Introduction to Computer Networks

1. Introduction to Computer Networks

2. The Purpose of Networking • Goals: Remote data exchange and remote process control. • A few desirable properties: • Interoperability, • Flexibility, • Geographical range, • Scalability, • Privacy and security.

3. Range of Coverage We can classify computer networks according to their geographical coverage: LAN: local area network WLAN: wireless local area network MAN: metropolitan area network WAN: wide area network (long haul network) Most commonly, we’re interested in the seamless integration of all these levels (as in the Internet). Note: Different levels use very different technologies.

4. Matters ofProtocol Everything in networking happens through protocols: • A protocol determines how hosts share and access the medium, • A protocol determines how hosts deal with the media bandwidth, errors, flow control, etc, • A protocol determines how connections between hosts are established and maintained, • A protocol determines how information is routed across short and long distances. • Etc, etc, etc… Question: Ok, but what is protocol?

5. The ISO/OSI Reference Model Source: Computer Networks, Andrew Tanenbaum ISO: International Standards Organization OSI: Open Systems Interconnection Application The protocol stack: Presentation Session The idea behind the model: Break up the design to make implementation simpler. Each layer has a well-defined function. Layers pass to one another only the information that is relevant at each level. Communication happens only between adjacent layers. Transport Network Data link Physical

6. The Layers in the ISO/OSI RF Model Physical: Transmit raw bits over the medium. Data Link: Implements the abstraction of an error free medium (handle losses, duplication, errors, flow control). Network: Routing. Transport: Break up data into chunks, send them down the protocol stack, receive chunks, put them in the right order, pass them up. Session: Establish connections between different users and different hosts. Presentation: Handle syntax and semantics of the info; examples: encoding, encrypting. Application: Protocols commonly needed by applications (cddb, http, ftp, telnet, etc).

7. Communication Between Layers within the Same Host It’s important to specify the services offered to higher layers in the hierarchy. What they are + how to use them = interface = API. Layer n+1 … SAP SAP Layer n SAPs (service access points) Note: This is ISO terminology. … SAP SAP Layer n-1

8. Communication Between Layers across Different Hosts receiver sender data data Application Application AH data PH Presentation Presentation data Session Session SH data TH Transport Transport data Network Network NH data Data link Data link DH DT data BITS Physical Physical

9. The Layers in the TCP/IP Protocol Suite Source: The TCP/IP Protocol Suite, Behrouz A. Forouzan Application FTP NFS HTTP DNS … Presentation Session Transport TCP UDP IP ICMP IGMP Network ARP RARP Data link Physical

10. Design Alternatives Point-to-point channels: physical links (as in wiring) connect every two communicating parties with a “private” channel. Broadcast channels: communicating parties are connected by a shared medium; hosts can hear transmissions not necessarily addressed to them.

11. Network Topology We can classify computer networks according to their topology: bus star mesh hypercube ring

12. Technology Network Interface Card (NIC): I/O device in the computer system that allows it to join a network. The NIC works with a specific medium (twisted-pair, coaxial cable, optic fiber, etc). As long as its bus allows, a host can have multiple NICs. Host NIC … NIC To the host, the NIC is just another I/O device, which has its own address. A protocol determines how the NIC accesses the medium.

13. Ethernet Ethernet has a bus topology. Bus Arbitration by Collision Detection: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Host A listens and finds the bus idle. Host A starts TX. Host B starts TX. Host A detects collision. Host B completes TX. Host A backs off. COLLISION! Host B backs off. time Host B listens and finds the bus idle. Host B detects collision. Host B listens and finds the bus idle. Host B starts TX.

14. COTS Ethernet Ethernet has a bus topology The medium could be anything that allows for a bus implementation. (some options are easier to work with than others) Host NIC Host NIC … Hub: An out-of-the box bus Host NIC

15. Switched LANs The bus bandwidth is limited, so go for a switched architecture. Host NIC Host NIC … Switch: allows for more than one pair to talk at the same time. Host NIC

16. Token Ring Hosts connect to a switch which closes the ring. Medium access is done by passing around a token. A C B D F E

17. Network Architecture for Performance The keyword is hierarchy: Subnet 1 hub Subnet 2 hub … Switch Subnet N hub

18. Network Architecture for Performance and Coverage Again, the keyword is hierarchy: Subnet 1 Subnet 1 … hub hub Switch Switch Subnet 2 Subnet 2 hub hub … … Subnet N Subnet N Router hub hub Connection to other networks

19. Security Issues Desirable properties: • Availability • Accessibility • Non-repudiation • Flexibility • Confidentiality • Authenticity • Integrity • Freshness • Scalability

20. Wireless Networks

21. Reasons to Go Wireless 1) 2) 3) … Challenges in Going Wireless 1) 2) 3) …

22. Medium: The Radio Spectrum Wireless communications use the electromagnetic spectrum, which is regulated by government institutions such as the Federal Communications Commission (FCC). Regulations specify what bands of frequency can be used for different applications. For instance: FM radio has 88-108MHz (200KHz) and AM radio has 540-1600KHz (10KHz bandwidth). Regulations also specify the transmission power that can be used in each band. There are portions of the spectrum that are UNLICENSED, however. The most popular wireless networks of today operate in unlicensed bands.

