Transcript Note

Chapter 3
Underlying
Technology
(Prepared by
Yu-Chee Tseng, CS/NCTU)
TCP/IP Protocol Suite
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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OBJECTIVES:
 To briefly discuss the technology of dominant wired LANs,
Ethernet, including traditional, fast, gigabit, and ten-gigabit
Ethernet.
 To briefly discuss the technology of wireless LANs, including
IEEE 802.11 LANs, and Bluetooth.
 To briefly discuss the technology of point-to-point WANs
including 56K modems, DSL, cable modem, T-lines, and SONET.
 To briefly discuss the technology of switched WANs including
X.25, Frame Relay, and ATM.
 To discuss the need and use of connecting devices such as
repeaters (hubs), bridges (two-layer switches), and routers
(three-layer switches).
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3-1 WIRED LOCAL AREA NETWORKS
(1) A local area network (LAN) is a computer network
that is designed for a limited geographic area such
as a building or a campus.
(2) Although a LAN can be used as an isolated network
to connect computers in an organization for the sole
purpose of sharing resources, most LANs today are
also linked to a wide area network (WAN) or the
Internet.
(3) The LAN market has seen several technologies such
as Ethernet, token ring, token bus, FDDI, and ATM
LAN, but Ethernet is by far the dominant technology.
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RJ45
• In 1985, the Computer Society of the IEEE
started a project, called Project 802, to set
standards to enable intercommunications.
– accepted by ANSI in 1987, resulting in functions of
PHY and Data Link Layers
• Mapping of OSI model to IEEE layering
Figure 3.1
IEEE standard for LANs
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Ethernet Frame Format
Figure 3.2
Figure 3.3
Ethernet Frame
Maximum and minimum lengths
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Note
Minimum length: 64 bytes (512 bits)
Maximum length: 1518 bytes (12,144 bits)
** Max. Length to reduce buffer requirement and
to prevent a station from monopolizing the shared medium.
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Addressing of Ethernet
• 6 bytes (48 bits)
• normally referred to as “data link address”,
“MAC address”, or “physical address”.
Figure 3.4
Ethernet address in hexadecimal notation
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Unicast, Multicast, and Broadcast
unicast: 0
Sent last
Figure 3.5
multicast: 1
Sent first
Unicast and multicast addresses
The broadcast destination address is a
special case of the multicast address
in which all bits are 1s.
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Example 3.1
Define the type of the following destination addresses:
a. 4A:30:10:21:10:1A
b. 47:20:1B:2E:08:EE
c. FF:FF:FF:FF:FF:FF
Solution
To find the type of the address, we need to look at the second
hexadecimal digit from the left. If it is even, the address is unicast. If it
is odd, the address is multicast. If all digits are F’s, the address is
broadcast. Therefore, we have the following:
a. This is a unicast address because A in binary is 1010 (even).
b. This is a multicast address because 7 in binary is 0111 (odd).
c. This is a broadcast address because all digits are F’s.
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Example 3.2
Show how the address 47:20:1B:2E:08:EE is sent out on line.
←
11100010 00000100 11011000 01110100 00010000 01110111
Solution
The address is sent left-to-right, byte by byte; for each byte, it is
sent right-to-left, bit by bit
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Ethernet的四個重要里程碑
Figure 3.6
Ethernet evolution through four generations
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Standard Ethernet (10 Mbps)
• now becomes history
• access method: CSMA/CD (carrier sense,
multiple access with collision detection)
• defined IEEE 802.3
– connected by a bus or a star topology
STA B 與 STA C碰撞的現象
A
C starts
at time t2
B starts
at time t1
B
C
D
Area where
A’s signal exists
C在此偵測到碰撞
B在此偵測到碰撞
Area where
both signals exist
Area where
B’s signal exists
Time
Figure 3.7
Time
Space/time model of a collision in CSMA
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Analysis of Collision Detection Delay in
CSMA/CD
• A starts sending at t1; C starts sending at t2; C
detects collision at t3; C aborts sending; A detects
collision at t4 and aborts sending.
Latency = t4 - t1
Figure 3.8
Collision of the first bit in CSMA/CD
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Example 3.3: Calculation of Min Frame Length
In the standard Ethernet, if the maximum propagation time is
25.6 μs, what is the minimum size of the frame?
Solution
(1)The frame transmission time is Tfr = 2 × Tp = 51.2 μs. This
means, in the worst case, a station needs to transmit for a
period of 51.2 μs to detect the collision.
(2)The minimum size of the frame is 10 Mbps × 51.2 μs = 512
bits or 64 bytes. This is actually the minimum size of a frame for
Standard Ethernet, as we discussed before.
