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物流管理系统--仓储管理子系统的实现

编辑:admin    来源:JSP毕业设计    作者:JSP毕业论文

  电路交换网与vb调用数据库
  切换电路的连接设置的结果是保留了从发送端到接收端的包所通过的线路带宽。其它的特性这一就是所有的包都沿同一个线路意味着如果包传输超过了时序那它将不能到达接收端.由于没有路径用来进行包交换,所以要想不同的包沿不同的路径传送就是能依靠网络的条件了。这样包即便是超过了时序它们也有可能到达的。
  包交换技术比线路交换技术更能容错。事实上,这也是它被发明的理由!当一个交换开头被拉下,则所有使用它的线路将被切断并且其余的包将不能再被传送。此时如果在进行包交换,包将在已选择的路径上停止交换。
  预先建立一条路径也开创预先保留带宽的可能性。如果带宽被保留,那么当一个包到达时,通过保留的带宽它可以被立即发送出去。假如没有带宽被保留用于包交换,那么所有的包将不得不等待着被转发。
  预先保留带宽意味着当一个包出现时不会发生拥塞(除非出现的包比预期的还多)。在另一方面,当尝试建立一条电路时,由于拥塞,尝试将会失败。因此,在线路交换(在设置时)和包交换(在包发送时)的不同时间拥塞都可能发生。
  如果为特定的用户建立一条线路那么当这个用户没有数据传送时,则这些保留下来的带宽将被浪费而不能用于其它用户的传输。而包交换则不会浪费带宽,因此它在系统宽带方面的应用更有前景。理解电路交换和包交换的这种商业差别是极其重要的。这种商业差别是在质保服务和浪费资源相对不质保服务和不浪费资源的。
  包交换是利用存储器并向前发送的。包是先存储在路由器的内存里然后再发送到下一个路由器的。和电路交换一样信息位也是连续不断地通过导线传输的。存储转发技术增加了延迟时间。
  另一个不同是电路交换是完全透明的。发送者和接收者可以使用他们想要的任一比特速率,格式和帧同步.这通信公司不知道也不关心这的。而在包交换中载体决定了基本的参数的。用一个简单比喻它们就像一个是公路一个是铁路。在以前,用户决定介质的大小,速度和特性。而后来这些都由通信公司做了的。这些都是透明的它允许声音,数据和传真共同存在于电话系统里。
  Electric circuit exchange network and vb transfer database
  The result of the connection setup with circuit switching is the reservation of bandwidth all the way from the sender to the receiver. All packets follow this path. Among other properties, having all packets follow the same path means that they cannot arrive out of order. With packet switching there is no path , so different packets can follow different paths, depending on network conditions at the time they are sent. They may arrive out of order.
  Packet switching is more fault tolerant than circuit switching. In fact, that is why it was invented. If a switch goes down, all of the circuits using it are terminated and no more traffic can be sent on any of them. With Packet switching, packets can be routed around dead switches.
  Setting up a path in advance also opens up the possibility of reserving bandwidth in advance. If bandwidth is reserved, then when a packet arrives, it can be sent out immediately over the reserved bandwidth. With packet switching, no bandwidth is reserved, so packets may have to wait their turn to be forwarded.
  Having bandwidth reserved in advance means that no congestion can occur when a packet shows up (unless more packets show up than expected).On the other hand, when an attempt is made to establish a circuit, the attempt can fail due to congestion. Thus, congestion can occur at different times with circuit switching(at setup time) and packet switching(when packets are sent).
  If a circuit has been reserved for a particular user and there is no traffic to send, the bandwidth of that circuit is wasted. It cannot be used for other traffic. Packet switching does not waste bandwidth and thus is more efficient form a system-wide perspective. Understanding this trade-off is crucial for comprehending the difference between circuit switching and packet switching. The trade-off is between guaranteed service and wasting resources versus not guaranteeing service and not wasting resources.
  Packet switching uses store-and-forward transmission. A packet is accumulated. in a router’s memory, then sent on to the next router. With circuit switching, the bits just flow through the wire continuously. The store-and-forward technique adds delay.
  Another difference is that circuit switching is completely transparent. The sender and receiver can use any bit rate, format, or framing method they want to. The carrier does not know or care. With packet switching, the carrier determines the basic parameters. A rough analogy is a road versus a railroad. In the former, the user determines the size, speed, and nature of the vehicle; in the latter, the carrier does. It is this transparency that allows voice, data, and fax to coexist within the phone system.

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