Transcript Lectures 26&27 - Electrical and Computer Engineering

Telecommunications Networking
II
Lectures 26-27
Local Area Networks
[Ref: “Computer Networks”
A.S. Tanenbaum]
Copyright 1999, S.D. Personick. All Rights Reserved.
Local Area Networks
• LAN Overview
• Link Layer- packet error control and media
access (coordination) protocols
• Network Layer: addressing and routing
protocols
• Higher layer protocols
• Examples of LAN standards: Ethernet,
token ring
Copyright 1999, S.D. Personick. All Rights Reserved.
What are we trying to do?
• We are trying to create a local
communications infrastructure which will
allow computing appliances to exchange
data in the form of packet data units
(packets)
• We want this local communications
infrastructure to be low in cost, and easy to
implement and maintain
Copyright 1999, S.D. Personick. All Rights Reserved.
A typical packet
Payload: 64-to-2048 bytes
Header
Header contains: source address, destination address,
special handling requirements, packet sequence number,
information about packet history, error control bytes, ...
Copyright 1999, S.D. Personick. All Rights Reserved.
What are we trying to do?
(cont’d)
• We would like our local communication
infrastructure to employ “open” interfaces at
appropriate points, so that portions of it can be
provided by different suppliers; and so that we can
accommodate multiple types of physical layer
transmission media: wires, coaxial cable, fiber,
wireless
• We would like our local communication
infrastructure to support a wide range of
applications
Copyright 1999, S.D. Personick. All Rights Reserved.
Issues we need to address
• When packet data units (PDU’s) are
transmitted across a physical medium (e.g.,
wire pairs), errors may occur
• We need to coordinate the actions of
multiple computing devices that may wish
to send PDU’s over the same medium
• We need a way to identify the intended
recipient(s) of a PDU
Copyright 1999, S.D. Personick. All Rights Reserved.
Issues We Need to Address
• We need to deal with the possibility that
PDU’s may be inadvertently lost,
duplicated, or delivered out of the order in
which they were sent
• We need to deal with the possibility that
some PDU’s may require expedited delivery
• We need to manage congestion when more
PDU’s are being sent than the network can
accommodate
Copyright 1999, S.D. Personick. All Rights Reserved.
“Higher Layer Issues”
• We need a methodology that enables computing
appliances to inform each other of their desire to
communicate (establish a “session”), and to accept
or decline such communication
• We may need to provide mechanisms for “secure”
communication
• We may need to provide mechanisms which
enable different higher layer protocols to interoperate (e.g., converting text to speech)
Copyright 1999, S.D. Personick. All Rights Reserved.
The Protocol Stack
Application Layer (e.g., E-mail or WWW)
Presentation Layer (e.g., Microsoft Word, plain text)
Session Layer (e.g., accepting or denying access)
Transport Layer (e.g., asking for retransmission of lost
packets; re-ordering packets that are out of order)
Network Layer (reading addresses and directing
packets toward their intended destinations)
Link Layer (coordinating access to the physical
medium, and trying to move packets across the
physical medium without making errors)
Physical layer (e.g., wire pairs or fiber or wireless)
Copyright 1999, S.D. Personick. All Rights Reserved.
Separation of Concerns
Each layer in the protocol stack should be
able to use the services of the layer(s) below
it, without having to be concerned with the
details of how those services are
implemented.
For example, the transport layer should not
be concerned with, or dependent upon the
details of how the network layer manages to
get the right packets to the right destinations
Copyright 1999, S.D. Personick. All Rights Reserved.
What each layer does
• The engineers who design physical layer
(e.g. wireless) solutions focus on the
problems of moving bits (1’s and 0’s) across
their physical layer... with a specified
probability of error, a specified data rate
capability, and specified delay
characteristics. E.g., wireless physical layer
designers worry about the stuff we discussed
in previous lectures
Copyright 1999, S.D. Personick. All Rights Reserved.
