CSE401N Computer Networks

Lecture-2 Network Structure[KR-1.2+1.3+1.4] S. M. Hasibul Haque Dept. of CSE BUET CSE401N 1

A closer look at network structure:

   network edge: applications and hosts network core:  routers  network of networks access networks, physical media: communication links CSE401N 2

The network edge:

   end systems (hosts):    run application programs e.g., WWW, email at “edge of network” client/server model   client host requests, receives service from server e.g., WWW client (browser)/ server; email client/server peer-peer model:   host interaction symmetric e.g.: Gnutella, KaZaA CSE401N 3

Network edge: connection-oriented service

Goal:

data transfer between end sys.



handshaking

: setup (prepare for) data transfer ahead of time   Hello, hello back human protocol

set up “state”

in two communicating hosts  TCP - Transmission Control Protocol  Internet’s connection oriented service  Why not connected?

TCP service    [RFC 793] reliable, in-order byte stream data transfer  loss: acknowledgements and retransmissions

flow control:

 sender won’t overwhelm receiver

congestion control:

 senders “slow down sending rate” when network congested CSE401N 4

Network edge: connectionless service

Goal:

data transfer between end systems  same as before!

 UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service    unreliable data transfer no flow control no congestion control App’s using TCP:  HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP (email) App’s using UDP:  streaming media, teleconferencing, Internet telephony CSE401N 5

The Network Core

  mesh of interconnected routers the fundamental question: how is data transferred through net?

  circuit switching: dedicated circuit per call: telephone net packet-switching: sent thru net in discrete “chunks” data CSE401N 6

Network Core: Circuit Switching

End-end resources reserved for “call”  link bandwidth, switch capacity    dedicated resources: no sharing circuit-like (guaranteed) performance call setup required CSE401N 7

Network Core: Circuit Switching

network resources (e.g., bandwidth) divided into “pieces”    pieces allocated to calls resource piece not used by owning call

(no sharing) idle

if dividing link bandwidth into “pieces”  frequency division  time division  dividing link bandwidth into “ pieces ”  frequency division  time division CSE401N 8

Network Core: Circuit Switching

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Circuit Switching: TDMA and TDMA

FDMA Example: 4 users frequency time TDMA frequency time CSE401N 10

Circuit Switching: Resources (Frequency and Time)

 Divide link bandwidth— the resource--into “ pieces ”  frequency division multiplexing (FDM)  time division multiplexing (TDM) CSE401N 11

Circuit Switching: The Process

 Three phases 1.

2.

3.

circuit establishment data transfer circuit termination  If circuit not available: “ busy signal ” CSE401N 12

Timing Diagram of Circuit Switching

Host 1 Host 2 Node 1 Node 2 processing delay at Node 1 circuit establishment propagation delay from Host 1 to Node 1 propagation delay from Host 2 To Host 1 data transmission circuit termination

DATA

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Delay Calculation in Circuit-Switched Networks  Propagation delay : delay for the first bit to go from source to destination  Transmission delay : time to pump data onto link at reserved rate Propagation delay:  d = length of physical link   s = propagation speed in medium (~2x10 5 km/sec) propagation delay = d/s Transmission delay:    R = bandwidth (bps) L = packet length (bits) time to send a packet into link = L/R CSE401N 14

An Example

   Propagation delay  suppose the distance between host 1 and host 2 is 4000 km, then one-way propagation delay is: 4000

km

200 , 000

km

/

s

 20

ms

Transmission delay   suppose we reserve one slot of a T1 line, which • has a bandwidth of 1.536 Mbps • is divided into 24 slots, and thus • each reserved slot has a bandwidth of 64 Kbps then the transmission delay of a file with 6.4 Kbits is 6 .

4

kbits

64

kbps

 100

ms

Suppose the setup message is very small, and the total setup processing delay is 200 ms CSE401N 15

An Example (cont.)

