三、多媒体通信的性能要求

三、多媒体通信的性能要求 1，网络基础计算机网络的概念及作用；计算机网络的分类；通信协议； 2，多媒体通信多媒体通信的特点；多媒体通信的性能要求；多媒体通信与各类网络平台；

计算机网络的概念及其应用 计算机网络的概念定义：计算机网络（computer network）是一个由若干互联且自主（autonomous）的计算机构成的集合。计算机：微机、工作站、基于计算机的设备（如：打印机、传真机、 router、gateway etc）；互联：计算机之间可以进行数据交换；自主：计算机之间无主/从（master/slave）关系；一台计算机可以强行启动/停止/控制另一台计算机如，主机/终端、diskless workstation and etc..

计算机网络的概念及其应用 计算机网络的作用资源共享（resource sharing）应用/设备/数据使资源具有高的可靠性（high reliability）冗余以提高可靠性节省开支设备开支（mainframe/terminal  client/server）商业开支系统具有良好的可扩展性（scalability） mainframe/terminal  client/server

Example: Two separate queues • Modelled as two separate M/M/1 queues, we get the following performance measures:

Example: Common pool • 36/day from Design + 36/day from sales = 72items/day =  • M/M/2 system with  = 72 items per day and =48 items per day.

计算机网络的分类 广播型网络和点到点型的网络（链路与设备的关系）广播型网络：网络上的各种设备共享同一物理传输介质；通信模式unicast/multicast/broadcast； arbitration mechanism； e.g. EthernetCSMA/CD(Carrier Sense, Multiple Access, with Collision Detection) FDDI, token ring; and etc. 点对点型网络：网络中任何一条物理链路均为某两台设备所独享。通信模式 unicast; multicast/broadcast(additional mechanism needed); e.g. PSTN, ISDN, ATM and etc.

计算机网络的分类 LAN/MAN/WAN/internet (覆盖范围) LAN (Local Area Network) <1>10m（room）、100m(building)、1km（campus）; <2>范围受限  worst-case transmission time is bounded and known in advance; this parameter is related to arbitration! <3>广播型网络 (exceptionATM LAN, switching LAN<miniature WAN>); <4>常见的拓扑结构总线/环行/星型（伪); <5>Ethernet,token ring(4－16M)，FDDI(100M); MAN (Metropolitan Area Network) <1>10km(city); <2>覆盖范围较大的LAN，技术与LAN相近 ;

计算机网络的分类 LAN/MAN/WAN/internet (覆盖范围) WAN(Wide Area Network) <1>100km（country）、1000km（Continent）; <2>点到点型网络，数据传输速率的范围较广； <3-1>主机（host）或端系统（end systerm） intended for running user programing； <3-2>通信子网（communication subnet）：报文的传输； transmission lines switching elements application aspects + communication aspects

计算机网络的分类 LAN/MAN/WAN/internet (覆盖范围) Host/Communication subnet Core network/Edge network Different requirements : • Edge routers; • Core routers; Internet <1>10,000kM（planet）; <2>Extended WAN; <3>heterogeneity;

计算机网络的分类 线路交换网和分组交换网（物理链路的使用方式）交换的概念 (Definition of Switching): 在特定的入口和出口之间，按需建立一条单一的联接以用于数据的传递；该联接持续存在直至数据传输完毕。 (equipment scale & network scale) 线路交换 (Circuit – Switching); 分组交换 (Packet – Switching);

1 1 2 2 3 3 4 4 计算机网络的分类线路交换网和分组交换网（物理链路的使用方式）线路交换：时分复用，易于QoS保障 • 通信资源被划分为若干基本单位(time slot)； • 基本单位被分配给 “calls”; • 无共享：任何两个“calls”不分享同一个基本单位； • 未被分配的基本单位被视为空闲； TDM

计算机网络的分类 线路交换网和分组交换网（物理链路的使用方式）分组交换：统计复用，易于提高通信资源的使用效率 • 数据流被划分为若干包（packets）; • 每个包使用全部通信资源； • 通信资源不被事先予留； • 每个包按存储转发（Store-and-Forward）的方式被传送，每次为一跳（hop）；

计算机网络的分类 线路交换网和分组交换网（物理链路的使用方式）分组交换：统计复用，易于提高通信资源的使用效率 • 数据包: • 具有相互独立的路径； • 到达次序可以有别于发送次序； • 可以被丢弃；

通信协议 协议（protocol） <1>通信双方就如何进行通信所达成的一种约定 <2>交互什么样的数据帧、包、消息等格式及含义 <3> 如何交互 e.g address, routing/switching, control and etc.

