-09牛志升◎清华大学8典型宽带业务特征（2）
分组语音及电视会议
(PacketizedVoice and
Videoconferences)
¾Two way communication (e.g., VoIP)
¾Sensitive to both maximum delay and delay
jitter
¾Transmission rate is 4-64Kbps (voice), 8Kbps-
1.3Mbps (CD quality audio), 1.15-3Mbps
(MPEG1), or 4Mbps (MPEG2)
¾Small packets are preferable
¾Some transmission errors are tolerable
52005-11-09牛志升◎清华大学9典型宽带业务特征（3）
World Wide Web (WWW)
¾A set of hyperlinked documents (web pages)
¾Each web page may contain text, pictures,
audio clips, video clips, and possible links
¾When you browse the web, you initiate a
sequence of file transfers
¾The size of a web page typically ranges from a
few kilobytesto a few megabytes
¾We expect web page transfers to be error-free
¾We also do not want to “World Wide Wait”!
2005-11-09牛志升◎清华大学102.2 宽带业务的性能要求
用户对通信网的基本要求
¾连通性：任意、快速
（无线网络尤为重要）
¾可靠性：
迂回路由、自愈恢复、信息安全
¾灵活性：突发业务、新业务
¾经济性：价格性能比、最优化
不同业务有着截然不同的性能要求
¾语音：非常小的延迟和抖动，但可以容忍一定的丢失
¾图像：非常小的延迟和抖动，
¾数据：尽量小的延迟和抖动，但非常小的信息丢失
62005-11-09牛志升◎清华大学11不同特性的网络用来承载不同种类的业务
电路交换网(delay-sensitive)
¾低延迟、高接通性（迂回中继）
¾连通期间有可能发生一定的信息丢失，且可能
存在较高的阻塞率
分组交换网(loss-sensitive)
¾低丢失率、高通过率（缓存、重传）
¾抖动可以通过虚电路方式予以保证，但难以保
证低延迟
ATM网络(delay-and-loss sensitive)
¾低丢失率、低延迟、低抖动、高通过率；
Differentiated QoS
2005-11-09牛志升◎清华大学12Quality of Service: What is it?
Multimedia applications:
network audio and video
network provides
application with level of
performance needed for
application to function.QoS
72005-11-09牛志升◎清华大学13QoS: is it really necessary?
No:
¾QoSguarantee by traffic control and resource management
istoo complicated to be practical
¾Over-provisionof bandwidth will provide QoS
Yes:
¾Traffic growth is always fasterthan bandwidth growth;
over provision can not solve the congestion completely
¾Bandwidth available can not be infinite, especially at the
edge(including wireless access)
¾QoScontrol should be end-to-end, otherwise you just move
the problem elsewhere
¾Seamless handover and roamingof mobile hosts need
strict QoS
¾QoSdifferentiation and tariffneed QoSguarantee
(“Concepts without intuitions are empty; intuitions without concepts are blind”)
2005-11-09牛志升◎清华大学14Why better than Best Effort?
To support a wider range of
applications
¾Real-time, Multimedia, etc
To develop sustainable economic
models and new private networking
services
¾Current flat priced models, and best-effort services do
not cut it for businesses
82005-11-09牛志升◎清华大学15What’s QoS?
ITU Definition: “QoSis the collective effect of service
performance which determine the degree of satisfaction of
a user of the service”
¾it is a measure of the satisfaction experienced by a person
while receiving a service
¾it is a user perception and different from network
performance (NP)!
¾service support, operability, servability, and security
IETF Definition: “A set of service requirements to be met
by the network while transporting a flow”
¾it is closely equivalent to the notion of NP! (much easier!)
¾it is a measure of network performance rather than the
user QoS
2005-11-09牛志升◎清华大学16What’s QoS?
Technically, we usually transfer the QoSlevel
into NP (intrinsic QoS) parameters
¾Bit Rate (bandwidth)
¾Blocking Prob. and/or Packet Loss Rate
¾Delay (mean and/or maximum) time,
¾Jitter
Additionally, intrinsic QoSmay have the
following attributes:
¾end-to-end (e.g., IntServ) or limited to a particular domain
