Data & Network Communication Final

Link layer

Responsible for transferring data grams from one node to another directly connected node through a link

Node

A device in the network - hosts and routers

Frame

Link-layer packet - takes datagram from network layer and wraps it in a frame

Framing

Encapsulating a datagram into a packet - add header/tailer

MAC address

link layer address used to identify devices on the same network - used to locally deliver frames vs. IP address global routing (think SSN vs postal address). EX: 1A-2F-BB…

• 48 bits long

• Usually written in hexadecimal (0-9, A-F)

half/full duplex

Half - both can transmit, but not at the same time. Full - both can transmit at the same time

Network Interface Card (NIC)

Where the link layer is implemented in a host, hardware that connects device to network

EDC

Error detection and correction bits - extra bits added to data so the receiver can check if the data was corrupted

Parity Bit

A single bit added to data to make the number of 1’s either even or odd

Single bit parity

Can detect a single bit error - if one bit flips, the number of 1’s will go from even to odd - usually even parity (adding a 1 to the end if its odd, and a 0 if its even already)

Two dimensional parity

Arranges data in rows and columns, then adds parity bits for each row and column. Allows the receiver to find the bad row AND the bad column to correct it

Internet checksum

Value calculated from the data. Sender sends data + checksum (ones complement), receiver recalculates checksum from data and compares w/ sent checksum. Goal is to detect flipped bits

Cyclic Redundancy Check (CRC)

More powerful at error detecting. Goal: Choose CRC bits, R, such that <D(the date), R> is exactly divisible by G (mod 2) - go over this arithmetic later

Point to point link

Connects one sender to one receiver (ex. host and Ethernet switch)

Broadcast links/shared medium

Link shared by multiple devices - when one device sends, other devices might hear it

Collision

When a node receives 2+ signals at the same time

Multiple access protocol

Determines when each node is allowed to transmit on a shared channel. Ideal: One node transmits at rate R, M nodes transmit at rate R/M

MAC (Medium Access Control) Protocol

Decides how devices share access to a communication medium. Three types:

1. Channel partitioning: divides channel into smaller pieces, and each node gets one for its personal use (TDMA, FDMA)

2. Random access: Devices transmit when they have data. Collisions can happen but there are protocols for recovery (ALOHA, CSMA, etc)

3. Taking turns: Nodes take turns transmitting, but those with more data can take longer turns (polling, token passing)

TDMA (Time Division Multiple Access)

Channel partitioning protocol. Divides channel by time - Access happens in rounds, each round has several slots, and each station gets its own slots. Avoids collisions, but some slots might get wasted.

FDMA (Frequency Division Multiple Access)

Channel partitioning protocol. Divides channel into frequency bands, and each station gets its own frequency band (think radios). Avoids collisions, but some frequency bands might get wasted if there is nothing to send.

Slotted ALOHA

Random access protocol. Time is divided into slots, and nodes may only start transmitting at the start of a slot. Node gets fresh frame —> transmits in next slot. If no collision, success. If colission, tries again in later slots with probability p. Simple and decentralized, but collisions waste slots and clock synchronization must occur

Pure ALOHA

Random access protocol. Like Slotted ALOHA, except devices don’t have to wait until the start of a new slot to start transmitting. Causes more collisions, less efficient than Slotted ALOHA

CSMA (Carrier Sense Multiple Access)

Random access protocol. Listen before transmit - a device checks whether channel seems idle before sending.

CSMA/CD (collision detection): Devices listening before transmitting AND detecting if a collision happened. Stops transmitting if there is one.

Binary Exponential Backoff

A retry strategy where, after more collisions, the device chooses from a larger range of random wait times.

Polling

Taking turns protocol. Master node asks each other node, one at a time, if it wants to transmit. Avoids collisions, but creates overhead, delay, and risk if master node fails.

Token passing

Taking turns protocol. A special control message, called a token, is passed from one node to the next. Node can only transmit when it has the token. Avoids collisions, but has overhead, can cause delay, and risky if token is broken.

ARP (Address Resolution Protocol)

Used to find a device’s MAC address when you already know its IP address.

