| Manufacture | Nokia | BenQ-Siemens |
| Model | N81 | P51 |
| Form factor | Slider | Block |
| Weight | 140 gram | 170.00 |
| Dimensions (HxWxD) | 102 x 50 x 18 | 122.00 x 60.00 x 20.00 |
| Operating System | Other | Other |
| Networks | 2G - 850/900/1800/1900
3G - 2100 | 2G - 900/1800
|
| HSCSD |  | |
| GPRS |  | |
| EDGE | | |
| UMTS | | |
| HSDPA | | |
| HSPA+ | | |
| WiMAX | | |
| LTE (4G light) | | |
| WLAN / Wi-Fi | | |
| Wi-Fi Hotspot | | |
| Wi-Fi Direct | | |
| SMS | | |
| MMS | | |
| Email | | |
| HDMI out | | |
| USB port | | |
| MHL | | |
| Bluetooth | | |
| Bluetooth version | NA | NA |
| DLNA | | |
| GPS | | |
| Glonass | | |
| ANT+ | | |
| IrDA | | |
| NFC | | |
| 3.5 mm audio connector | | |
| Camera | | |
| Camera resolution | 2 Megapixels, 20x digital zoom | 1.3 Megapixels |
| Image Stabilization | | |
| BSI sensor | | |
| Camera Flash/Light | | |
| Flash type | NA | NA |
| Video recording | | |
| Video capture resolution | NA | NA |
| Second Camera | NA | NA |
| Display size | 2.4 inches | 2.80 |
| Touchscreen | | |
| Colour display | | |
| Number of colours | 16700000 | 65536 |
| Display width | 240 | 240 |
| Display height | 320 | 320 |
| PPI | 166.67 | 142.86 |
| Second Display | NA | NA |
| CPU | NA | NA |
| GPU | NA | NA |
| Memory | 18MB. Expandable up to 4GB with miniSD card | 128 MB, 128 MB Flash |
| Micro SIM | | |
| Dual SIM | | |
| MP3 | | |
| Ringtones / Music file format | MP3, AAC, eAAC+, AAC+, WMA, WAV | Polyphonic 64 voices |
| Java | | |
| Games | Space Impact Light, Asphalt demo, Fifa Soccer 2007 demo | Yes |
| FM Radio | | |
| Battery | 1050 mAh | Li-Ion 1370 mAh |
| Standby time (h) | 408 | 150 |
| Talktime (m) | 240 | 240 |
| Wireless Charging | | |
| Browser | 2.0 XHTML | 2.0 |
| Calendar | | |
| Voicedialing | | |
| IP Code | NA | NA |
| Predictive text | | |
| RSS reader | | |
| Voicememo | | |
| PC Synchronization | | |
| SyncML | | |
| Website | Website | Website |
