1. Link Layer & Physical Layer CPE 400 / 600 Computer Communication Networks Lecture 24

2. 2 Lecture 24: Outline r 5.1 Introduction and Services r 5.2 Error-detection and Error-correction r 5.3 Multiple Access Protocols r 5.4 Link-layer Addressing r 5.5 Ethernet r 5.6 Link-layer Switches r 5.7 Point to Point Protocol r 5.8 Link Virtualization m ATM, MPLS r Physical Layer m Data and Signals

3. DataLink Layer3 Point to Point Data Link Control r one sender, one receiver, one link: easier than broadcast link: m no Media Access Control m no need for explicit MAC addressing m e.g., dialup link, ISDN line r popular point-to-point DLC protocols: m PPP (point-to-point protocol) m HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!)

4. DataLink Layer4 PPP Design Requirements [RFC 1557] r packet framing: encapsulation of network-layer datagram in data link frame m ability to demultiplex upwards r bit transparency: must carry any bit pattern in the data field r error detection (no correction) r connection liveness: detect, signal link failure to network layer r network layer address negotiation: endpoint can learn/configure each other’s network address Error recovery, flow control, data re-ordering all relegated to higher layers!

5. DataLink Layer5 PPP Data Frame r Flag: delimiter (framing) r Address: does nothing (only one option) r Control: does nothing; in the future possible multiple control fields r Protocol: upper layer protocol to which frame delivered (eg, IP, PPP-LCP, IPCP, etc) r info: upper layer data being carried r check: cyclic redundancy check for error detection

6. DataLink Layer6 PPP Data Control Protocol Before exchanging network- layer data, data link peers must r configure PPP link (max. frame length, authentication) r learn/configure network layer information m for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

7. DataLink Layer7 Virtualization of networks Virtualization of resources: powerful abstraction in systems engineering: r computing examples: virtual memory, virtual devices m Virtual machines: e.g., java m IBM VM os from 1960’s/70’s r layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly

8. DataLink Layer8 The Internet: virtualizing networks ARPAnet satellite net gateway Internetwork layer (IP): r addressing: internetwork appears as single, uniform entity, despite underlying local network heterogeneity r network of networks Gateway: r “embed internetwork packets in local packet format or extract them” r route (at internetwork level) to next gateway

9. DataLink Layer9 Cerf & Kahn’s Internetwork Architecture What is virtualized? r two layers of addressing: internetwork and local network r new layer (IP) makes everything homogeneous at internetwork layer r underlying local network technology m cable m satellite m telephone modem m today: ATM, MPLS … “invisible” at internetwork layer. Looks like a link layer technology to IP!

10. DataLink Layer10 ATM and MPLS r ATM, MPLS separate networks in their own right m different service models, addressing, routing from Internet r viewed by Internet as logical link connecting IP routers m just like dialup link is really part of separate network (telephone network)

11. DataLink Layer11 Asynchronous Transfer Mode: ATM r 1990’s/00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture r Goal: integrated, end-end transport of carry voice, video, data m meeting timing/QoS requirements of voice, video (versus Internet best-effort model) m “next generation” telephony: technical roots in telephone world m packet-switching (fixed length packets, called “cells”) using virtual circuits

12. DataLink Layer12 ATM architecture r adaptation layer: only at edge of ATM network m data segmentation/reassembly m roughly analagous to Internet transport layer r ATM layer: “network” layer m cell switching, routing r physical layer physical ATM AAL physical ATM AAL physical ATM physical ATM end system switch

13. DataLink Layer13 ATM Adaptation Layer (AAL) r Different versions of AAL layers, depending on ATM service class: m AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation m AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video m AAL5: for data (eg, IP datagrams) AAL PDU ATM cell User data small payload -> short cell-creation delay for digitized voice

14. DataLink Layer14 ATM Layer: Virtual Circuits r VC transport: cells carried on VC from source to dest m call setup, teardown for each call before data can flow m each packet carries VC identifier (not destination ID) m every switch on source-dest path maintain “state” for each passing connection m link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf. r Permanent VCs (PVCs) m long lasting connections m typically: “permanent” route between to IP routers r Switched VCs (SVC): m dynamically set up on per-call basis

15. DataLink Layer15 ATM VCs r Advantages of ATM VC approach: m QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) r Drawbacks of ATM VC approach: m Inefficient support of datagram traffic m one PVC between each source/dest pair) does not scale (N*2 connections needed) m SVC introduces call setup latency, processing overhead for short lived connections

16. DataLink Layer16 ATM cell header r 5-byte ATM cell header r VCI: virtual channel ID m will change from link to link thru net r PT: Payload type (e.g. RM cell versus data cell) r CLP: Cell Loss Priority bit m CLP = 1 implies low priority cell, can be discarded if congestion r HEC: Header Error Checksum m cyclic redundancy check

