1

Physical and Data Link Layer

Computer Network SystemSirak Kaewjamnong

2

Network Devices

3

Circuit SwitchingA B

Source

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.

4

Circuit Switching : Telephone Network

Source“Caller”

Central Office“C.O.”

Destination

“Callee”

Central Office“C.O.”

TrunkExchange

Each phone call is allocated 64kb/s. So, a 2.5Gb/s trunk line can carry about 39,000 calls.

5

Packet Switching

A

R1

R2

R4

R3

B

Source

Destination

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.

6

Packet Switching : Simple router model

R1Link 1

Link 2

Link 3

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

7

Why does the Internet use packet switching?

• Efficient use of expensive links:– 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:– “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

8

Packet Switching

Host A

Host B

R1

R2

R3

A

R1

R2

R4

R3

B

TRANSP1

TRANSP2

TRANSP3

TRANSP4

PROP1

PROP2

PROP3

PROP4

Source

Destination

“Store-and-Forward” at each Router

( )i ii

TRANSP PROP Minimum end to end latency

9

Packet SwitchingWhy not send the entire message in one packet?

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.

Host A

Host B

R1

R2

R3

M/R

min/ ii

M R PROP Latency

Host A

Host B

R1

R2

R3

( / )i ii

PROP M R Latency

M/R

10

Common Data Link

11

LAN and WAN Layers

Physical and Data Link layer for LAN and WAN

12

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)

13

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

14

Token Ring / IEEE 802.5

15

IEEE 802.5 Operation

Token ring LANs continuously pass a token or a token ring frame

16

Fiber Distributed Data Interface (FDDI)

Devices on FDDI maintain connectivity on dual ring

17

WAN Protocols• SDLC : Synchronous Data Link

Controlfor IBM SNA network

• HDLC : High Level Data Link Controlcommon WAN data link

• LAPB : Link Access Protocol, BalancedDTE to DCE data link for X.25

• Frame Relayframing for higher speed, reduce checking

for higher speed • PPP : Point to Point Protocol

part of TCP/IP stack for WAN links

18

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

19

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

20

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

21

PPP Advantage

• Support multiple protocols• CRC for every frame• Dynamic negotiation of the IP

address for each end• Link control protocol for

negotiating data-link options

22

Loopback Interface

• 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 wi th the name “ localhost ”

• Loopback interface appears as anot her link layer to the network layer

23

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)