Short Message Service Virtual Laboratory

Explore the underlying architecture, PDU encoding, and network signaling of SMS systems. Simulate transmission from Mobile Station to SMSC in real-time.

Learning Objectives

PDU Structure

Understand the Protocol Description Unit (PDU) format, including SCA, PDU-Type, MR, DA, PID, DCS, VP, UDL, and UD fields.

Network Architecture

Analyze the signaling path (SS7) from MS through BTS, BSC, MSC to the SMSC (Short Message Service Center).

Encoding Schemes

Differentiate between 7-bit GSM Alphabet (160 chars), 8-bit Data (140 bytes), and 16-bit UCS2 (70 chars).

Theoretical Background

SMS Network Architecture

MS
Mobile Station
BTS
Base Station
BSC
Controller
MSC
Switching Ctr
SMSC
Msg Center

Signaling Path: SMS uses the control channel (SDCCH) rather than the traffic channel (TCH) used for voice. This allows transmission even during an active call. The message travels via the SS7 (Signaling System No. 7) protocol stack.

MO-SMS PDU Structure (Mobile Originated)

SCA Service Center Address
PDU-Type Message Type Indicator (MTI), VPF...
MR Message Reference (0-255)
DA Destination Address
PID Protocol Identifier
DCS Data Coding Scheme (7bit/8bit/UCS2)
VP Validity Period
UDL User Data Length
UD User Data (The Message)

Key Concepts

  • 7-bit Packing: The GSM alphabet uses 7 bits per character. To save bandwidth, these are packed into 8-bit octets. A 160-character message fits into 140 octets.
  • Concatenated SMS (SMS-COM): If a message exceeds the single SMS limit (160 chars for 7-bit), it is split into multiple PDUs. A User Data Header (UDH) is added to indicate the sequence.
  • Validity Period (VP): Defines how long the SMSC should attempt delivery before discarding the message.

Interactive SMS Simulator

SYSTEM ONLINE

Configuration

0/160

System Log

> System initialized.
> Waiting for input...

Network Transmission Visualization

MS
BTS
BSC
MSC
SMSC
1

Current Phase

IDLE

Latency

-- ms

PDU Inspector (Hex)

// PDU will be generated here after transmission...

Transmission Analysis

Total Octets 0
Character Count 0
Segments (Concatenated) 1
Encoding 7-bit GSM

Channel Usage

SDCCH Load 0%

Did you know?

SMS uses the signaling channels that phones use to communicate with cell towers (the control channel), not the voice channels. This is why you can sometimes send a text even when you have no signal bars for a call!

Lab Report Guidelines

1. Objective

State the purpose of the experiment: To understand SMS PDU structure, encoding schemes, and network architecture.

2. Theory

Summarize the following concepts based on the theory section above:

  • Structure of a Mobile Originated SMS PDU.
  • Difference between 7-bit default alphabet and UCS2.
  • Role of the SMSC in store-and-forward architecture.

3. Procedure & Observations

Document your simulation steps:

  • Send a standard 7-bit message. Record the PDU length and hex string.
  • Send a message with > 160 characters. Observe how the PDU changes (UDH insertion) and note the number of segments.
  • Change encoding to UCS2. Send the same message as step 1 and compare the PDU size increase.

4. Calculations

Include manual calculations for:

  • Number of septets required for a given text string.
  • Conversion of phone number to BCD (Binary Coded Decimal) format.

5. Conclusion

Discuss the efficiency of 7-bit packing versus 8-bit/16-bit. Explain why SMS has a character limit.