●●● ●●●● ●●●●● ●●●● 虚电路子网和数据报子网的比较 ●●● ●● Issue Datagram subnet Virtual-circuit subnet Circuit setup Not needed Required Addressing Each packet contains Each packet contains a the full source and short vc number destination address State information Routers do not hold Each Vc requires router state information about connections table space per connection Routing Each packet is Route chosen when Vc routed independently is set up; all packets follow it Effect of router failures None except for packets All VCs that passed ost during the crash through the failed router are terminated Quality of service Difficult Easy if enough resources can be allocated in advance for each Vc Congestion control Difficult Easy if enough resources can be allocated in advance for each vclinwei@bbi.edu.cn 10 虚电路子网和数据报子网的比较 5-4