EE5412 Telecommunication Networks--ATM rev1

EE5412 Telecommunication Networks--ATM rev1

1520年是馬丁路德正式與中世紀羅馬天主教決裂的年分，那年他出版了三本

立場明確的「 改教運動論文 」。

第一篇是《訴諸平信徒》（Appeal to the Laity），他要求德國諸侯起來改革教會，並且拒絕納貢給羅馬，廢掉教士守獨身、為死人舉行彌撒、朝聖和天主教其他的建制和做法。

第二篇是《教會被巴比倫俘擄》（On the Babylonian Captivity of the Church）。從積極一面，他提出了新約的聖禮（Sacrament）神學，和兩種團契；從消極一面，他反對變質說（Transubstantiation），和彌撒中的獻祭觀，以及一切與之有關的邪惡（參聖餐，Eucharist）。

第三篇是《基督徒的自由》（The Freedom of a Christian Man），這是一篇不牽涉任何辯論、上乘的靈修作品，解釋靠恩稱義和藉行為稱義的分別。

1520年 六月十五日，教宗發表一道教諭（Exsurge, Domine），把路德所有的著作判為異端，又下命令要焚毀他的作品；教宗給他六十天時間收回自己的作品。

路德公然把這教諭和一些過時的教會律例（Canon Law）焚燒，羅馬教廷立刻把路德開除教籍（Decet Romanum，1521年 一月三日）。

Tutorial - ATM Ref0_ATM.doc

1. What is ATM?
Ans.

ATM is the switching and multiplexing technique chosen by CCITT for the broadband access to ISDN.

User network interface offers one physical channel over which connections are multiplexed using short, fixed length packets called cells.
Cells are switched in the network based on the routing information contained in their headers.

In B-ISDN, ATM layer is in charge of transporting cells to their destinations. The cells are carried by the physical layer transmission system. ATM is intended to transport all services with the same format. Switching fabrics need not be aware of the services being transported.

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2. What is the advantages of using ATM in providing various services such as speech and video?

Characteristic of ATM –
Ans; Bandwidth on demand; variable bit rate; better bandwidth utilization; same interface and transport system for all services; QOS guaranteed.

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3. In ATM, small fixed cell size is used.
What is the purpose of using fixed size cell?
What is the purpose of using small cell size?

Ans : Fast hardware switching.
Reduce node delay.

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4. ATM service is classified into four different categories and is classified as “service access point 1” to “service access point 4” to the adaptation layer. How are the services classified? Give an example to each class of service.

Ans: Service class – constant bit rate – eg. Fixed access, voice circuit
Variable bit rate – eg. Video
Connection oriented data – dg. X.25
Connection-less data – eg. Virtual LAN, close user group

The above 4 services are classified as SAP1 to SAP4 (service access point) to the adaptation layer.

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5. The ATM cell header consists of a few bytes which is obviously not able to carry full destination information.
How routing can be done in the switches?
Give an example with a few switching nodes.

Ans: Label switching technique is used.
Label carries the routing information and is not an explicit address.
Explicit Address is not possible because of the short fixed cell size.

Switch keeps routing table
Routing table set up in advance
Cell label used for switching and will be modified by switch on routing to its destination
Label consists of VPI, virtual path identifier and VCI, virtual channel identifier.

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6. Cell loss is one of the problems with ATM switching. What is cell loss and how it happens?

Ans : Dropping of cells due to system buffer overflow.

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The role of AAL layer in ATM network
AAL is responsible to provide mapping between the ATM layer and the next higher layer.

Service dependent, supports information transfer protocols not based on ATM
Maps high-layer information into ATM cells
Collects information from ATM cells for delivery to higher layers

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ATM gives better bandwidth utilization in a multi-rate environment. What does multi-rate means?

Ans – Multi-rate is to group different class of services together into families.
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