1. EECS 122: Introduction to Computer Networks Network Service and Applications
Computer Science Division
Department of Electrical Engineering and Computer Sciences
University of California, Berkeley
Berkeley, CA

2. Overview
Taxonomy of Communication Networks
Services and Applications

3. Taxonomy of Communication Networks
Communication networks can be classified based on the way in which the nodes exchange information:
Communication Network
Switched Communication Network
Broadcast Communication Network
Packet-Switched Communication Network
Circuit-Switched Communication Network
Datagram Network
Virtual Circuit Network

4. Broadcast vs. Switched Communication Networks
Broadcast Communication Networks
Information transmitted by any node is received by every other node in the network
Examples: usually in LANs (Ethernet, WiFi)
Problem: coordinate the access of all nodes to the shared communication medium (Multiple Access Problem)
Switched Communication Networks
Information transmitted to a sub-set of designated nodes
Examples: WANs (Telephony Network, Internet)
Problem: how to forward information to intended node(s)?
Done by special nodes (e.g., routers, switches) executing routing protocols

5. Taxonomy of Communication Networks
Communication networks can be classified based on the way in which the nodes exchange information:
Communication Network
Switched Communication Network
Broadcast Communication Network
Packet-Switched Communication Network
Circuit-Switched Communication Network
Datagram Network
Virtual Circuit Network

6. Circuit Switching Three phases If circuit not available: “Busy signal”
circuit establishment
data transfer
circuit termination
If circuit not available: “Busy signal”
Examples
Telephone networks
ISDN (Integrated Services Digital Networks)

7. Telegraph Network Alexander Graham Bell
1876: Demonstrates the telephone at US Centenary Exhibition in Philadelphia

8. Telephone Network Almon Brown Strowger (1839 - 1902)
1889: Invents the “girl-less, cuss-less” telephone system, aka the mechanical switching system

9. Timing in Circuit Switching
Host 1
Host 2
Node 1
Node 2
DATA
processing delay at Node 1
propagation delay
between Host 1
and Node 1
Circuit Establishment
Data Transmission
Circuit Termination
propagation delay
between Host 2
and Node 1

10. Circuit Switching Node (switch) in a circuit switching network
incoming links
Node
outgoing links

11. Circuit Switching: Multiplexing/Demultiplexing
Frames
Slots = 1 2 3 4 5 1 2 3 4 5
Time divided in frames and frames into slots
Relative slot position inside a frame determines to which conversation data belongs
E.g., slot 0 belongs to red conversation
Requires synchronization between sender and receiver—surprisingly difficult!
In case of non-permanent conversations
Needs to dynamically bind a slot to a conservation
How to do this?
If a conversation does not use its circuit the capacity is lost!

12. Taxonomy of Communication Networks
Communication networks can be classified based on the way in which the nodes exchange information:
Communication Network
Switched Communication Network
Broadcast Communication Network
Packet-Switched Communication Network
Circuit-Switched Communication Network
Datagram Network
Virtual Circuit Network

13. Packet Switching Data sent as formatted bit-sequences (Packets)
Packets have following structure:
Header and Trailer carry control information (e.g., destination address, check sum)
Each packet traverses the network from node to node along some path (Routing)
At each node the entire packet is received, stored briefly, and then forwarded to the next node (Store-and-Forward Networks)
Typically no capacity is allocated for packets
Header
Data
Trailer

14. Packet Switching Node in a packet switching network incoming links
outgoing links
Memory

15. Packet Switching: Multiplexing/Demultiplexing
Data from any conversation can be transmitted at any given time
Single conversation can use the entire link capacity if it is alone
How to tell them apart?
Use meta-data (header) to describe data

16. Taxonomy of Communication Networks
Communication networks can be classified based on the way in which the nodes exchange information:
Communication Network
Switched Communication Network
Broadcast Communication Network
Packet-Switched Communication Network
Circuit-Switched Communication Network
Datagram Network
Virtual Circuit Network

17. Datagram Packet Switching
Each packet is independently switched
Each packet header contains destination address
No resources are pre-allocated (reserved) in advance
Example: IP networks

18. Timing of Datagram Packet Switching
Host 1
Host 2
Node 1
Node 2
propagation
delay between
Host 1 and
Node 2
transmission
time of Packet 1
at Host 1
Packet 1
Packet 2
Packet 3
processing delay of Packet 1 at Node 2
Packet 1
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3

19. Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Host E
Node 7
Node 6
Node 4

20. Taxonomy of Communication Networks
Communication networks can be classified based on the way in which the nodes exchange information:
Communication Network
Switched Communication Network
Broadcast Communication Network
Packet-Switched Communication Network
Circuit-Switched Communication Network
Datagram Network
Virtual Circuit Network

