B.数据包将被传递通过一个又一个2路由器所以时间完全转移的第一个数据包离开路由器Router1是15×10 ^ 6/5000 /（3 * 10 * 2 = 0.002s ^ 6）；
1. Dealing with complex systems: explicit structure allows identification, relationship of complex system’s pieceslayered reference model for discussion
2.Modularization eases maintenance, updating of system: change of implementation of layer’s service transparent to rest of system(e.g., change in gate procedure doesn’t affect
rest of system)
1. May take much effect to consider the layers in the beginning.
2. Would take a long time for all the communities to agree on the standard layers.
Application: supporting networkapplications e.g. FTP, SMTP, HTTP
Transport: process-process data transfer e.g. TCP, UDP
Network: routing of datagrams from source to destination e.g. IP, routing protocols
Link: data transfer between neighboring network elements e.g. PPP, Ethernet
Physical: bits “on the wire”
Presentation: allow applications to interpret meaning of data, e.g., encryption, Compression, machine-specific conventions
Session: synchronization, check pointing, recovery of data exchange
The link layer contains communication technologies for a single network segment (link) of a local area network.
The internet layer (IP) connects independent networks, thus establishing internetworking.
The transport layer handles host-to-host communication.
The application layer contains all protocols for specific data communications services on a process-to-process level. For example, the Hypertext Transfer Protocol (HTTP) specifies the web browser communication with a web server.
The Internet application layer includes the OSI application layer, presentation layer, and most of the session layer. Its end-to-end transport layerincludes the graceful close function of the OSI session layer as well as the OSI transport layer. The internetworking layer (Internet layer) is a subset of the OSI network layer (see above), while the link layer includes the OSI data link and physical layers, as well as parts of OSI's network layer. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the network layer document.
1. Nodal processing e.g. check bit errors and determine output link
2. Queueing e.g. time waiting at link for transmit and depends on conglevel of router
3. Transmission delay:
R=link bandwidth (bps)
L=packet length (bits)
time to send bits into link = L/R
4. Propagation delay:
d = length of physical lin
s = propagation speed in medium (~2x108 m/sec)
propagation delay = d/s
La/R ~ 0: average queueing delay small
La/R -> 1: delays become large
La/R > 1: more “work” arriving than can be serviced, average delay infinite!
Host A just send the entire packet into the line so it will be right at the end of A;
The first bit will be 12/15*m = 4/5m meters from A;
according to specification, we should find m so that the equality:
m/S = L/R
so m = SL/R = 2.5 * 10^8 * 100 / 28 = 8.92 * 10^8 meters;