Network Communication

Wireless data transmission

Information encoded as variability of electromagnetic fields:

• frequency (f): wave oscillation speed, ==higher f $\to$ shorter range==
• wavelength ($\lambda$): inverse of frequency, ==higher $\lambda$ $\to$ longer antenna==
• bandwidth: width of a range of frequencies used, ==more bandwidth $\to$ higher throughput==

Sender (Tx) includes oscillator DAC, receiver (Rx) includes oscillator ADC

Smartphone wireless interfaces

Wireless interface selection impact:

• capabilities - transfer speed, coverage area and maximum range
• monetary cost
• power consumption

	Near Field Communication (NFC)	Bluetooth	WiFi	Cellular network
Power	Low / no	Low (~10mW)	Medium (~100mW)	High (~200mW)
Range	Short (~10cm)	Short (~10m)	Medium (~100m)	Long (~1km)
Throughput	Low (~400kbps)	Low (~1Mbps)	High (~100Mbps)	Varying (~40kbps with 2G, ~1Gbps with 5G)
Usage	Security tags, location-based services, payment systems	Peripherals and wearables, medical equipment, vehicle systems	Large downloads, home entertainment	Real-time updates
Interface management: NfcManager, BluetoothManager, WifiManager, TelephonyManager, …
ConnectivityManager: network connections monitoring, connectivity changes notifications

Networking abstractions

Sockets

Socket - TCP, DatagramSocket - UDP

Http(s)URLConnection

HTTP(S) connection and transfers:

• connection pooling $\to$ socket reuse
• response caching
• cookie management

OkHttp (3rd party library)

Advanced HTTP client:

• all features of HttpsURLConnection
• automatic network connection recovery, retries
• data compression

Retrofit (3rd party library)

REST client using (OkHttp under the hood):

• define model and REST operations
• define converter and adapter
• define authentication mechanisms

Best practices

• Run network operations on a separate thread
• Reduce amount of data transfers:
  • compress data / use low resolution content when possible
  • query only what’s needed
  • cache static (and dynamic as much) content
• Get notified on remote data change instead of polling
• Reuse network connections
• Secure connection (SSL) and minimize sensitive data transfers
• Energy-efficient networking:
  • bundle data $\to$ send data less frequently
  • use WorkManager
• Adapt to available connection:
  • slower network $\to$ reduce transfers
  • faster network $\to$ prefetch

Backend for mobile apps

Firebase (Google-developed):

• Authentication with Google ID
• Notifications
• Crashlytics

Parse server (open source backend as a service):

• building REST APIs
• cron jobs on server
• user management