2013年5月24日

How [ RFC1483 Bridging ] work ?

The Cisco 6xx CPE defaults to routing mode. Therefore, when it is configured for bridging mode and installed at the subscriber's location with the necessary splitters/microfilters, it trains up automatically upon power up. When the CPE trains up, it indicates that the physical layer between the CPE and DSLAM is fine. Depending on how the end station?s IP address is configured (that is, whether it is assigned via a DHCP server or it is a static IP address with default gateway information), it can then communicate with the service destination.

Following is a description of the flow of packets.

The user's data is encapsulated in IEEE 802.3 from the PC and enters the Cisco 6xx CPE. It is then encapsulated into an Logical Link Control/Subnetwork Access Protocol (LLC/SNAP) header, which in turn is encapsulated in ATM adaptation layer 5 (AAL5) and handed over to the ATM layer.

routed_bridged_encap_1


The ATM cells are then modulated by the ADSL transmission technology, Carrierless Amplitude and Phase (CAP) modulation or Discrete Multi-Tone (DMT), and sent over the wire to the DSLAM. At the DSLAM, these modulated signals are first received by the POTS splitter, which checks whether the frequency of the signal is below or above 4 kHz. After it identifies the signals as above 4 kHz, it passes them to the ADSL Transmission Unit - Central Office (ATU-C) in the DSLAM.

The ATU-C demodulates the signal and retrieves the ATM cells, which are then passed to the network interface card (NIC) in the multiplexing device (MUX). The NIC looks at the subscriber side VPI/VCI information in the ATM header and makes the switching decision to another VPI/VCI which will be forwarded to the service destination router. After the service destination router receives these cells on a particular ATM interface, it re-assembles them, looks at the upper layer, and passes the information to the BVI interface. The BVI interface looks at the Layer 3 information and decides where the packet is to be delivered.

2013年5月23日

31

從家裡到公司大約7.5公里。

假設從出發到終點是一條直線,且沒有任何停留:
以騎機車、開車來算,時速50耗時9分鐘,
以騎單車來算,時速30耗時15分鐘,
若以跑步來算,配速5.5分鐘/公里耗時41分鐘。

可是事實上,我到公司必須經過的紅綠燈,總共有31個!
如果一個紅綠燈要停留30秒,那總共得停留15.5分鐘!

根據之前看到的調查,台灣人平均每天來回通勤24公里。
換句話說,就是每天花費將近兩個小時在通勤上。
塞車、發呆,嚴然成為台灣人的第二專長...

20年內台灣的交通狀況不會改善到哪裡去,
既然兩個小時的通勤免不了,那我能做的就是想辦法提升這兩個小時的附加價值。

山不轉,幹,我轉。


001
(這不是擷取畫面,這是我們家小朋友畫的!是不是很有天分?)