STM0, STM1, STM4, STM16, STM64, STM256: Capacity and Speed Comparison

This article compares the capacity and speed of STM0, STM1, STM4, STM16, STM64, and STM256. We’ll explore the differences between these Synchronous Transport Modules (STMs) concerning their data-carrying capabilities.

CCITT (now ITU-T) defined a new multiplexing hierarchy called SDH (Synchronous Digital Hierarchy). Both PDH (Plesiochronous Digital Hierarchy) and SDH are used with digital multiplexers in telephone exchanges to combine different hierarchies with varying bit rates.

STMs are fiber optic network levels defined within SDH for different bit rates.

Understanding E1 Frames

Let’s first understand the E1 frame, as it’s used as a reference point for STM capacity.

Figure 1: E1 Frame

The E1 frame takes 8 bits from each of the 64 kbps channels sequentially and retransmits these channels at 2048 kbps (2.048 Mbps). An E1 frame accommodates 30 voice channels, plus two additional 64 kbps channels. These two extra channels transmit synchronization, service, and signaling data.

Therefore, the E1 transmission rate is 2.048 Mbps, equivalent to 32 * 64 kbps. The E4 rate corresponds to 140 Mbps, as you’ll see later.

SDH and STM

SDH uses a basic digit rate of 155.52 Mbps and multiples of this rate by factors of 4*n. This basic SDH signal is called STM-1. Other signals include STM-4, STM-16, STM-64, and STM-256.

The following sections detail the speed and capacity of these STM levels relative to E1 and E4:

• E1 = 2.048 Mbps
• E2 = 4 x E1 (8.448 Mbps)
• E3 = 4 x E2 (34.368 Mbps)
• E4 = 4 x E3 (139.264 Mbps)
• E5 = 4 x E4 (565.148 Mbps)

STM Level Specifications

STM0 Speed | STM0 Capacity

• STM0: Synchronous Transport Module at level 0
• Speed (Bit Rate): 51.84 Mbits/sec
• Abbreviated bit rate: 51 Mbps
• Capacity: 21 * E1

STM1 Speed | STM1 Capacity

• STM1: Synchronous Transport Module at level 1
• Speed (Bit Rate): 155.52 Mbits/sec
• Abbreviated bit rate: 155 Mbps
• Capacity: 63 * E1 or 1 * E4

STM4 Speed | STM4 Capacity

• Speed (Bit Rate): 622.08 Mbits/sec
• Abbreviated bit rate: 622 Mbps
• Capacity: 252 * E1 or 4 * E4

STM16 Speed | STM16 Capacity

• Speed (Bit Rate): 2488.32 Mbits/sec
• Abbreviated bit rate: 2.4 Gbps
• Capacity: 1008 * E1 or 16 * E4

STM64 Speed | STM64 Capacity

• Speed (Bit Rate): 9953.28 Mbits/sec
• Abbreviated bit rate: 10 Gbps
• Capacity: 4032 * E1 or 64 * E4

STM256 Speed | STM256 Capacity

• Speed (Bit Rate): 39813.12 Mbits/sec
• Abbreviated bit rate: 40 Gbps
• Capacity: 16128 * E1 or 256 * E4