What each layer does (cont’d)
• The engineers who design (and agree on standards
for) link layer protocols are concerned with:
- how to detect errors that occur when PDU’s are
communicated across a physical layer; and how
such detected errors might also be corrected (error
detecting codes, error correcting codes,
retransmission)
-how to coordinate access to the physical layer by
multiple computing appliances which are using
the same physical layer medium at the same time
(“media access control”)
Copyright 1999, S.D. Personick. All Rights Reserved.
What each layer does (cont’d)
• Engineers who design (and agree on standards for)
network layer protocols are concerned with:
-how destination addresses (and possibly specific
routing requests) are incorporated into PDU’s
-how routers (network packet switches) coordinate
to establish routes over which packets can be
moved toward their intended destinations
-how requests to reserve network capacity are
specified and accepted (or denied)
Copyright 1999, S.D. Personick. All Rights Reserved.
What each layer does (cont’d)
• Engineers who design (and agree on standards for)
transport layer protocols are concerned with
-how to ensure end-to-end PDU transport integrity
by: detecting lost or duplicated packets; requesting
retransmissions of lost packets; re-ordering
packets that are received out-of-order; adjusting
the rate at which packets are sent across the
network to accommodate congestion and other
problems; storing packets for retransmission or
initial transmission, if required.
Copyright 1999, S.D. Personick. All Rights Reserved.
The protocol “hour glass”
E-mail, WWW, Telephony, ...
TCP, UDP
IP
Fewer is better: Token passing
CSMA-CD
CSMA-CA
HDLC
Wire pairs, Fiber, Coax, Wireless, ...
Copyright 1999, S.D. Personick. All Rights Reserved.
HDLC
(see Tanenbaum pp 175-239)
01111110 | Address | Control | Data | Checksum | 01111110
8
8
8
> or = 0
16
Copyright 1999, S.D. Personick. All Rights Reserved.
8
HDLC
Each packet data unit (frame) includes a “cyclic redundancy
check” (CDC) sequence that allows for the detection of most
single or multiple bit errors. Frames with detected errors are
discarded.
The header of each frame also includes a “frame sequence
number” that is typically between 0-7, but could be between 0
and (2**n)-1, where n is between 3 and 7
Frames are acknowledged after they are correctly received
The maximum number of outstanding, unacknowledged
frames is: (2**n) -1 (e.g., 7 )
Retransmission is used to replace unacknowledged or rejected
frames
Copyright 1999, S.D. Personick. All Rights Reserved.
HDLC
2
1
0
Frames (includes
retransmissions)
NR=3*
Acknowledgements
*NR=3 indicates that the next expected
frame sequence number = 3
Copyright 1999, S.D. Personick. All Rights Reserved.
Cyclic Redundancy Check
Examples of error detecting methods:
Parity bit = sum of the binary digits in the part of
the packet you want to protect ( modulo 2). A
single bit error causes the parity bit not to match
the parity calculated at the receiver
CRC-16 or CRC-32: Divide the data field by a
binary polynomial with either 2**16 or 2**32 as
its most significant bit. The CRC is the remainder
Copyright 1999, S.D. Personick. All Rights Reserved.
Cyclic Redundancy Check
(cont’d)
Example: At the transmitter….
Data sequence to be protected is: (4 byte
example)….
10110111 00001001 11011010 11100110= DS
Polynomial is: CRC- CCITT (16 bit CRC):
10001000000100001= P
Divide DS 00000000 00000000 by P; the
remainder is the CRC
Copyright 1999, S.D. Personick. All Rights Reserved.
CRC (binary long division)
10001000000100001 | 101
101101110000100111011010111001100000000000000000
10001000000100001
________________
01011101111100101 (First remainder + next digit)
00000000000000000
_________________
10111011111001010 (Second remainder + next digit)
10001000000100001
Copyright 1999, S.D. Personick. All Rights Reserved.
