Transcript Physical and Data Link Layer

Physical and Data Link Layer
Computer Network System
Sirak Kaewjamnong
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Network Devices
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Circuit Switching
A
Source
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B
Destination
It’s the method used by the telephone network.
A call has three phases:
1. Establish circuit from end-to-end (“dialing”),
2. Communicate,
3. Close circuit
Originally, a circuit was an end-to-end physical wire.
Nowadays, a circuit is like a virtual private wire: each call has its
own private, guaranteed data rate from end-to-end.
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Circuit Switching : Telephone Network
Each phone call is allocated 64kb/s. So, a
2.5Gb/s trunk line can carry about 39,000 calls.
Destination
“Callee”
Source
“Caller”
Central
Office
“C.O.”
Central
Office
“C.O.”
Trunk
Exchange
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Packet Switching
A
Source
B
R2
R1
R3
Destination
R4
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It’s the method used by the Internet.
Each packet is individually routed packet-by-packet, using the
router’s local routing table.
The routers maintain no per-flow state.
Different packets may take different paths.
Several packets may arrive for the same output link at the same
time, therefore a packet switch has buffers.
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Packet Switching : Simple router model
Link 1
Link 2
Link 3
R1
Link 4
Link 1, ingress
Link 1, egress
Link 2, ingress
Link 2, egress
Link 3, ingress
Link 3, egress
Link 4, ingress
Link 4, egress
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Why does the Internet use packet switching?
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Efficient use of expensive links:
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The links are assumed to be expensive and scarce.
Packet switching allows many, bursty flows to share the same link
efficiently.
“Circuit switching is rarely used for data networks, ... because of very
inefficient use of the links” - Gallager
Resilience to failure of links & routers:
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“For high reliability, ... [the Internet] was to be a datagram subnet, so if
some lines and [routers] were destroyed, messages could be ... rerouted” Tanenbaum
Source: Networking 101
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Packet Switching
A
B
R2
Source
R1
Destination
R3
R4
Host A
TRANSP1
R1
R2
R3
Host B
“Store-and-Forward” at each Router
TRANSP2
PROP1
TRANSP3
PROP2
TRANSP4
PROP3
PROP4
Minimum end to end latency   (TRANSPi  PROPi )
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Packet Switching
Why not send the entire message in one packet?
M/R
M/R
Host A
Host A
R1
R1
R2
R2
R3
R3
Host B
Latency   ( PROPi  M / Ri )
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Host B
Latency  M / Rmin   PROPi
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Breaking message into packets allows parallel transmission across all links, reducing
end to end latency. It also prevents a link from being “hogged” for a long time by
one message.
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Common Data Link
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LAN and WAN Layers
Physical and Data Link layer for LAN and WAN
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Medium Access Control
• Protocol for controlling access to transmission medium
• Defined as part of Data Link layer
• The protocol performs:
– Perform functions related to medium access (MAC sublayer)
– Concerned with the transmission of a link-Level between two
node (LLC sublayer)
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IEEE 802.3 Operation
• Every node can receive a transmission by all other nodes
• Need addressing scheme to identify destination
• Only destination copies frame to it, all other nodes discarded the frame
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Token Ring / IEEE 802.5
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IEEE 802.5 Operation
Token ring LANs continuously pass a token or a token ring frame
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Fiber Distributed Data Interface (FDDI)
Devices on FDDI maintain connectivity on dual ring
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WAN Protocols
• SDLC : Synchronous Data Link Control
for IBM SNA network
• HDLC : High Level Data Link Control
common WAN data link
• LAPB : Link Access Protocol, Balanced
DTE to DCE data link for X.25
• Frame Relay
framing for higher speed, reduce checking for higher speed
• PPP : Point to Point Protocol
part of TCP/IP stack for WAN links
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SLIP : Serial Line IP
• Method for encapsulation IP datagrams on serial line
• Popular for connecting home computer to Internet, via modem
• RFC 1005 de facto standard
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SLIP Deficiency
• Need manual configuration of IP address both side (no negotiation
mechanism )
• Only one protocol can be used because there is no field to specify
type of protocol
• No checksum, bad for protocol that depend on CRC
• Slow line make inefficient to carry only 1 byte data with 40 bytes
overhead(IP + TCP header)
– CSLIP (compress SLIP) reduces 40 bytes header to 3-5 bytes
– smaller header greatly improve the interactive respond time
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PPP : Point to Point Protocol
• Methods for encapsulation IP datagrams on serial line, correct all
deficiency in SLIP
• Support either an asynchronous link with 8N1 or bit oriented
synchronous link
• Two parts:
– LCP :(Link Control Protocol ) to establish, configure and test
connection
– NCP :(Network Control Protocol) support different network layer
protocols
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PPP Advantage
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Support multiple protocols
CRC for every frame
Dynamic negotiation of the IP address for each end
Link control protocol for negotiating data-link options
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Loopback Interface
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Channel for client and server on the same host use to communicate
Class A network ID 127 is reserve for the loopback interface
Most system assigns IP 127.0.0.1 with the name “ localhost ”
Loopback interface appears as another link layer to the network
layer
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Summary
• The physical layer provides access to the medium of an
internetwork
• The data link layer provides support for communications over
several types of data links:
– LAN (Ethernet, IEEE 802.X, FDDI)
– Dedicated WAN (SDLC, HDLC,PPP,LAPB)
– Switches LAN (X.25, Frame Relay, ISDN)
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