Course Content#
OSI 7-Layer Model#
Note the data units corresponding to each layer.
Data Encapsulation#
- Before sending, continuously add headers: Message 👉 Segment 👉 Datagram 👉 Frame
- Upon reaching the switch, find the router's address from the frame header
- Upon reaching the router, strip the header, add a new header
- When matched to a specific host, continuously strip the header to obtain the data
Packet Switching VS. Circuit Switching#
Packet Switching Network#
The general internet is based on packet switching.
Messages are divided into multiple packages for transmission, and routing protocols determine the route of the packages.
Advantages: Bandwidth sharing, simplicity, low cost.
Disadvantages: There is latency, and packet loss may occur.
Store-and-Forward Mechanism#
Routers will only forward packages once they are all received.
PS: In case of anomalies, timeout retransmissions or 3 redundant ACK retransmissions may be triggered.
Queuing Delay and Packet Loss#
Queuing delay is the most uncontrollable and frequently fluctuating delay among all delays.
The waiting time that triggers router timeout retransmissions will adjust based on the last loss situation. When a timeout retransmission occurs, the router will increase the expected waiting time.
PS: Routers have multiple ports, each with input and output buffers. When the router cannot accommodate more, packet loss is likely to occur.
Forwarding Table and Routing Protocol#
Micro: Forwarding table, each destination IP address corresponds to an output link.
Macro: Routing protocol, routing decisions are made based on the forwarding table.
Circuit Switching Network#
Similar to a telephone, both parties establish an independent physical channel, maintained by the router's connection state.
Advantages: Efficient, secure, reliable.
Disadvantages: Limited resources, a single line can only have one connection.
Connections can be increased through the following two methods.
Frequency Division Multiplexing#
Different frequencies are responsible for transmitting different types of information, such as downstream data and upstream data.
Time Division Multiplexing#
Divides 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 package/unit, rather than the total transmission time of all data, otherwise it is too affected by the data size.
Queuing Delay and Packet Loss#
The average queuing delay has an exponential relationship with traffic intensity. As traffic intensity approaches 1, the queuing delay increases exponentially, leading to 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 segment.
Additional Knowledge Points#
- Multi-layer Network Model
- Benefits of layering: Independence between layers; good flexibility; easy maintenance; promotes standardization of work.
- Functions of each layer: Error control, flow control, segmentation multiplexing and demultiplexing, connection establishment and release.