AAL - ATM Adaptation Layer

In order for ATM to support many kinds of services with different traffic characteristics and system requirements, it is necessary to adapt the different classes of applications to the ATM layer. This function is performed by the AAL, which is service-dependent. Four types of AAL were originally recommended by CCITT. Two of these (3 and 4) have now been merged into one, AAL 3/4.

Briefly the four AALs are:

• AAL1 - Supports connection-oriented services that require constant bit rates and have specific timing and delay requirements. Example are constant bit rate services like DS1 or DS3 transport.
• AAL2 - Supports connection-oriented services that do not require constant bit rates. In other words, variable bit rate applications like some video schemes.
• AAL3/4 - This AAL is intended for both connectionless and connection oriented variable bit rate services. Originally two distinct adaptation layers AAL3 and 4, they have been merged into a single AAL which name is AAL3/4 for historical reasons.
• AAL5 - Supports connection-oriented variable bit rate data services. It is a substantially lean AAL compaired with AAL3/4 at the expense of error recovery and built in retransmission. This tradeoff provides a smaller bandwidth overhead, simpler processing requirements, and reduced implementation complexity. Some organizations have proposed AAL5 for use with both connection-oriented and connectionless services. See rfc1483 for a description of encaspsulation of layer 3 protocols and bridged PDUs over AAL5. rfc1483 also contains a description of the AAL5 frame format.

Via: http://cell.onecall.net/cell-relay/FAQ/dictionary/AAL.html

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Wireless Mesh

Wireless Mesh is mesh networking implemented over a wireless LAN. 其核心是让网络中的每个节点都发送和接收信号，任何无线设备节点都可以同时作为AP和路由器，每个节点都可以与一个或多个对等节点进行直接通信。Each node need only transmit as far as next node. Node acts as repeaters to transmit data from nearby nodes to pear that are too far away to reach, resulting in network taht can span large distance. This type of infrastructure is decentralized, inexpensive, very reliable and resilient. It is non Line-of-Sight.

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WiMAX与Wi-Fi

WiMAX和Wi-Fi是两种无线技术。WiMAX (World Interoperability for Microwave Access), 即IEEE 802.16. WiMAX是一项无线城域网(MAN)技术, 可在更大的范围内提供宽带无线连接, WiMAX的有效范围是以英里记的，而不是英尺。Wi-Fi, 即IEEE 802.11b，是一种局域网(LAN)技术, 属于在办公室和家庭中使用的短距离无线技术.

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Optical Access Network

纯光纤接入网(Optic Access Network ,OAN)是指传输介质全部采用光纤连接到用户的网络, 又称为光纤用户环路(FTTL), 它是未来接入网的主要实现技术。为此，需要在本地交换局(LE) 设置光纤线路终端OLT,在用户侧设置光纤网络单元ONU。从应用角度分类，光纤接入网可划分为两种不同的类型：光纤到路边(FTTC)及光纤到大楼(FTTB)；光纤到家庭(FTTH)及光纤到办公室(FTTO).

将光纤网络单元(ONU)放置路边，ONU和用户之间用同轴电缆或双绞线相连，ONU置放在大楼内，用铜线或同轴电缆延伸到用户。用户可以广泛使用高速数据、电子检索、电子邮件、可视图文、远程教育等宽带业务，非常适合现代智能大楼。

将ONU放到用户室内，称为光纤到家(FTTH). FTTH和FTTO为一种全光纤的网络结构，用户与业务节点之间以全光纤作传输。因此无论在带宽方面还是 在传输质量和维护方面都十分理想，适合各种交互式业务，是接入网发展的最终目标。但因FTTH要为每个用户提供光纤和光端机，短期内经济上难以承受，所以 是长远的发展方向。

光纤接入网可划分为无源光网络(PON)和有源光网络(PON)和有源光网络(AON)。利用ATM的集中和统计复用的无源光纤接入网络(APON)前途最为看好，是面向21世纪宽带接 入技术的最新发展方向。由于APON是无源系统，因而安装、开通和运行维护的成本大为降低，将会逐步成为真正的全业务接入网系统。但需改进的是要使 APON能够支持IP接入。

光纤用户网的主要技术是光波传输技术，目前光纤传输的复用技术用得最多的是时分复用(TDM), 波分复用(WDM), 频分复用(FDM), 码分复用(CDM)等。

Via: http://cne2345.spaces.live.com/blog/cns!D267D10611CBC1FF!105.entry

从总的发展趋势看,在接入网中馈线光纤必将代替馈线电缆，直接向用户挺进，实现纯光纤接入，即光纤到家 （FTTH）方式。这种方式光纤直通用户家中，一般仅需要一至二条用户线，是长远的发展方向和最终的接入网解决方案。目前的实际应用中，部分实现光纤化， 光纤只到路边的业务接入点(SAP)。在该处通过光网络单元（ONU），完成光/电转换和分接等功能，然后通过多种方式接入用户家中。光纤到大楼后可以采 用ADSL、Cable、LAN，即FTTx+ADSL、FTTx+Cable和FTTx+LAN等方式接入用户家中。

