What’s going on here? Leo Koppel Jan

The OSI Model LayersData unitFunctionExamples Host layers 7. ApplicationApplication Data High-level APIs, including resource sharing, remote file access, directory servicesand virtual terminalsAPIsdirectory servicesvirtual terminals HTTPHTTP, FTP, SM TPFTPSM TP 6. PresentationPresentation Translation of data between a networking service and an application; includingcharacter encoding, data compression and encryption/decryptioncharacter encodingdata compressionencryption/decryption ASCIIASCII, EBCDIC, JPEGEBCDIC JPEG 5. SessionSession Managing communication sessions, i.e. continuous exchange of information in the form of multiple back-and-forth transmissions between two nodessessions RPCRPC, PAPPAP 4. TransportTransportSegments Reliable transmission of data segments between points on a network, includingsegmentation, acknowledgement and multiplexingsegmentationacknowledgementmultiplexing TCPTCP, UDP, L2TPUDPL2TP Media layers 3. NetworkNetwork PacketPacket/Dat agramDat agram Structuring and managing a multi-node network, including addressing, routing andtraffic controladdressingroutingtraffic control IPv4IPv4, IPv6, IPs ec, AppleTalkIPv6IPs ecAppleTalk 2. Data linkData linkBitBit/FrameFrame Reliable transmission of data frames between two nodes connected by a physical layer PPPPPP, IEEE 802.2IEEE PhysicalPhysicalBit Transmission and reception of raw bit streams over a physical medium DSLDSL, USBUSB

What’s the point? Encapsulation: Each layer only uses the service provided by the layer below. It doesn’t need to be aware of lower layers. Each layer provides a service through a specified interface. The service is implemented using a protocol – the rules for communication on that layer. Ion Stoica:

Example: physical layer This is one transmission medium for the Ethernet physical layer. Cat5 RJ45/ 8P8C The physical layer specifies voltage levels, timings, coding scheme, etc. This is the protocol. The service it provides is sending bits between two systems. The interface says how to send the bit. Ethernet PHY transceiver

Data link layer The data link layer starts caring about the meaning of the bits sent. Cat5 Ethernet Network Interface Controller The service is sending data units between two nodes sharing a physical connection. The protocol includes addresses and frame structure (added bits around the payload you want to send). Also does low-level error detection, access control and some other things

Network layer Now we can think of machines not physically connected to each other. This router actually combines tasks of all 3 media layers This layer puts information in packets and moves it between destinations on different networks. One protocol: IPV4 Defines addresses, more headers

By the way... A hub is a signal repeater. Layer 1 A switch handles MAC addresses. Layer 2 A router handles IP addresses to connect different networks. Layer 3 A “router” is a router, switch, and access point (sometimes combined with a modem)

Transport layer Only at the fourth level we can talk about reliable, end-to- end connections. Most common: TCP, UDP Includes acknowledgement of received messages, guaranteed arrival (in the case of TCP) Can be message (UDP) or stream (TCP) oriented The service TCP provides to us is a stream of bytes: we know they will (eventually) arrive, in order. We don’t have to worry about lower levels.

Application Layer Includes protocols used directly by software applications, e.g. HTTP. What happened to Session & Presentation layers? They’re not present in the IP suite’s (TCP/IP - which we use) model - so I won’t talk about them.

So…. The point is, we don’t have to care about everything, all at once!

Our rover: TCP/IP over Wi-fi Physical layer: g “Wi-fi” –Specifies physical medium (2.4 Ghz radio) –Specifies signal modulation (frequency- division multiplexing) and encoding New concerns: interference, attenuation, directional antennas – concerns for comm team Ethernet on board the rover. See

Our rover: software The computers on the rover and base station already have network controllers built in, which take care of layers 1-3. The operating system gives us an interface to the Transport Layer. So we only use the interface to the transport layer, and have to use a protocol at the application layer.

Our rover: software Currently we use JSON as an application protocol. Does not care about wired vs. wireless. Does not even know! Example: {‘motor_fl’:12, ‘motor_fr’:0}