Computer Communication Networks Link Layer Physical Layer Outline

Computer Communication Networks Link Layer & Physical Layer

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 2

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!) Data. Link Layer 3

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! Data. Link Layer 4

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 Data. Link Layer 5

PPP Data Control Protocol Before exchanging networklayer 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 Data. Link Layer 6

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 Data. Link Layer 7

The Internet: virtualizing networks 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 ARPAnet satellite net Data. Link Layer 8

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! Data. Link Layer 9

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) Data. Link Layer 10

Asynchronous Transfer Mode: ATM r 1990’s/00 standard for high-speed (155 Mbps 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/Qo. S 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 Data. Link Layer 11

ATM architecture AAL ATM ATM physical end system switch end system 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 Data. Link Layer 12

ATM Adaptation Layer (AAL) r Different versions of AAL layers, depending on ATM service class: m AAL 1: for CBR (Constant Bit Rate) services, e. g. circuit emulation m AAL 2: for VBR (Variable Bit Rate) services, e. g. , MPEG video m AAL 5: for data (eg, IP datagrams) User data small payload -> short cell-creation delay for digitized voice AAL PDU ATM cell Data. Link Layer 13

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 Data. Link Layer 14

ATM VCs r Advantages of ATM VC approach: m Qo. S performance guarantee for connection mapped to VC (bandwidth, 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 Data. Link Layer 15

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 Data. Link Layer 16

IP-Over-ATM app transport IP Eth phy IP datagrams into ATM AAL 5 PDUs IP AAL Eth ATM phy app transport IP AAL ATM phy IP addresses to ATM addresses Data. Link Layer 17

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 MPLS header label 20 IP header remainder of link-layer frame Exp S TTL 3 1 5 Data. Link Layer 18

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 Data. Link Layer 19

MPLS forwarding tables in label out label dest 10 12 8 out interface A D A 0 0 1 R 4 R 6 in label out label dest out interface 10 6 A 1 12 9 D 0 R 3 0 0 D 1 1 R 5 0 0 R 2 in label 8 out label dest 6 A out interface 0 A R 1 in label 6 out label dest - A out interface 0 Data. Link Layer 20

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 m Ethernet switched LANs PPP virtualized networks as a link layer: ATM, MPLS Data. Link Layer 21

Physical Layer Slides are modified from Behrouz A. Forouzan 22

TCP/IP and OSI model 23

Source-to-destination delivery 24

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

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

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

Comparison of analog and digital signals Physical Layer 28

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

Signal amplitude Physical Layer 30

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

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

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

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

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

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

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

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

Frequency Domain q The frequency domain is more compact and useful when we are dealing with more than one sine wave. q 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. Physical Layer 39

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

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

Time and frequency domains of a nonperiodic signal q 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 Øbecause that implies that we are repeating the same word or words with exactly the same tone. Physical Layer 42

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

Example A nonperiodic composite signal has a bandwidth of 200 k. Hz, with a middle frequency of 140 k. Hz 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 k. Hz and the highest at 240 k. Hz. Physical Layer 44