9618 · 14.1

Protocols — practice questions

Practice and worked examples for 9618 Protocols. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

A client wants to establish a TCP connection with a server. Describe the three steps of the TCP three-way handshake, identifying the flags (SYN, ACK) used in each packet.

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The TCP three-way handshake ensures both client and server are ready to communicate.

Step 1: SYN The client initiates the connection by sending a TCP packet to the server with the SYN (Synchronise) flag set. This packet essentially says, 'I want to start a connection.'

Step 2: SYN-ACK If the server is ready, it responds with a TCP packet that has both the SYN and ACK (Acknowledge) flags set. This means, 'I acknowledge your request and I also want to establish a connection.'

Step 3: ACK Finally, the client sends a third packet back to the server with the ACK flag set. This acknowledges the server's response and completes the establishment of the connection. 'I acknowledge your readiness, the connection is now open.'

At this point, data transfer can begin.

Worked example 2

A communication system uses a modulation technique where each signal change can represent one of eight different voltage levels. If the system has a baud rate of 4800 baud, what is the bit rate in kilobits per second (kbps)?

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Step 1: Find the number of bits per symbol. The number of bits per symbol depends on the number of possible levels (N). The formula is $log_2(N)$. In this case, N = 8. Number of bits per symbol = $log_2(8) = 3$ bits.

Step 2: Calculate the bit rate. Using the formula: Bit Rate = Baud Rate × Number of bits per symbol. Bit Rate = 4800 × 3 = 14,400 bps.

Step 3: Convert to kbps. To convert from bps to kbps, divide by 1000. Bit Rate = 14,400 / 1000 = 14.4 kbps.

Final Answer: The bit rate is 14.4 kbps.

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