Cyclic Redundancy Check
(cont’d)
Append CRC (remainder) to data being protected:
[DS CRC]
…….
At the receiver... Divide [DS CRC] by P;
The result should be a zero remainder
Copyright 1999, S.D. Personick. All Rights Reserved.
Media Access Protocols
• Sharing a physical link among multiple
(simultaneous) users
• Objectives are: to minimize waiting time to
obtain access to a link; to get a high level of
achievable throughput on the link; and to
provide a “fair” sharing of the link capacity
among users. These objectives are
sometimes at odds with each other
Copyright 1999, S.D. Personick. All Rights Reserved.
Aloha* Protocol
Transmit your packet, if it gets
through…fine. If it collides with another
packet, then try again.
Used to share links with long end-to-end
delay (vs. packet duration)
*Early radio network in the Hawiian
Islands
Copyright 1999, S.D. Personick. All Rights Reserved.
Aloha Protocol
(with fixed length packets)
N users; each user sending k packets per second; fixed packet
length = T (seconds). Channel bit rate = B (bits/sec)
The probability that any transmitter packet does NOT
experience a collision= exp [-2(N-1)kT]; I.e. none of the other
N-1 users transmits a packet within T seconds (before or after)
of the time I start sending my packet.
Throughput = BT x Nk x exp [-2(N-1)kT]
For N>>1, maximum throughput = B/(2e)
Copyright 1999, S.D. Personick. All Rights Reserved.
Carrier Sense Multiple Access
A variation on Ahola, where you listen to see
if the “channel” is being used (e.g., listen for
a “carrier”) before transmitting.
If the end-to-end delays are short compared
to the length of a transmitted packet, this can
result in high utilization of the channel
If the point-to-point (through the medium)
delay between users is large compared to the
duration of a transmitted packet, then this
becomes the same as Aloha
Copyright 1999, S.D. Personick. All Rights Reserved.
CSMA/CD
CSMA with collision detection
If a collision is detected, the colliding
transmitters “back off” for a randomly
selected amount of time, in order to try to
prevent another collision when the retransmit
Copyright 1999, S.D. Personick. All Rights Reserved.
Token Passing
Each transmitter has an opportunity (a
“turn”) to send packets... defined as holding
the “token”. After a predefined length of
time or number of transmitted packets, it
must pass the token to another transmitter.
Copyright 1999, S.D. Personick. All Rights Reserved.
Internetworking Protocol (IP)
Networking Layer
Issues are:
Addressing
Creating routing tables (in routers)
Segmentation and re-assembly of IP packets
Domain name service
Copyright 1999, S.D. Personick. All Rights Reserved.
IP Addressing (IPv4)
0-255, 0-255, 0-255, 0-255
4 bytes of address: e.g., 144.128.32.1
Class A: 1 byte network/ 3 bytes subnet
Class B: 2 bytes network/ 2 bytes subnet
Class C: 3 bytes network/ 1 byte subnet
Copyright 1999, S.D. Personick. All Rights Reserved.
Routing
backbone router
local router
Bellatlantic
.net
Source Host
(computer)
Drexel.edu
local router
Destination Host
(computer)
Copyright 1999, S.D. Personick. All Rights Reserved.
Setting Up Routing Tables
• Each router (packet switch) communicates with its
neighbors, using a complex protocol to determine
where it should route packets to
• Routing tables are continuously updated
• When an incoming packet arrives, the router
compares the destination address of the packet to
the listings in its routing table to determine which
output to route the packet to
Copyright 1999, S.D. Personick. All Rights Reserved.
Domain Name Service
• The domain name service complements IP
routing by providing a mechanism for
translating domain “names” into IP
addresses
ece.drexel.edu => 144.128.32.1
Copyright 1999, S.D. Personick. All Rights Reserved.
TCP (Transport Layer)
• Keeps track of incoming packets to detect
lost packets
• Stores transmitted packets for possible
retransmission
• Acknowledges (or NACKS) receipt of
packets
• Adjusts window size to accommodate endto-end packet loss and delay conditions
Copyright 1999, S.D. Personick. All Rights Reserved.
