Feature:
The ESP-01S WiFi module is based on ESP8266 chip, so ESP-01S has all the functions and performance of ESP8266.
Comparing with former version ESP-01, ESP-01s has below advantages:
The PCB layout has changed for an improvement in WiFi radio performance.
4MB Flash Memory, upgraded from 512KB.
Latest version ESP-01s, with stronger signal
4MB Flash chip: XMC 0H32BHIG, XM25QH32CJIG or XM25QH32CHIGU10Q, wide chip or narrow chip, ship by random.

Specification:
Wifi standard: 802.11 b/g/n
Wake up and transmit packets in Standby power consumption of SDIO 2.0, SPI, UART
ESP8266 Chip in 32-pin QFN package
Wi-Fi Direct (P2P), soft-AP
Integrated TCP/IP protocol stack
Integrated TR switch, balun, LNA, power amplifier and matching network
Integrated PLLs, regulators, DCXO and power management units
+19.5dBm output power in 802.11b mode
Power down leakage current of Integrated low power 32-bit CPU could be used as application processor
STBC, 1×1 MIMO, 2×1 MIMO
A-MPDU & A-MSDU aggregation & 0.4ms guard interval
Integrated RF switch, balun, 24dBm PA, DCXO, and PMU
Integrated RISC processor, on-chip memory and external memory interfaces
Integrated MAC/baseband processors
Quality of Service management
I2S interface for high fidelity audio applications
On-chip low-dropout linear regulators for all internal supplies
Proprietary spurious-free clock generation architecture
Integrated WEP, TKIP, AES, and WAPI engines