• Each node has an ARP table, which stores mappings between IP addresses and MAC addresses.

Host A will broadcast an ARP request to everyone on the LAN. Every device on the LAN receives the request, but only Host B recognizes the target IP address as its own, and sends an ARP reply back with the MAP address. A stores that in its ARP table.

bus topology

Formerly used by Ethernet. All devices share the same communication medium, like one shared cable. All can collide with each other.

switched topology

Currently used by Ethernet. Device connects to central switch, and each device has its own separate link to the switch.

Switch

Link-layer deicde that receives Ethernet frames, looks at the MAC addresses, and decides where to forward them. Allow multiple simultaneous transmissions and reduce collisions.

• Each switch has a switch forwarding table that maps a host’s MAC address to the switch interface needed to reach that host. Each entry usually has a MAC address, interface #, and timestamp.

Flooding

When the switch forwards the frames out of all interfaces except the one it came in on.

Switches vs routers

Switches: Work at link layer and use MAC addresses

Routers: Work at the network layer and use IP addresses

Both: Store and forward devices - receive data, examine it, and forward it to the correct interface

VLAN (Virtual LAN)

A way to split one physical LAN into multiple logical LANS.

Trunk port

Switch port that carries traffic for multiple VLANs between switches.

802.1Q

The standard that adds VLAN tag information to Ethernet frames.

Infarstructure Mode

Wireless device connects through a base station/access point (how normal Wi-Fi works)

Ad hoc mode

Wireless devices communicate directly with each other without base stations

CDMA (Code Division Multiple Access)

A technique where different users share the same frequency at the same time, but each user has a unique code.

BSS (Basic Service Set)

A group of wireless hosts connected to the same access point.

802.11

Family of Wi-Fi standards

• Passive scanning: Access point sends beacon frames, and the host listens for them

• Active scanning: Host sends a probe request, and access points respond

CSMA/CA (Carrier Sense Multiple Access w/ Collision Avoidance)

• Devices listen before sending and try to avoid collisions before they happen

• Used by Wi-Fi

Rate adaptation

Wi-Fi can change its transmission speed depending on the quality of the wireless signal.

Power management

Allows a device to go to sleep when it does not need to send or receive right away

Beacon frame

A message sent by the access point to announce information about the wireless network

Base station

The cellular network device that mobile users connect to over the wireless link. Connects into the core network, which handles authentication, mobility, etc

Data plane

Carries the user’s actual data, like videos, messages, etc

Control plane

Carries management information, like authentication, mobility updates, etc

Indirect routing

Traffic goes through the mobile device’s home network first, then gets forwarded to wherever the device currently is.

Direct routing

The sender learns the mobile device’s current temportary address and sends directly to that visited network,

Home network

Network where the mobile device “belongs” (ex. If your cellular provider is Verizon, that is your home network)

Visited network

A different network the device is using while away from home

Registration

When the mobile device tells the network where it currently is

Triangle Routing

When traffic takes an indirect path through the home network instead of going directly between the sender and the mobile device.

Network security

Four big goals - confidentiality, authentication, messeage integrity (msg wasn’t changed), access/availability

Symmetric key cryptography

Sender and receiver use the same key to decrypt

DES (Data Encryption Standard)

Uses a 56-bit symmetric key and works on 64-bit plaintext input blocks - no longer considered secure b/c key is too short

AES (Advanced Encryption Standard)

Uses 128-biy blocks and can use 128, ,192, or 256-bit keys. Stronger than DES

Public key cryptography

Uses two different keys: a public and a private. If someone wants to send Bob a message, they can encrypt it with Bob’s public key, but only his private key can decrypt it.

RSA

Public key encryption method based on the difficulty of factoring large numbers.

• Mod = 14 mod 10 = 4 —> 14/10 = remainder 4

• choose two very large prime numbers, p and q. p * q = n

• public key = n and another number e

• private key = n and another number d

• ciphertext = message^e mod n

• message = c^d mod n

Digital signature

A cryptographic way to prove who created a message and that the message was not changed.

Message digest

Fixed size output of a hash function - acts like a fingerprint of the message