光纤接入网（OAN）是采用光纤传输技术的接入网，即本地交换局和用户之间全部或部分采用光纤传输的通信系统。光纤接入网又可划分为无源光网络(PON)和有源光网络(AON)，其中无源光网络PON发展更快些。

PON(无源光网络)是指ODN(光配线网)中不含有任何电子器件及电子电源，ODN全部由光分路器(Splitter)等无源器件组成，不需要贵重的有 源电子设备。PON网络的突出优点是消除了户外的有源设备，所有的信号处理功能均在交换机和用户宅内设备完成。而且这种接入方式的前期投资小，大部分资金 可以等到用户真正接入时才投入。它的传输距离比有源光纤接入系统的短，覆盖的范围较小，但它造价低，无需另设机房，维护容易。因此这种结构可以经济地为居 家用户服务。PON的复杂性在于信号处理技术。在下行方向上，交换机发出的信号是按广播式发给所有的用户。在上行方向上，各ONU必须采用某种多址接入协 议，如TDMA(Time Division Multiple Access)协议，才能完成共享传输通道信息访问。

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VPN types - Secure and Trusted

Via: Wikipedia

Secure VPNs use cryptographic tunneling protocols to provide the intended confidentiality (blocking snooping and thus Packet sniffing), sender authentication (blocking identity spoofing), and message integrity (blocking message alteration) to achieve privacy. When properly chosen, implemented, and used, such techniques can provide secure communications over unsecured networks. This has been the usually intended purpose for VPN for some years.

Because such choice, implementation, and use are not trivial, there are many insecure VPN schemes available on the market.

Secure VPN technologies may also be used to enhance security as a "security overlay" within dedicated networking infrastructures.

Secure VPN protocols include the following:

• IPsec (IP security) - commonly used over IPv4, and an obligatory part of IPv6.
• SSL used either for tunneling the entire network stack, as in the OpenVPN project, or for securing what is, essentially, a web proxy. SSL is framework more often associated with e-commerce, but it has been built-upon by vendors like Aventail and Juniper to provide remote access VPN capabilities.
• PPTP (point-to-point tunneling protocol), developed jointly by a number of companies, including Microsoft.
• L2TP (Layer 2 Tunnelling Protocol), which includes work by both Microsoft and Cisco.
• L2TPv3 (Layer 2 Tunnelling Protocol version 3), a new release.
• VPN-Q The machine at the other end of a VPN could be a threat and a source of attack; this has no necessary connection with VPN designs and has been usually left to system adminstration efforts. There has been at least one attempt to address this issue in the context of VPNs. On Microsoft ISA Server, an applications called QSS (Quarantine Security Suite) is available.

Some large ISPs now offer "managed" VPN service for business customers who want the security and convenience of a VPN but prefer not to undertake administering a VPN server themselves. In addition to providing remote workers with secure access to their employer's internal network, other security and management services are sometimes included as part of the package. Examples include keeping anti-virus and anti-spyware programs updated on each client's computer.

Trusted VPNs do not use cryptographic tunneling, and instead rely on the security of a single provider's network to protect the traffic. In a sense, these are an elaboration of traditional network and system administration work.

• Multi-protocol label switching (MPLS) is often used to build trusted VPN.
• L2F (Layer 2 Forwarding), developed by Cisco, can also be used.

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ATM Traffic contract/Service contract

Via: Wikipedia

Another key ATM concept is that of the traffic contract. When an ATM circuit is set up each switch is informed of the traffic class of the connection.

ATM traffic contracts are part of the mechanism by which "Quality of Service" (QoS) is ensured. There are four basic types (and several variants) which each have a set of parameters describing the connection.

• CBR - Constant bit rate: you specify a Peak Cell Rate (PCR), which is constant.
• VBR - Variable bit rate: you specify an average cell rate, which can peak at a certain level for a maximum interval before being problematic.
• ABR - Available bit rate: you specify a minimum guaranteed rate.
• UBR - Unspecified bit rate: your traffic is allocated all remaining transmission capacity.

VBR has real-time and non-real-time variants, and is used for "bursty" traffic.

Most traffic classes also introduce the concept of Cell Delay Variation Tolerance (CDVT) which defines the "clumping" of cells in time.

Traffic contracts are usually maintained by the use of "Shaping", a combination of queuing and marking of cells, and enforced by "Policing".

Traffic shaping

Traffic shaping is usually done at the entry point to an ATM network and attempts to ensure that the cell flow will meet its traffic contract.

Traffic policing

To maintain network performance it is possible to police virtual circuits against their traffic contracts. If a circuit is exceeding its traffic contract, the network can either drop the cells or mark the Cell Loss Priority (CLP) bit (to identify a cell as discardable further down the line). Basic policing works on a cell by cell basis, but this is sub-optimal for encapsulated packet traffic (as discarding a single cell will invalidate the whole packet). As a result, schemes such as Partial Packet Discard (PPD) and Early Packet Discard (EPD) have been created that will discard a whole series of cells until the next frame starts. This reduces the number of redundant cells in the network, saving bandwidth for full frames. EPD and PPD work with AAL5 connections as they use the frame end bit to detect the end of packets.