17. DataLink Layer17 IP-Over-ATM AAL ATM phy Eth IP ATM phy ATM phy app transport IP AAL ATM phy app transport IP Eth phy IP datagrams into ATM AAL5 PDUs IP addresses to ATM addresses

18. DataLink Layer18 Multiprotocol label switching (MPLS) r initial goal: speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding m borrowing ideas from Virtual Circuit (VC) approach m but IP datagram still keeps IP address! PPP or Ethernet header IP header remainder of link-layer frame MPLS header label Exp S TTL 20 3 1 5

19. DataLink Layer19 MPLS capable routers r a.k.a. label-switched router r forwards packets to outgoing interface based only on label value (don’t inspect IP address) m MPLS forwarding table distinct from IP forwarding tables r signaling protocol needed to set up forwarding m RSVP-TE m use MPLS for traffic engineering m forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !! r must co-exist with IP-only routers

20. DataLink Layer20 R1 R2 D R3 R4 R5 0 1 0 0 A R6 in out out label label dest interface 6 - A 0 in out out label label dest interface 10 6 A 1 12 9 D 0 1 in out out label label dest interface 8 6 A 0 0 in out out label label dest interface 10 A 0 12 D 0 8 A 1 MPLS forwarding tables

21. DataLink Layer21 Chapter 5: Summary r principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m link layer addressing r instantiation and implementation of various link layer technologies m Ethernet m switched LANs m PPP m virtualized networks as a link layer: ATM, MPLS

22. Physical Layer Slides are modified from Behrouz A. Forouzan 22

23. TCP/IP and OSI model 23

24. Source-to-destination delivery 24

25. Physical layer 25Physical Layer To be transmitted, data must be transformed to electromagnetic signals.

26. Physical Layer Chapter 3: Data and Signals Chapter 4: Digital Transmission Chapter 5: Analog Transmission 26

27. 3-1 ANALOG AND DIGITAL Data can be analog or digital  Analog data refers to information that is continuous  Analog data take on continuous values  Analog signals can have an infinite number of values in a range  Digital data refers to information that has discrete states  Digital data take on discrete values  Digital signals can have only a limited number of values In data communications, we commonly use periodic analog signals and nonperiodic digital signals. 27Physical Layer

28. Comparison of analog and digital signals 28Physical Layer

29. 3-2 PERIODIC ANALOG SIGNALS Periodic analog signals can be classified as simple or composite.  A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals.  A composite periodic analog signal is composed of multiple sine waves. 29Physical Layer

30. Signal amplitude 30Physical Layer

31. Frequency is the rate of change with respect to time.  Change in a short span of time means high frequency.  Change over a long span of time means low frequency.  If a signal does not change at all, its frequency is zero  If a signal changes instantaneously, its frequency is infinite. Frequency 31Physical Layer

32. Frequency and period are the inverse of each other. Units of period and frequency Frequency and Period 32Physical Layer

33. Two signals with the same amplitude, but different frequencies 33Physical Layer

34. The power we use at home has a frequency of 60 Hz. What is the period of this sine wave ? Examples The period of a signal is 100 ms. What is its frequency in kilohertz? 34Physical Layer

35. Phase describes the position of the waveform relative to time 0 Phase Three sine waves with the same amplitude and frequency, but different phases 35Physical Layer

36. A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in degrees and radians? Example Solution We know that 1 complete cycle is 360°. Therefore, 1/6 cycle is 36Physical Layer

37. Wavelength and period 37Physical Layer Wavelength = Propagation speed x Period = Propagation speed / Frequency

38. Time-domain and frequency-domain plots of a sine wave 38Physical Layer A complete sine wave in the time domain can be represented by one single spike in the frequency domain.

39. Frequency Domain  The frequency domain is more compact and useful when we are dealing with more than one sine wave.  A single-frequency sine wave is not useful in data communication o We need to send a composite signal, a signal made of many simple sine waves. 39Physical Layer

40. According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases. Fourier analysis  If the composite signal is periodic, the decomposition gives a series of signals with discrete frequencies;  If the composite signal is nonperiodic, the decomposition gives a combination of sine waves with continuous frequencies. 40Physical Layer

41. A composite periodic signal Decomposition of the composite periodic signal in the time and frequency domains 41Physical Layer

42. Time and frequency domains of a nonperiodic signal  A nonperiodic composite signal o It can be a signal created by a microphone or a telephone set when a word or two is pronounced. o In this case, the composite signal cannot be periodic  because that implies that we are repeating the same word or words with exactly the same tone. 42Physical Layer

43. The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. Bandwidth 43Physical Layer

44. A nonperiodic composite signal has a bandwidth of 200 kHz, with a middle frequency of 140 kHz and peak amplitude of 20 V. The two extreme frequencies have an amplitude of 0. Draw the frequency domain of the signal. Solution The lowest frequency must be at 40 kHz and the highest at 240 kHz. Example 44Physical Layer