Ethernet Standards (802.3)
see Tanenbaum pp 276-287
• 802.2 is an IEEE standard for the use of
CSMD/CD on a shared medium (e.g.,
coaxial cable, wire pairs cable, fiber optic
cable, wireless)
• “Ethernet” is the commonly used name for
802.2 LANs; but it really refers to a specific
product that was developed at Xerox PARC
Copyright 1999, S.D. Personick. All Rights Reserved.
802.3 Layer 1
• 802.3 is specified for various types of cable:
-10Base5: Thick coax, 500m per segment*
-10Base2: Thin coax, 200m per segment*
-10BaseT: Twisted pair, 100m per segment
-10BaseF: Fiber optics, 2000m per segment
-higher speed versions of 802.3
*electronic repeaters, bridges or hubs allow
concatenation of segments
Copyright 1999, S.D. Personick. All Rights Reserved.
802.3 Layer 1 (cont’d)
• Manchester bipolar coding is used to avoid
ambiguities between 0 volts and a logical
“0”, and also to produce “d.c. balance”,
which is helpful for designing the
transceivers that interface to 802.3 LANs
Binary “1” => high-low (+.85v, -.85v)
Binary “0” => low-high (-.85v, +.85v)
Copyright 1999, S.D. Personick. All Rights Reserved.
802.3 Layer 2
• Layer 2 frames consist of:
-7 bytes of preamble to allow for transceiver
synchronization (10101010 x 7)
-1 byte “start of frame” (10101011)
-6 byte (48 bit) destination (global) address
-6 byte (48 bit) source (global) address
-2 byte “length of frame”
- Between 0 and 1500 bytes of data
-Up to 46 bytes of “pad” (if needed)
- 4 byte (16 bit) checksum (CRC)
Copyright 1999, S.D. Personick. All Rights Reserved.
802.3 Layer 2 (cont’d)
• When a station detects a collision, it sends
out a 48 bit “noise burst” to warn all other
stations
• As per CSMD-CD definition, stations listen
before they transmit. In the event of a
detected collision, stations “back off” and
wait a random amount of time before
sending again. Frames must be longer than
2T, where T is the maximum 1-way delay.
Minimum frame duration = 51,200 ns
Copyright 1999, S.D. Personick. All Rights Reserved.
Token Ring Standards (802.5)
see Tanenbaum pp 292-299
• Physical layer is formed by point-to-point
links between adjacent stations, connected
to form a closed ring
-Each received bit is buffered, examined,
possibly changed, and re-transmitted by
each station
• The token consists of a 3-byte pattern.
Changing one bit of this pattern converts it
to the first 3 bytes of a normal data frame
Copyright 1999, S.D. Personick. All Rights Reserved.
802.5 (continued)
• In “listen mode” each node on the ring
inserts 1 bit of buffer delay as part of the
total delay around the ring
• In “transmit mode” a node inserts its frames
into the ring, and removes its inserted bits
as they come back around the ring. When
finished transmitting, a transmitter reinserts
the token on to the ring
Copyright 1999, S.D. Personick. All Rights Reserved.
802.5 Physical layer
• 1, 4 or 16 Mbps on shielded twisted pairs
• Differential Manchester bipolar coding: +/3.0-4.5 volts
• “Bypass center” (central hub) can be used to
bypass failed nodes (star-shaped ring)
Copyright 1999, S.D. Personick. All Rights Reserved.
802.5 Layer 2
• Token holding time is 10 msec (100,000 bits
@ 10 Mbps)
• The data field of a frame has no byte
number limit (except as limited by the
allowed token holding time)
• Frames are acknowledged by the receiving
station by changing a bit in the “frame
status” byte (after the CRC)
Copyright 1999, S.D. Personick. All Rights Reserved.