What’s going on here?

Leo KoppelJan 22 2015

The OSI ModelLayers Data unit Function Examples

Hostlayers

7. Application

Data

High-level APIs, including resource sharing, remote file access, directory servicesand virtual terminals

HTTP, FTP, SMTP

6. PresentationTranslation of data between a networking service and an application; includingcharacter encoding, data compression and encryption/decryption

ASCII, EBCDIC, JPEG

5. SessionManaging communication sessions, i.e. continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes

RPC, PAP

4. Transport SegmentsReliable transmission of data segments between points on a network, includingsegmentation, acknowledgement and multiplexing

TCP, UDP, L2TP

Medialayers

3. Network Packet/Datagram

Structuring and managing a multi-node network, including addressing, routing andtraffic control

IPv4, IPv6, IPsec, AppleTalk

2. Data link Bit/Frame Reliable transmission of data frames between two nodes connected by a physical layer

PPP, IEEE 802.2

1. Physical Bit Transmission and reception of raw bit streams over a physical medium DSL, USB

http://en.wikipedia.org/wiki/OSI_model

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: http://www.cs.berkeley.edu/~istoica/classes/cs194/05/notes/2-NetRPC.pdf

Example: physical layer

http://www.cablera.org/cat5-cable/ http://www.ti.com/product/DP83848C/description

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

http://commons.wikimedia.org/wiki/File:Ethernet_NIC_100Mbit_PCI.jpg

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

http://www.tp-link.com/

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 3A “router” is a router, switch, and access point (sometimes combined with a modem)

Transport layerOnly 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.

http://www.hardwaresecrets.com/article/The-OSI-Reference-Model-for-Network-Protocols/431/4

So….

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

http://www.tcpipguide.com

Our rover: TCP/IP over Wi-fi

• Physical layer: 802.11g “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 http://www.scs.stanford.edu/08sp-cs144/notes/l12.pdf

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}