Chapter 7 - Wireless and Mobile Networks

Reading: Kurose & Ross, chapter 7

Two separate problems that get taught together: the link is wireless, and the host may move. They are independent, and the chapter treats them separately.

7.2 What makes wireless different

Wired links are boring in a good way. Wireless links are not:

• Decreasing signal strength. Attenuation with distance, through walls.
• Interference from other sources. 2.4 GHz is shared with a great deal else.
• Multipath propagation. The signal takes several paths of different lengths and arrives blurred.

Consequences: bit errors are common rather than rare, and the error rate varies over time. SNR and BER trade off against each other. A higher SNR gives a lower BER; increasing transmit power raises SNR but also interferes with everyone else.

WARNING

The critical structural difference: on a wireless link, a node cannot detect collisions while transmitting, because its own transmission drowns out everything else at its antenna. That single fact is why 802.11 uses CSMA/CA rather than the CSMA/CD that Ethernet uses.

Hidden terminals. A and C can both reach B but cannot hear each other. Carrier sensing therefore fails: both hear silence, both transmit, and they collide at B. Signal fading produces the same effect for a different reason.

7.3.1 Sharing the wireless channel

CDMA lets multiple senders share the same band by encoding with orthogonal chipping sequences, and the receiver decodes with the same sequence.

7.3.2 WiFi: 802.11

Architecture. A basic service set is one access point plus its associated hosts. Hosts must associate with an AP before sending anything.

Association. APs send periodic beacon frames with their SSID and MAC address. A host scans channels (passive scanning), or sends probes (active scanning), picks one, and associates. Then usually DHCP for an address.

CSMA/CA. Sense the channel. If idle for a DIFS interval, transmit the whole frame, because there is no collision detection to abort on. If busy, back off, and count down only while the channel is idle.

ACKs are required, unlike Ethernet, because the sender has no other way to know whether the frame arrived intact on a link this error-prone.

RTS/CTS. An optional exchange of short control frames that reserves the channel. The CTS is heard by every node in range of the receiver, including hidden terminals, so they stay quiet. It costs two extra frames, so it pays only for large data frames.

Frame format. Four address fields, which is more than Ethernet's two, because a frame may pass between a host, an AP, and a router, and all of those need naming.

Rate adaptation. The sender lowers the modulation rate as the channel degrades and raises it as the channel improves.

7.3.3 and 7.4 Cellular networks

Architecture: base stations, the radio access network, and the core network. 4G LTE separates the data plane from the control plane and is entirely packet switched, with an all-IP core.

5G adds millimeter wave bands: far more bandwidth, far less range and penetration, so cells get much smaller.

7.5 Mobility management

Home network and home agent. Visited network and foreign agent. A care-of address in the visited network.

Two ways to route to a mobile node:

• Indirect routing. Everything goes to the home agent, which tunnels to the care-of address. Simple, but triangle routing is inefficient.
• Direct routing. The correspondent asks the home agent for the care-of address and sends directly. Efficient, but it breaks when the node moves again mid-session.

Handoff. Moving between base stations without dropping the connection.

Wireless and mobility: impact on higher layers

The 8th edition gave this its own section. The 9th edition does not, so these notes are the reference for it.

TCP interprets loss as congestion. On a wireless link, most loss is corruption, not congestion. TCP therefore cuts its window for the wrong reason, and throughput collapses on a link that is merely noisy. The fixes (local recovery, split connections, explicit notification) all involve breaking a layer boundary somewhere, which is why none of them is clean.