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Figure 3.9 CSMA/CD flow diagram
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Standard Ethernet Implementation
Topology : bus
Legend : 10Base5
傳輸距離=約500m
或是type
10Mbps
Baseband
10Base-F
Fiber
(unshielded
twisted pair,
UTP)
(optical
fiber)
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Fast Ethernet (100 Mbps)
• compatible with standard Ethernet
• keep the same min and max frame lengths
• only allows star topology
– half duplex: connected via a hub
– full duplex: connected via a switch with buffers at each port
• access method:
– half-duplex: CSMA/CD
– full-duplex: no need for CSMA/CD (for backward compatibility only)
• auto-negotiation
– negotiate mode
– negotiate rate (so 10M Ethernet is acceptable)
– allow a station to check a hub’s capabilities
Fast Ethernet Implementation
• Define by IEEE 802.3u
4 pairs
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Gigabit Ethernet (1 Gbps)
• Defined by 802.3z
• Half duplex(少見) or full duplex (no CSMA/CD)
• 容許carrier extension (8倍長frame) 或frame burst(連續傳多
個frame)
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• Lack of collision implies the max length of the
cable is determined by signal attenuation, not
by collision detection latency.
Note
In the full-duplex mode of Gigabit
Ethernet, there is no collision;
the maximum length of the cable is
determined by the signal attenuation
in the cable.
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Ten-Gigabit Ethernet (10 Gbps)
• Defined by 802.3ae
• Only full duplex, no CSMA/CD
Allow interconnecting
LANs into MAN or WAN
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3-2 WIRELESS LANS
(1) Wireless communication is one of the fastest
growing technologies.
(2) The demand for connecting devices without the
use of cables is increasing everywhere. Wireless
LANs can be found on college campuses, in
office buildings, and in many public areas.
(3) In this section, we concentrate on two wireless
technologies for LANs: IEEE 802.11 wireless
LANs, sometimes called wireless Ethernet, and
Bluetooth, a technology for small wireless LANs.
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IEEE 802.11
• Architecture:
– ad hoc network
– infrastructure
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Extended Service Set (ESS)
• two or more BSSs, connected by a “distributed
system”
(4)
(3)
(1)
Figure 3.14
(2)
(Note: Labels will be explained by Table 3.7.)
Extended service sets (ESSs)
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Figure 3.15
CSMA/CA flow diagram
• CA=collision
avoidance
• Can not implement
CD because
– 無法同時send 和
receive
– Hidden station (無
法察覺collision,
explained later)
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Frame Exchange Time Line
Source
Destination
All other stations
•••
DIFS
1
RTS
SIFS
CTS
2
SIFS
3
NAV
(No carrier sensing)
Data
SIFS
ACK
Time
Figure 3.16
4
Time
Time
Time
CSMA/CA and NAV (Network Allocation Vector)
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Figure 3.17
Frame format
•有fragmentation機制: 容許分割為較小frame(用於noisy environment)
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Figure 3.18
Control frames
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(1)
(2)
(3)
(4)
(使用參考 Fig.3.14)
永遠是下一個
接收者
永遠是目前
傳送者
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當真實src/dst
被佔用時，則退
至此
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Hidden Station Problem
Figure 3.19
Hidden station problem
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Figure 3.20 Use of handshaking to
prevent hidden station problem
B
A
C
RTS
CTS
Time
CTS
Time
Time
Note
The CTS frame in CSMA/CA handshake
can prevent collision from a hidden
station.
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Exposed Station Problem
• 問題: A-> B和 C->D可同時存在，但是C暴露在A的signal範圍內
• cannot be resolved by RTS_CTS mechanism
Figure 3.21
Exposed station problem
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Figure 3.22
Use of handshaking in exposed station problem
RTS
RTS
CTS
Data
RTS
RTS
Data
CTS
Collision
here
Note : C無法判讀D的狀態，故RTS/CTS仍解決不了問題。
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Bluetooth Technology
Figure 3.23
Piconet
• Each piconet has its clock and hopping
sequence
• 若需要更多node，可用
TCP/IP Protocol Suite ”parked” state
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Merging Two Piconets
Figure 3.24 Scatternet
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• hopping by slot
– one slot = 625 us
– a frame can be 1-slot, 3-slot, or 5-slot
* Note: Primary 送出的frame長度永遠是奇數，
Secondary回的frame長度永遠是奇數，
來回總和為偶數。
72 bits
54 bits
Access code
Header
Figure 3.25 Frame format types
0 to N bits
Data
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3-3 POINT-TO-POINT WANS
(1) A second type of network we encounter in the
Internet is the point-to-point wide area network.
(2) A point-to-point WAN connects two remote
devices using a line available from a public
network such as a telephone network.
(3) We discuss traditional modem technology, DSL
line, cable modem, T-lines, and SONET.
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56K Modem
Downloading,
no quantization noise
Uploading,
quantization noise
• Rate = 56k
– The telephone companies sample voice 8000
times per sec, with 8 bits per sample, but 1 bit
used for control.
– 8000 * (8-1) = 56k
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ADSL
• ADSL = Asymmetric Digital Subscriber Line
ADSL is an asymmetric communication
technology designed for residential users;
it is not suitable for businesses.