通信协议 协议的分层结构 For each layer: • entity --- the active unit indulged in communications; • The ID issue; • layer N entity --- layer N protocol; • peer entity; • virtual communication/actual communication; Benefits of layering  simplicity  easily implementation  easily adjustment/upgrade  …

通信协议 协议的分层结构

通信协议 协议的分层结构 service provider/service user SAP (Service Access Point) Interface  a set of SAPs ICI(Interface Control Information) SDU(Service Data Unit) the data to be transmitted PDU(Protocol Data Unit) the transmission unit

通信协议 协议的分层结构 Service  primitives (basic operations)

通信协议 协议的分层结构 Confirmed service: include all previously mentioned primitives e.g the connection setup service CONNECT.request  Request a connection to be established; CONNECT.indication  Signal the callee; CONNECT.response  Used by the callee to accept/reject calls; CONNECT.confirm  Tell the caller if the call is accepted. Unconfirmed service: include only request and indication e.g. the disconnect service DISCONNECT.request  Request that a connection be released; DISCONNECT.indication  Signal the peer about the event.

通信协议 OSI_RM • This model is an attempt by the International Standards Organization (ISO) to standardize various protocols • The models called the ISO OSI Reference Model (Open System Interconnection), or simply OSI/RM • OSI/RM defined a 7-layer reference model (OSI) as a guide to the design of a network protocol suite • ISO has also produced standards for all the layers of the model. • Many modern protocols do not conform to the OSI reference model such as Broadband ISDN networks. • Protocol translations

Application Presentation Session Transport Network Data Link Physical LAYER 7 LAYER 6 LAYER 5 LAYER 4 LAYER 3 LAYER 2 LAYER 1 通信协议 OSI_RM Application-oriented Layers Network-dependent Layers

通信协议 Application Layer • Distributed System Model; • A collection of user and network applications. • provides user interface to the OSI environment • Provides management functions and mechanism that support distributed systems and general-purpose applications such as FTP, SMTP (electronic mail), and remote login (virtual terminal protocol) • Distributed systems hides system details and allow multiple stations to run the same software and to access common resources without knowing where they are located

通信协议 Presentation Layer • Provides syntax (presentation) of information during transfer between 2 communicating applications (i.e. common formats for representation of inf.) • This layer provides code conversion, data encryption, and data compression: • Converts the syntax if different (ex. ASCII , EBCDIC) • Provide security by encrypting data (Int. 64 bits or US128 bits) • Provide data compression: reduce the no of bits to be stored/transmitted while retaining their meaning (no of 1s, 0s…); ex. Lempel-Ziv algorithm • These Functions are often provided as a collection of library routines.

通信协议 Session Layer • The Session Layer allows users on different machines to establish session between them, provides token management and synchronization in case of crashing (buffering &recovery) • Establish and manage a reliable exchange of messages (session): • Connect: establish the session • Supervise the data transfer: regular, expedited, control • Clear the connection • Supervise dialogue by exchanging tokens • Recover after error: insert synchronization points

通信协议 Transport Layer • Provide application-oriented layers with a message transfer facility that is independent of the network type (interface): • Fragmentation of messages into packets and reassembly • Re-sequencing • Flow control • Multiplex several low-rate sessions into one • Split a high-rate session into multiple sessions, • Multiplexing & de-multiplexing • ISO has 5 transport protocol standards with dif. QOS (Class 0,1,…4) • TCP/IP provides 2 common transport layer protocols: • Connection-oriented Transmission Control Protocol (TCP) • Connectionless UDP Protocol (User datagram protocol)

通信协议 Network Layer • Establish and clear a network-wide connection between 2 transport layer protocol entities • The Network Layer controls naming, addressing, routing, and congestion: • Naming: nodes have symbolic address (user@domain) • Addressing: network connections have network and physical addresses (198.109.25.23) • Routing: hierarchical • Congestion control:commonly use window flow control • The Internet Sub-layer (top) is used when a gateway is used to connect two different sub-nets

通信协议 Network Layer (cont’d) • Directory service is used to translate the name into network address • Name servers maintain a list of network addresses of nodes with known names • ARP translates the network address into physical address • The Internet network layer protocol, IP, uses datagram transport

通信协议 Data Link Layer • Provide the network layer with a reliable and ordered packet transfer facility: • Error detection & retransmission • Frame synchronization • Flow & error control • Addressing • The Data Link Control layer (DLC) provides a virtual error-free packet link to higher layers and regulates data flow rate. The MAC sublayer (medium access control) allocates the multiaccess channel so that each node can transmit its frames without interference from the other nodes • Format the bit pipe of data provided by the physical layer into a packet link in frames and delivery of frames through network interface

通信协议 Data Link Layer(cont’d) • Provide 2 types of services: • Connection-oriented: provide an error free data trans • Connectionless: frame is transmitted using a best try approach (if error detected, discard): • Unacknowledged connectionless service • Acknowledged connectionless service.