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Connection Admission Control

Via: Wikipedia: http://en.wikipedia.org/wiki/Connection_Admission_Control

Connection admission control is an admission control in ATM networks. It is defined as ‘the set of actions taken by the network during the call set-up phase (or during the call renegotiation phase) to establish whether a VC/VP connection can be accepted’ [2][3].

In principle, in a connection-oriented network, the role of CAC is to decide whether there are sufficient free resources on the requested link to allow a new connection [4]. A connection can only be accepted if sufficient resources are available to establish the connection end-to-end with its required QoS. The agreed QoS of existing connections in the network must not be affected by the new connection [2].

If the network has the required resources, the CAC may allow a connection request to proceed; if not, the CAC will indicate this and notify the originator of the request that the request has been refused [4].

Via: 赛迪网

中文译名: 连接接纳控制

解释: 异步转移模式（ATM）技术的一种流量控制标准。在开始建立连接时分配网络资源，判断是否可以满足连接申请的各种要求并达成某种协约。用户也可以根据连接接纳控制（CAC）期间达成的协约中分析网络的拥塞情况。

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Circuit switching

Via: Circuit switching - Wikipedia, the free encyclopedia

In telecommunications, a circuit switching network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may communicate. Each circuit that is dedicated cannot be used by other callers until the circuit is released and a new connection is set up. Even if no actual communication is taking place in a dedicated circuit then, that channel still remains unavailable to other users. Channels that are available for new calls to be set up are said to be idle.

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Virtual Circuit 虚电路

虚电路是在分组交换散列网络上的两个或多个端点站点间的链路。它为两个端点间提供临时或专用面向连接的会话。它的固有特点是，有一条通过多路径网络 的预定 路径。提前定义好一条路径，可以改进性能，并且消除了帧和分组对头的需求，从而增加了吞吐率。从技术上看，可以通过分组交换网络的物理路径进行改变，以避 免拥挤和失效线路，但是两个端系统要保持一条连接，并根据需要改变路径描述。

具有永久性和交换型的虚电路如下所述：

永久性虚电路（PVC）是一种提前定义好的，基本上不需要任何建立时间的端点站点间的连接。在公共－长途电信服务，例如异步传输模式（ATM）或帧 中继 中，顾客提前和这些电信局签订关于PVC的端点合同，并且如果这些顾客需要重新配置这些PVC的端点时，他们就必须和电信局联系。
交换型虚电路（SVC）是端点站点之间的一种临时性连接。这些连接只持续所需的时间，并且当会话结束时就取消这种连接。虚电路必须在数据传送之前建立。一些电信局提供的分组交换服务允许用户根据自己的需要动态定义SVC。

在ATM环境，在端点站点之间的逻辑连接称为虚拟信道（VC）。虚拟路径（VP）是一个或多个VC通过一个散列网络到达相同目的地的一条定义好的路 径，虽 然每个VC可以连接到不同的端系统或在这个目的地的应用处理。可以将VP想象为包含一束电线的电缆。在这个比喻中，该电缆将两点和此电缆 连接端系统内的独立电路相连。它的优点是，共享穿越网络的相同路径的连接被组织在一起，并使用相同的管理功能。如果已经建立了VP，就可以很轻易地增加新 的VC，这时因为已经完成了定义穿越这个网络路径的工作。另外，如果这个网络为了避免拥挤或失效的线路而需要改变一条路径时，所有为这个VP建立的VC都 被定向到这个新的路径。

PVC是在帧中继环境中传统的连接方式，虽然1993年末SVC被支持加入这个规范。PVC具有特定服务特点。在与提供商建立了服务时，就定义 PVC和这 里列出的服务特点。这个链路的服务特点：承约信息大小（committed burst size）、承约信息率（committed information rate）、过量信息大小（excess burst size）和帧大小（frame size）。它们是在协商时定义的。

承约信息大小（CBS）是网络提供商同意在时间间隔内的正常网络状态进行传输的最大数据（按位）的数量。

承约信息率（CIR）是网络提供商同意的在一个PVC的正常网络状态期间传输CBS承约数据的传输率。

过量信息大小（EBS）是最大允许的超出CBS的未承约数据（按位）的数量，这个网络将试图在一个时间间隔期间传送它们。EBS数据将在网络拥挤期间被网络按照可抛弃数据对待。

帧大小是传送顾客数据穿越分组交换网络的帧的体积。

参考资料：
1.赛迪网 http://www0.ccidnet.com/school/dict/explain/V_408.html

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Exponential backoff

Exponential backoff is an algorithm that uses feedback to multiplicatively decrease the rate of some process, in order to gradually find an acceptable rate. It is often used in network congestion avoidance to help determine the correct sending rate. For example, a sender might send a message, set a timer to wait 0.25 seconds for an acknowledgment, and if none arrives, retransmit the message and wait 0.5 seconds for an acknowledgment. It will continue to retry until it receives an acknowledgement and will wait, 1s, 2s, 4s, 8s, etc. each time before retrying.

Via: Wikipedia

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