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异步传输模式 .
异步传输模式 ( Asynchronous Transfer Mode,ATM )是由美国国家标准协会 和ITU-T(前身为CCITT)定义的一种电信标准,用于多种类型流量的传输。ATM采用电路交换 的方式,以信元 (cell)为单位。每个信元长53字节,其中头部资讯占了5字节。
ATM能够比较理想地实现各种QoS,既能支持有连接的业务,又能支持无需连接的业务。它是宽频ISDN(B-ISDN)技术 的典范。ATM是一种交换技术,将数码数据切割成多个固定长度的分组,在光纤或DS1/DS3中进行传输。到达目的地后再重新组合。ATM网络可以同时集成声音、影像和数据。针对各种资讯类型,提供最佳的传输环境。
ATM架构几乎被以IP为主的下一代网络取代,在历史上从未真正获取重要地位。
协议架构
ATM结合了电路交换系统和基于数据包的系统的优点。 但是与以太网数据包不同的是,ATM信元有固定的长度,因此很容易通过识别小区的头部来减少交换系统中的时间损耗。此外,它并不像电路交换那样占用线路。
ATM 信元构造
ATM信元由5个八位位组标头和48个八位位组有效负载组成。
ATM有两种不同的信元标准: 网络到网络接口 (Network-Network Interface,NNI)和用户到网络接口(User-Network Interface,UNI)。大多数ATM链路使用UNI信元格式。
UNI ATM信元图
7
4
3
0
GFC
VPI
VPI
VCI
VCI
VCI
PT
CLP
HEC
必要时的有效负载和填充(48字节)
NNI ATM信元图
7
4
3
0
VPI
VPI
VCI
VCI
VCI
PT
CLP
HEC
必要时的有效负载和填充(48字节)
GFC:通用流量控制
VPI:虚拟路径标识符
VCI:虚拟频道标识符
PT:有效载荷类型
CLP:拥塞丢失优先级
HEC:错误控制
部署与应用
ATM在20世纪90年代初受到电信公司和部分电脑制造商的青睐。然而,即使到了90年代末,基于互联网协议的产品仍比ATM有更好的性价比。[1]
FORE Systems等公司专注于ATM产品[2] ,而其他大型厂商,如思科 等,则仅将ATM视作一种额外的选择。
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