• 1 slot for voice, 25 slots for up (1 control +24 data = 1.44Mbps),
224 slots for down (13.4 Mbps)
• 由於抗noise因素，實際rate通常低於上述數字
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ADSL Topology
• 在 Telephone company 有一個DSLAM(digital
subscriber line access multiplexer)作分工。
• 其他另有SDSL (symmetric), HDSL (high-bitrate), VDSL (very-high-bit-rate) 等不同技術。
Figure 3.28
ADSL and DSLAM
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Cable Modem
• based on Cable TV
• Upstream 和 downstream 均需與他人分享。
• Upstream 須有CSMA 解決contention問題。
約12Mbps
Figure 3.29
Cable bandwidth
約30Mbps
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Figure 3.30
Cable modem configuration
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T Lines
• T lines are standard digital telephone carriers
designed to carry data from a home or an
organization
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SONET
• SONET = Synchronous Optical Network
• Use fiber-optic cable to carry high-rate data
• OC = Optical Carrier
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3-4 SWITCHED WANS
(1) A switched WAN is a wide area network that
covers a large area (a state or a country) and
provides access at several points to the users.
(2) Inside the network, there is a mesh of point-topoint networks that connects switches.
(3) A switch has multiple port connectors allowing
the connection of several inputs and outputs.
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3 Switched WANs
X.25 : (1) 不相容於IP網路(很早)
(2) 3-layer
(3) 太多error control
Frame Relay : (1) 取代X.25
(2) 提供burst data
(3) 較少overhead
ATM : (1) a cell relay network
(2) Asynchronous time-division
multiplexing
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ATM multiplexing
A3
A2
B2
B1
C3
C2
A1
C3
B2
A3
C2
B1
A2
C1
A1
C1
A cell network uses the cell as the basic
unit of data exchange.
A cell is defined as a small, fixed-size
block of information.
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Figure 3.33
Architecture of an ATM network
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Virtual Connection
• Connection between two end points is
through TPs, VPs, and VCs (see next page)
• Each VC is identified by a pair (VPI, VCI)
VC
VP
TP
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TP, VP, and VC
• TP = transmission path
– the physical connection between two switches
– (ex: the set of highways that directly connect two cities)
• VP = virtual path
– a TP is divided into several VPs
– (ex: a highway that connects two cities)
• VC = virtual circuit
– a VP is divided into several VCs
– a VC is where cells are transmitted
– (ex: a lane of a highway)
ATM layers
AAL = application adaptation layer,
(used only by end points)
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Figure 3.36
Use of the layers
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Figure 3.37 AAL5
Note
The IP protocol uses the AAL5 sublayer.
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Figure 3.38 ATM layer
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3-5 CONNECTING DEVICES
(1) LANs or WANs do not normally operate in
isolation. They are connected to one another or
to the Internet via connecting devices.
(2) Connecting devices can operate in different
layers of the Internet model. We discuss three
kinds of connecting devices: repeaters (or hubs),
bridges (or two-layer switches), and routers (or
three-layer switches).
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Mapping to Protocol Layers
Figure 3.40
Connecting devices
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Repeater and Hub
• Repeater : to regenerate the signal
• Hub : a star topology
Sent
Maintained
A repeater forwards every bit; it has no
filtering capability.
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Bridge
Bridge table
Address
Port
1
71:2B:13:45:61:41
2
71:2B:13:45:61:42
3
64:2B:13:45:61:12
4
64:2B:13:45:61:13
具有filtering的功能
A bridge has a table used in filtering
decisions.
A bridge does not change the physical (MAC)
addresses in a frame.
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Functionality of Bridge
• filtering (as shown in Fig. 3.42)
• transparent: (defined in IEEE 802.1d)
– forward frames
– learn forwarding table automatically (see Fig. 3.43)
– prevent loops
• Whenever a bridge has no entry about a
destination, it uses “flooding”.
– ex: b and c in Fig. 3.43
Learning of MAC Addresses
Address
Port
a. Original
Address
71:2B:13:45:61:41
64:2B:13:45:61:13
Port
1
4
c. After D sends a frame to B
M
Address
Port
71:2B:13:45:61:41
1
64:2B:13:45:61:13
4
71:2B:13:45:61:42
2
d. After B sends a frame to A
M
M
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Address
Port
71:2B:13:45:61:41
1
64:2B:13:45:61:13
4
71:2B:13:45:61:42
2
64:2B:13:45:61:12
3
e. After C sends a frame to D
M
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Router
A router is a three-layer (physical, data link,
and network) device.
A router changes the physical addresses in a
packet.
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Features of a Router
• A router typically has an interface connected to a
bridge.
• A router has a physical address and a logical (e.g., IP)
address for each of its interfaces.
• A router acts only on those packets in which the
physical destination address matches the physical
address of the interface from which the packet
arrives.
• A router changes the physical address of the packet
(both source and destination) when it forwards the
packet.
Comparison (Repeater, Bridge, and
Router)
Note
(1)A repeater or a bridge connects
segments of a LAN.
(2)A router connects independent LANs
or WANs to create an internetwork
(internet).
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Summary
•
•
•
•
•
Ethernet
Wireless LAN
point-to-point WAN
switched WAN
devices (repeater, bridge, and router)