Course Content#
OSI 7-Layer Model#
Pay attention to the data units corresponding to each layer.
Data Encapsulation#
- Before sending, continuously add headers: message -> message segment -> datagram -> frame.
- When reaching a switch, find the router's address from the frame header.
- When reaching a router, remove the header and add a new header.
- When matching a specific host, continuously remove headers to obtain the data.
Packet Switching vs. Circuit Switching#
Packet Switching Network#
The common Internet is based on packet switching.
Messages are divided into multiple packages for transmission, and the routing protocol determines the route of the packages.
Advantages: Bandwidth sharing, simplicity, low cost.
Disadvantages: Delay, potential packet loss.
Store-and-Forward Mechanism#
Routers wait for all packages to arrive before forwarding them.
PS: In case of exceptional situations, it may trigger timeout retransmission or 3-time redundant ACK retransmission.
Queuing Delay and Packet Loss#
Queuing delay is the most unpredictable and fluctuating delay among all delays.
The waiting time for triggering router timeout retransmission will be adjusted based on the previous loss situation. When timeout retransmission occurs, the router will increase the expected waiting time.
PS: The router has multiple ports, and each port has input and output buffers. When the router is full, it is easy to cause packet loss.
Forwarding Table and Routing Protocol#
Microscopic: Forwarding table, each destination IP address corresponds to an output link.
Macroscopic: Routing protocol, based on the forwarding table for routing selection.
Circuit Switching Network#
Similar to telephone, both parties establish an independent physical channel, maintained by routers.
Advantages: Efficiency, security, reliability.
Disadvantages: Limited resources, only one connection per line.
Two ways to increase the number of connections:
Frequency Division Multiplexing#
Different frequencies are responsible for transmitting different types of information, such as downstream data and upstream data.
Time Division Multiplexing#
Divide time into segments to achieve multiplexing.
Challenge: Granularity of segmentation.
Delay, Packet Loss, and Throughput#
Total Node Delay#
Transmission delay mainly considers the transmission time of one packet/unit, rather than the transmission time of the entire data, otherwise it will be greatly affected by the data size.
Queuing Delay and Packet Loss#
The average queuing delay is exponentially related to the traffic intensity. When the traffic intensity approaches 1, the queuing delay exponentially increases, resulting in packet loss.
Note: Traffic intensity $=La/R$, where $L$ is the number of bits per group, $a$ is the rate at which traffic arrives at the queue, and $R$ is the rate at which bits are pushed out of the queue, i.e., the transmission rate of the link.
Throughput#
The throughput of a link depends on the minimum throughput of each link in the link.
Additional Knowledge#
- Network Layer Models
- Benefits of layering: Independence between layers; good flexibility; easy maintenance; promotion of work standardization.
- Functions of each layer: Error control, flow control, segmentation multiplexing and demultiplexing, connection establishment and release.