通信协议 Physical Layer • Concerned with transmitting raw bits over the physical transmission medium between two nodes or sites • The design issues here deal with mechanical, electrical, procedural interface between the network terminating equipment and the user equipment • Concerned with the physical and electrical interfaces

通信协议 Physical Layer (cont’d) • Basic network hardware: RS-232, FDDI, Token Ring, Token bus or Ethernet • The x.21 physical layer interface, similar in function to RS-232-C, is used as the physical layer for the x.25 protocol. Input Device Source System g(t) Transmitter s(t) Medium r(t) Receiver Destination System g’(t) Output Device

通信协议 Internetworking

通信协议 TCP/IP The reference model

通信协议 TCP/IP The protocol stack

通信协议 TCP/IP • The core components in the protocol suite: • Internet Protocol (IP) • Transmission Control Protocol (TCP) • User Datagram Protocol (UDP) • Address Resolution Protocol (ARP) • Internet Control Message Protocol (ICMP) • Point to Point Protocol (PPP) • Serial-Line IP (SLIP)

通信协议 TCP/IP---IP • Internet Protocol (IP) 为传输层协议（TCP、UDP）提供数据通信服务； • 服务特点：best-effort, connectionless • Best-effort: • 对通信资源、延时、数据传输的可靠性无保障 • 包可以具有很长的延时； • 包可以被丢弃 (packet loss)； • 包重复 (packet duplication)； • 包破损 (packet broken)； • 包的接收次序可以有别于发送次序 (out-of-order)； • 全局性地址空间 • 所有的IP节点共享同一个地址空间； • 唯一性

TCP Characteristics • TCP is connection-oriented. • 3-way handshake used for connection setup/teardown. • TCP provides a stream-of-bytes service. • TCP is reliable: • Acknowledgements indicate delivery of data. • Checksums are used to detect corrupted data. • Sequence numbers detect missing, or mis-sequenced data. • Corrupted data is retransmitted after a timeout. • Mis-sequenced data is re-sequenced. • (Window-based) Flow control prevents over-run of receiver. • TCP uses congestion control to share network capacity among users. • Not suitable for real-time audiovisual communication.

(Active) Client (Passive) Server Fin (Data +) Ack Fin+Ack Ack Connection Close/Teardown 2 x 2-way handshake TCP is connection-oriented (Active) Client (Passive) Server Syn Syn + Ack Ack Connection Setup 3-way handshake

TCP supports a “stream of bytes” service Host A Byte 0 Byte 1 Byte 2 Byte 3 Byte 80 Host B Byte 0 Byte 1 Byte 2 Byte 3 Byte 80

…which is emulated using TCP “segments” Host A Byte 0 Byte 1 Byte 2 Byte 3 Byte 80 Segment sent when: • Segment full (MSS bytes), • Not full, but times out, or • “Pushed” by application. TCP Data TCP Data Host B Byte 0 Byte 1 Byte 2 Byte 3 Byte 80

The TCP Segment Format IP Data IP Hdr TCP Data TCP Hdr 0 15 31 Src port Dst port Sequence # Src/dst port numbers and IP addresses uniquely identify socket Ack Sequence # TCP Header and Data + IP Addresses Flags Window Size RSVD 6 HLEN 4 SYN PSH URG RST FIN ACK Checksum Urg Pointer (TCP Options) TCP Data

Sequence Numbers Host A ISN (initial sequence number) Sequence number = 1st byte TCP HDR TCP Data Ack sequence number = next expected byte TCP HDR TCP Data Host B

Initial Sequence Numbers (Active) Client (Passive) Server Syn +ISNA Syn + Ack +ISNB Ack Connection Setup 3-way handshake

TCP Sliding Window • How much data can a TCP sender have outstanding in the network? • How much data should TCP retransmit when an error occurs? Just selectively repeat the missing data? • How does the TCP sender avoid over-running the receiver’s buffers?

window Sent but not acked Not yet sent Sequence numbers Sender Receive buffer Next to be sent Acked but not delivered to user Sequence numbers window The Sliding Window Receiver

TCP Sliding Window Window Size Data ACK’d Outstanding Un-ack’d data Data OK to send Data not OK to send yet • Retransmission policy is nearly “Go Back N”. • Current window size is “advertised” by receiver (usually 4k – 8k Bytes when connection set-up).

Go-back-n Sender Receiver DATA1 DATA2 DATA3 Noise DATA4 ACK1 DATA5 ACK2 DATA6 NAK DATA7 DATA3 DATA4 DATA5 DATA6 ACK3 ACK4 Time

三、多媒体通信的性能要求

三、多媒体通信的性能要求

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