21. Virtual-Circuit Packet Switching
Hybrid of circuit switching and packet switching
Data is transmitted as packets
All packets from one packet stream are sent along a pre-established path (=virtual circuit)
Guarantees in-sequence delivery of packets
However, packets from different virtual circuits may be interleaved
Example: ATM networks

22. Virtual-Circuit Packet Switching
Communication with virtual circuits takes place in three phases
VC establishment
data transfer
VC disconnect
Note: packet headers don’t need to contain the full destination address of the packet

23. Timing of Virtual-Circuit Packet Switching
Host 1
Host 2
Node 1
Node 2
propagation delay
between Host 1
and Node 1 VC
establishment
Packet 1
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
Data
transfer
Packet 1
Packet 2
Packet 3 VC
termination

24. Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Host E
Node 7
Node 6
Node 4

25. Packet-Switching vs. Circuit-Switching
Most important advantage of packet-switching over circuit switching: Exploitation of statistical multiplexing:
Efficient bandwidth usage; ratio between peek and average rate is 3:1 for audio, and 15:1 for data traffic
However, packet-switching must handle congestion:
More complex routers
Harder to provide good network services (e.g., delay and bandwidth guarantees)
In practice they are combined:
IP over SONET, IP over Frame Relay

26. Overview
Taxonomy of Communication Networks
Services and Applications

27. The Internet Protocol (IP)
Problem:
Many different network technologies
e.g., Ethernet, Token Ring, ATM, Frame Relay, etc.
How can you hook them together?
n x n translations?
IP was invented to glue them together
n translations
Minimal requirements (datagram)
The Internet is founded on IP
“IP over everything”

28. Addressing Every Internet host has an IP address
e.g.,
Packets include destination address
Network is responsible for routing packet to address
Host-view:
Network
Source
Destination

29. IP-centric View Host C Host D Host A Router 1 Router 2 Router 3
Host B
Host E
Router 7
Router 6
Router 4

30. Physical View A big mess!
Every “link” could be a whole network of ATM, frame relay, ethernet, DSL, etc.
Beauty of IP: you can ignore these different network technologies
In many networks, IP is used only at the edge

31. Back to IP Host C Host D Host A Router 1 Router 2 Router 3 Router 5
Host B
Host E
Router 7
Router 6
Router 4

32. Routing Routers have “routing tables”
Tables mapping each destination with an outgoing link
Requires that routing table is highly compressible!
Implications for address assignment, mobility, etc.
Routing decisions made packet-by-packet
Routers keep no connection state
Question: Why have the network do routing?
Why not the hosts?
Compare delivery-by-hand to FedEx

33. Internet Service “Best-Effort” service Why this service model?
No guarantees about packet delivery
Hosts must cope with loss and delay
Why this service model?
Why not guarantee no loss and low delay?

34. Domain Name Service (DNS)
Humans/applications use machine names
e.g.,
Network (IP) uses IP addresses
e.g.,
DNS translates between the two
An overlay service in its own right
Global distribution of name-to-IP address mappings—a kind of content distribution system as well
Unsung hero of the Internet

35. File Transfer (FTP, SCP, etc.)
Get file from soup.cs.berkeley.edu
Get file
Your PC
file
soup.cs.berkeley.edu
Get address for
soup.cs.berkeley.edu
DNS

36. Email Email message exchange is similar to previous example, except
Exchange is between mail servers
DNS gives name of mail server for domain

37. Get www.icir.org/file.html
Web
Get
Your PC
Proxy
Get file.html
file.html
Get address for
DNS

38. Caching Caches can be visible or transparent Visible: Transparent:
Client is configured to ask cache
Transparent:
Cache intercepts packet on its way to web server
Example of a “application-aware middlebox”
Violates purity of architecture, but are prevalent...

39. Content Distribution Network (CDN)
How to get closest copy of replicated content?
CDNs have mirror servers distributed globally
CDN customers allow CDN to run their DNS
“Smart” DNS server returns results based on requester’s IP address

40. Dashed lines are “virtual links”
Gnutella (P2P)
Overlay Network
Dashed lines are “virtual links”

41. Gnutella (cont’d) User asks for file (by metadata)
Each host sends request to its “neighbors” in overlay network
Responses sent back to original requesting node
Many variations on P2P file sharing.....

42. Overlay Networks Create a set of “virtual links” between hosts
Communication between neighbors on overlay is done by IP
But the overlay can use different routing, or application-specific processing, at overlays nodes
IP is often overlay on circuit-switched network
App-specific networks increasingly overlaid on IP

43. Architecture The assignment of tasks and knowledge
Who does what, and where is the state kept?
How they do it: algorithms and implementation

44. Internet Architecture
Routers do routing, and almost nothing else
No application-specific functions
Hosts do all application-specific processing
Allowed wide variety of applications to flourish on Internet