 Then the delay to transfer a 6.4 Kbits file from host 1 to host 2 (from the beginning until host receives last bit of the file) is: 20  200  20  20  100  360

ms DATA

20 + 200 20 20 100 CSE401N 16

Network Core: Packet Switching

each end-end data stream divided into packets  user A, B packets

share

network resources   each packet uses full link bandwidth resources used as

needed

, Bandwidth division into “pieces” Dedicated allocation Resource reservation resource contention:  aggregate resource demand can exceed amount available   congestion : packets queue, wait for link use store and forward at a time : packets move one hop   transmit over link wait turn at next link CSE401N 17

Packet Switching

Each end-to-end data

flow

 divided into

packets

Packets have the following structure:

Header

Data

Trailer

  • Header and Trailer carry control information (e.g., destination address, check sum) • (

where is the control information for circuit switching?

) At each node the entire packet is received, stored briefly, and then forwarded to the next node (

Store-and-Forward Networks

) Each packet is passed through the network from node to node along some path (

Routing

) CSE401N 18

Packet Switching

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Inside a Packet Switching Router

 A node in a packet switching network incoming links node Memory outgoing links CSE401N 20

Packet Switching: Resources

     Each packet waits for its turn at the output link On its turn, a packet uses full link bandwidth Resources used as needed Aggregate resource demand can exceed amount available Congestion: packets queue, wait for link use Bandwidth division into “pieces” Resource reservation Dedicated allocation CSE401N 21

A Taxonomy of Packet-Switched Networks According to Routing

 

Goal:

move packets among routers from source to destination  we’ll study several routing algorithms later in the course Two types of packet switching  datagram network • each packet of a flow is switched independently  virtual circuit network: • all packets from one flow are sent along a pre-established (= virtual circuit) path CSE401N 22

Datagram Packet Switching

  Example: IP networks Each packet is independently switched   each packet header contains

complete destination address

receiving a packet, a router looks at the packet’s destination address and searches its current routing table to determines the next hop   routes may change during session routers do not keep any state about a flow

An example of datagram-style routing in daily life?

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Datagram Packet Switching

Host C Host A Host D Node 1 Node 2 Node 3 Node 5 Host B Host E Node 6 Node 7 Node 4 CSE401N 24

Timing Diagram of Datagram Switching

Host 1 Host 2 Node 1 Node 2 transmission time of Packet 1 at Host 1 Packet 1 Packet 2 Packet 3 propagation delay from Host 1 to Node 1 Packet 1 Packet 2 Packet 3 processing and queueing delay of Packet 1 at Node 2 Packet 1 Packet 2 Packet 3 CSE401N 25

Virtual-Circuit Packet Switching

   Example: Asynchornous Transfer Mode ( ATM) networks Hybrid of circuit switching and datagram switching  each packet carries a short tag (virtual-circuit (VC) #), tag determines next hop Incoming Interface Incoming VC# Outgoing Interface   fixed path determined at Virtual Circuit setup time, remains fixed thru flow routers maintain per-flow state 1 1 2 … 12 16 12 2 3 3 What advantages do virtual circuit have over datagram?

  Guarantees in-sequence However: Packets from interleaved delivery of packets different virtual circuits may be Outgoing VC# 22 1 22 CSE401N 26

Virtual-Circuit Switching

Host C Host A Node 1 Node 2 Node 5 Node 3 Host D Host B Host E Node 6 Node 7 Node 4 CSE401N 27

Virtual-Circuit Packet Switching

 Three phases 1.

2.

3.

VC establishment Data transfer VC disconnect CSE401N 28

Timing Diagram of Virtual-Circuit Switching Host 1 Host 2 Node 1 Node 2 VC establishment propagation delay between Host 1 and Node 1 data transfer Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 VC termination CSE401N 29

Discussion: Datagram Switching vs. Virtual Circuit Switching  What are the benefits of datagram switching?

 What are the benefits of virtual circuit switching?