23. Omnidirectional Antenna Radiates in all azimuth directions.

24. Directional Antenna Radiates in a “cone”.

25. Design Alternatives Point-to-point channels: Information flows in “beams” that connect communicating parties. signal received signal not received The antennas on the transmitter and receiver need to be properly aligned for signals to go through. On the flip-side, directional antennas have great power efficiency and range. Directional antennas are a good choice for systems with fixed infrastructure. They introduce additional difficulties in infrastructure-less systems or when transmitters and receivers can move around, but offer reduced power consumption.

26. Design Alternatives Broadcast channels: Information radiates in all possible directions from the transmitter. signal received There’s no need to align antennas on the transmitter and receiver. If signals radiate in all directions, a receiver will the transmitters independently of their relative positions. Because power radiates all around, omnidirectional antennas can’t reach as far as directional antennas. Note also that quite a bit of power can be “wasted”. signal received This is a good choice for mobile systems. At the expense of increased power consumption, coverage reaches 360o.

27. Concepts in Radio Communications Coverage = f(PowerTX) Interference D A E B C Multipath Noise

28. Multiple Access to the Radio Spectrum Spatial Division Multiple Access (SDMA): each pair of nodes communicates through a tight beam that takes a portion of space. Frequency Division Multiple Access (FDMA): each pair of nodes uses a distinct subrange of the total frequency band for the application. T0 T1 T2 T3 T4 T5 T6 T7 T8 A B frequency spectrum

29. Multiple Access to the Radio Spectrum Time Division Multiple Access (TDMA): each pair of nodes uses a different time slot to communicate. During its time, the pair can use the entire frequency band allocated for the application. T2 T4 T0 T3 T4 T1 T0 T1 T3 T2 T0 T1 time one time unit one time unit one time unit Code Division Multiple Access (CDMA): can be seen as a combination of FDMA and TDMA. Frequency Hopping Spread Spectrum (FHSS): transmitters use each frequency band for a random time then move to another randomly chosen – TX and RX must agree to a hopping sequence. T0 time T1 time T2 time

30. Remember Ethernet?When Problems Get Worse… Assume that all wireless devices use the same channel. Arbitration of access to the medium (Medium Access Control, or MAC, a protocol in the Data Link layer) is similar to Ethernet’s CSMA/CD. Most radios in wireless networking can’t transmit and receive at the same time, so we can’t detect collisions. Instead, we’ll do CSMA/CA (collision avoidance). Collisions are bad because they reduce the effective bandwidth and also because they cause waste of power. Even when two transmissions do not collide, they may still interfere with each other causing bit error rates to rise.

31. Types of Wireless Networks Fixed Infrastructure wired backbone BS BS BS Ad Hoc • Easy to deploy. • Good in changing environments. • Allows for node mobility. • Can be designed for self-configurability. • Can be designed for scalability.

32. Data Link Layer: Medium Access Control (Coordinated access to a shared resource) • Power is a scarce resource; so is the RF spectrum. • Collisions lead to wasted power and wasted spectrum. • Need to impose some kind of access discipline so as to avoid collisions.

33. The MAC Layer Challenge Maximize throughput: • Minimize collisions. • Avoid exposed nodes. An interesting option: schedule medium access. Related challenges: • Clock synchronization. • Distributed coordination for determining schedule.

34. IEEE 802.11 DCF (CSMA/CA) start NAV starts with the Duration field value of the last transmission sensed on the medium and counts down to zero. NO NAV=0 YES Sense Medium Medium Idle Random Backoff Time NO YES Transmit Frame Collision? YES NO

35. The Hidden Node Problem • Station C can sense stations A and B. • Stations A and C can’t sense each other. • Problem: coordinate transmissions from A and C so as to avoid collisions. A B C

36. The Hidden Node Problem • Station C can sense stations A and B. • Stations A and C can’t sense each other. • Problem: coordinate transmissions from A and C so as to avoid collisions. A B C • Solution:RTS/CTS/DATA/ACK handshake – A sends RTS • to B, B sends CTS to A, C hears CTS and stays quiet, A sends DATA • to B, B replies to A with an ACK.

37. The Exposed Node Problem A B C D An exposed node is one that is in range of the transmitter, but outside range of the receiver. Problem:exposed nodes reduce bandwidth.

38. The Network Layer Challenge • How do we build routes dynamically? • Pro-active algorithms. • Reactive algorithms. • Will the routing protocol scale up to LARGE networks? • Can routing adapt to changes in network traffic, propagation conditions, etc.? • Packet forwarding costs power. Can we do routing in a way that balances power consumption?

39. Security Issues • Desirable properties: • As of today, the network can be vulnerable at multiple levels: • PHY: radio jamming, physical actions to the node. • MAC: DoS via fake requests or schedules. • NET: fake route advertisements (black hole attack). • Availability • Accessibility • Self-organization • Non-repudiation • Flexibility • Confidentiality • Authenticity • Integrity • Freshness • Scalability