Wireless and Mobile Networks Highlights

NOTE

• First reading: p. 531 to p. 564
• Second reading: p. 564 to p. 596

0. Sources

1. James Kurose & Keith Ross, Pearson+, “Computer Networking: A Top-Down Approach” - 2020-07-24

1. Core Terminologies

Terminology	Description
Code Division Multiple Access (CDMA)	shared medium access protocol used in wireless network
IEEE 802.11 (WiFi)	- wireless LAN standard known as WiFi - 802.11 b, g, n, ac, ax are its successive generations - WiFi 4, 5, 6 are 802.11 n, ac, and ax
infrastructure mode	mode host uses to indicate that it is associated with a base station (for accessing traditional network services)
ad hoc networks	doesn’t have an infrastructure like base station to facilitate network services (must do it themselves). they don’t have any connection to the larger outside network
handoff/handover	host changes the base station it is associated with by moving to the range of another base station and outside of its initial base station
mobile ad hoc networks (MANETs)	node connectivity changes among other nodes
vehicular ad hoc network (VANE T)	mobile networks like MANETs but the nodes are vehicles
multipath propagation	changes in the transmission path as a result of electromagnetic wave reflecting off objects/ground
signal-to-noise ratio	ratio between received signal strength and noise (measured in dB). Bigger SNR = easier to extract received signal
bit error rate (BER)	probability that the transmitted bit will be received wrong
hidden terminal problem	a problem where physical obstructions make a specific transmitting station not detect another transmitting station with a common destination station, resulting in an interference at the destination
fading	fading of wireless signal strength as it propagates through a medium, resulting in undetected collision from two transmitting stations with a common receiving station and signals which cannot be detected but can interfere one another
basic service set (BSS)	fundamental building block of a wireless LAN architecture, containing one or more wireless stations, and a central base station (access point)
access point	alongside a router, it connects BSS to the internet
infrastructure wireless LANs	WLANs with APs

2. Details

2.1. Elements in a Wireless Network

Element	Description
Wireless hosts	- may or may not be mobile - examples: smartphone, laptop, IoT devices (e.g., sensors, appliances, and auto-mobiles)
Wireless links	- the wireless communication link is the connection between a host and base station/another host
Base station	- responsible for sending/receiving data to/from a wireless host associated with it1 - examples: cell towers for cellular network and *access points for 802.11 wireless LANs
Network infrastructure	the larger network

2.2. Wireless Network Classifications

Two criteria:

1. Amount of wireless hops
2. If there is an infrastructure used

	infrastructure-based	infrastructure-less
single-hop	there is a base station directly connected to the larger wired network. communication between host and base station occurs in a single hop	no base station to connect to wireless network, instead one node facilitates the transmission in the network for the other devices
multi-hop	uses base station to communicate with large network, however, may contain multiple hops for some wireless nodes.2	no base station to connect to wireless network, nodes instead relay messages among other nodes to reach a destination. they may be mobile or their connectivity changes among nodes

2.3. Differences of Wireless Links from Wired Links

• Signal strength gets weaker as it passes through matter and also as it disperses (path loss)
• It can be interfered by other radio sources sharing the same frequency band. It can also be interfered by other electromagnetic noise in the environment.
• Multipath propagation may occur
• higher and time-varying BER

2.4. Important Physical Layer Characteristics

They specifically help understand higher-layer wireless communication protocols. For each modulation technique (e.g., BSPK, QAM16, and QAM256),

• higher SNR = higher BER³
• higher bit transmission rate = higher BER
• dynamic selection of the physical-layer modulation technique can be used to choose the appropriate technique depending on the channel conditions

2.5 CDMA

• Prevents interference through channel partitioning (for wireless transmission)
• For detailed explanation check p. 539 to p. 542

Sender side:

$$z_{i,m}=d_i\cdot c_m$$

• $z$ is the CDMA encoder output
• $d_i$ is the data bit at position $i$
• $c_m$ is the code at $m$ (mini-slot)

In ideal conditions, data recovery of original bit is computed using,

$$d_i=\frac{1}{M}\sum _{m=1}^M{Z}_{i,m}\cdot c_m$$

• $M$ is the number of mini slots total for each slot

Receiver side:

if multiple interfering senders at $m$ mini-slot,

• additive, meaning each sender output at the receiver will be added
• and receiver’s received output will be

$$Z_{i,m}^{\ast }=\sum _{s=1}^N{Z}_{i,m}^s$$

Original bit can still be recovered from aggregate signal using,

$$d_i=\frac{1}{M}\sum _{m=1}^M{Z}_{i,m}^{\ast }\cdot c_m$$

2.6. 802.11

Frequency Ranges:

2.4 GHz	5 GHz
unlicensed and therefore competing for the frequency range against other 2.4 GHz devices	greater power but shorter distance
	suffers from multipath propagation

Wireless LAN Architecture:

Footnotes

1. associated wireless host is wireless within its range and that wireless uses that base station to send data to the larger network (and vice versa) ↩

2. Multiple hops because a wireless host may be too far to the base station and is therefore connected to an intermediate relay device for relaying via base station instead ↩

3. higher SNR can be achieved by higher transmission power ↩