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Delay at a Router in Packet Switching

  A packet experiences delay at each Four types of delay at each hop     hop nodal processing delay : check errors & routing queueing : time waiting for its turn at output link transmission delay : time to pump packet onto a link at link speed propagation delay : router to router propagation CSE401N 31

Delay in Datagram Networks transmission time of Packet 1 at Host 1 Host 1 Packet 1 Packet 2 Packet 3 Node 1 Node 2 propagation delay between Host 1 and Node 2 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Host 2 nodal processing and queueing delay of Packet 1 at Node 2 CSE401N 32

Network Core: Packet Switching

A 10 Mbs Ethernet statistical multiplexing C B 1.5 Mbs queue of packets waiting for output link 45 Mbs D E Packet-switching versus circuit switching: human restaurant analogy  other human analogies?

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Packet switching versus

circuit switching Packet switching allows more users to use network!

    1 Mbit link each user:   100Kbps when “active” active 10% of time circuit-switching:  10 users packet switching:  with 35 users, probability > 10 active less than .0004

N users 1 Mbps link CSE401N 34

Packet switching versus circuit switching

Is packet switching a “slam dunk winner?”    Great for bursty data  resource sharing  Excessive congestion:  no call setup packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior?

 bandwidth guarantees needed for audio/video apps  still an unsolved problem (chapter 6) CSE401N 35

Access networks and physical media

Q: How to connection end systems to edge router?

 residential access nets   institutional access networks (school, company) mobile access networks

Keep in mind:

 bandwidth (bits per second) of access network?

 shared or dedicated?

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Residential access: point to point access

   Dialup via modem  up to 56Kbps direct access to router (conceptually) ISDN: integrated services digital network: 128Kbps all digital connect to router ADSL: asymmetric digital subscriber line  up to 1 Mbps home-to-router   up to 8 Mbps router-to-home ADSL deployment: happening CSE401N 37

Residential access: cable modems

   HFC: hybrid fiber coax  asymmetric: up to 10Mbps upstream, 1 Mbps downstream network of cable and fiber attaches homes to ISP router   shared access to router among home issues: congestion, dimensioning deployment: available via cable companies, e.g., MediaOne CSE401N 38

Residential access: cable modems

Diagram: http://www.cabledatacomnews.com/cmic/diagram.html

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Institutional access: local area networks

    company/univ local area network (LAN) connects end system to edge router Ethernet:  shared or dedicated cable connects end system and router  10 Mbs, 100Mbps, Gigabit Ethernet deployment: institutions, home LANs happening now LANs: chapter 5 CSE401N 40

Wireless access networks

   shared wireless access network connects end system to router wireless LANs:   radio spectrum replaces wire e.g., Lucent Wavelan 11 Mbps wider-area wireless access  CDPD: wireless access to ISP router via cellular network router base station mobile hosts CSE401N 41

Home networks

Typical home network components:  ADSL or cable modem    router/firewall Ethernet wireless access point to/from cable headend cable modem router/ firewall Ethernet (switched) wireless laptops wireless access point CSE401N 42

Physical Media

   physical link: transmitted data bit propagates across link guided media:  signals propagate in solid media: copper, fiber unguided media:  signals propagate freely, e.g., radio Twisted Pair (TP)  two insulated copper wires   Category 3: traditional phone wires, 10 Mbps Ethernet Category 5 TP: 100Mbps Ethernet CSE401N 43

Physical Media: coax, fiber

Coaxial cable:    wire (signal carrier) within a wire (shield)   baseband: single channel on cable broadband: multiple channel on cable bidirectional common use in 10Mbs Ethernet Fiber optic cable:    glass fiber carrying light pulses high-speed operation:   100Mbps Ethernet high-speed point-to-point transmission (e.g., 5 Gps) low error rate CSE401N 44

Physical media: radio

    signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects:    reflection obstruction by objects interference Radio link types:     microwave  e.g. up to 45 Mbps channels LAN  (e.g., WaveLAN) 2Mbps, 11Mbps wide-area  (e.g., cellular) e.g. CDPD, 10’s Kbps satellite   up to 50Mbps channel (or multiple smaller channels) 270 Msec end-end delay  geosynchronous versus LEOS CSE401N 45

Thank YOU

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