Transcript LANs and Ethernet

Local Area Networks
Includes some material from
Forouzan ‘Data Communications’
Defining Features
• Scope – building or campus, private
• PDUs are called frames
• Shared Medium (multi-access) or point to point
(e.g. Switched Ethernet )
• High speed – up to 10 Gbps
• Low error rates – 1 in 1010
• Wired or wireless
• Ring, bus, tree, star and extended star topology
LAN Protocol Architecture
OSI Layers
3 (Network) and 4 (Transport)
2 (Data Link)
1 (Physical)
TCP/IP protocol family
IPX/SPX protocol family
LLC
MAC
Physical
LLC = Logical Link Control
MAC = Media Access control
Physical Layer Functions
•
•
•
•
Encoding/decoding of signals
Preamble generation/removal for synchronisation
Bit transmission, reception
Transmission medium specification
(baud rate, distance limitations)
MAC functions
•
•
•
•
•
Medium Access Control
Frame assembly, disassembly
Frame transmission and reception
Checksum (re)computation
Address recognition
LLC Functions
• Abstracts the higher layer protocols from the
details of the physical link technology and access
method.
• Provide one or more Service Access Points (SAPs)
for user protocols (higher layer protocols).
• Addressing (SAPs) of user processes.
• Control and handshaking:
–
–
–
–
Connection Management
Frame sequencing
Error recovery (ACK etc., not checksum)
Flow control
MAC Protocols
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•
•
•
Contention – Ethernet (CSMA/CD)
Round Robin – Token passing
Polling – VGAnyLAN (no longer used)
Reservation – WLAN & satellite
IEEE and CSMA/CD / Ethernet
Overview of operation
IEEE 802 Committees
The LAN / MAN Standards Committee (LMSC) (or IEEE Project 802) develops LAN
and MAN standards, mainly for the lowest 2 layers of the OSI Reference Model.
Active Working Groups
802.1 High Level Interface (HILI)
802.3 CSMA/CD
802.11 Wireless LAN (WLAN)
802.15 Wireless Personal Area Network (WPAN)
802.16 Broadband Wireless Access (BBWA)
802.17 Resilient Packet Ring (RPR)
802.18 Radio Regulatory Technical Advisory Group
802.19 Coexistence Technical Advisory Group
802.20 Mobile Wireless Access
Hibernating Working Groups (standards published, but inactive)
802.2 Logical Link Control (LLC)
802.4 Token Bus
802.5 Token Ring
802.6 Metropolitan Area Network (MAN)
802.7 BroadBand Technical Adv. Group (BBTAG)
802.9 Integrated Services LAN (ISLAN)
802.10 Standard for Interoperable LAN Security (SILS)
802.12 Demand Priority
802.2 Logical Link Control (LLC)
Provides an interface between the various network technologies and the upper layers.
(Hides the differences between the technologies from the upper layers).
LLC
802.2
Data-Link
Layer
MAC
Physical
Layer
802.3
802.4
802.5
802.6
CSMA/CD
Token Bus
Token Ring
MAN
LLC frame:
DSAP
Address
(1 byte)
SSAP
Address
(1 byte)
Control
(1 or 2 bytes)
Information
(0 or more bytes)
Logical Link Control (LLC)
Layer 3
Layer 2
MAC
header
Layer 1
LLC
header
Packet
Layer 3 PDU
Payload
LLC PDU
Payload
01101011101100 … Bit stream … 00111010100011
MAC PDU (Frame)
Ethernet Technology (Introduction)
The dominant LAN technology in the world.
Operates in two areas of the OSI model:
MAC sublayer of the data link layer, and the physical layer.
Not one technology but a family of LAN technologies:
Various specifications support different media, bandwidths,
and other Layer 1 and 2 variations.
However, all the specifications are essentially compatible with the
original Ethernet standard.
Essentially: frame format and addressing scheme the same for all varieties.
The same protocol that transported data at 3Mbps in 1973 is carrying data at 10GbpS.
Ethernet is considered very scalable:
The bandwidth of the network can be increased many times without
changing the underlying Ethernet technology.
E.g.
An Ethernet frame can be created by an older coax 10-Mbps NIC,
during its journey travel over a 10-Gbps Ethernet fiber link,
and be eventually delivered to a 100-Mbps NIC.
As long as the packet stays on Ethernet networks it is not changed.
Collision Domains (and relationship to Layer 1 / Layer 2 devices)
Collision domain: part of the network bounded by layer 2 (or higher) devices.
A collision:
Will travel across a Repeater or Hub (Layer 1 devices).
Will not pass across a Bridge or Switch (Layer 2 devices).
Will not pass across a Router (Layer 3 device).
Router
Switch
Hub
Hub
Hub
Hub
Switch
Hub
Hub
Hub
Router
Hub
Hub
Hub
Hub
Hub
Hub
Hub
Media Access Control - Addresses
Ethernet MAC addresses are:
48 bits in length
Expressed as twelve hexadecimal digits,
e.g. 05CA64FF7EA1
Burned into read-only memory; referred to as burned-in addresses (BIA)
Copied into random-access memory (RAM) when the NIC initializes
Organizational Unique Identifier (OUI)
The first six hexadecimal digits, (administered by the IEEE) identify the manufacturer
Vendor assigned part
The remaining six hexadecimal digits represent the interface serial number
On receipt of a frame the NIC checks to see if its MAC address
matches the destination MAC address in the frame:
If it matches the frame is passed to the upper layers
If it does not match, the frame is discarded
Layer 2 Framing
Framing means providing a standard representation of information passed over
network links.
i.e.
Framing is the Layer 2 encapsulation process,
so
A Frame is the Layer 2 Protocol Data Unit (PDU).
The 802.3 / Ethernet Frame:
Preamble
7 bytes of
10101010
Start-of-frame Destination Source
Delimiter
MAC
MAC
1 byte
Address Address
10101011
6 byte
6 byte
Length
Data
/ Type 0 – 1500
2 bytes
bytes
Pad
0 – 46
bytes
Frame Check
Sequence
(FCS)
4 bytes
(Preamble fields shown yellow, header fields shown blue, payload fields shown red)
Notes:
1 The Frame is considered to start at the Information-carrying portion,
i.e. from the Destination Address Field, for frame size calculations.
2 The header is considered to include the FCS, even though it is at the end of the frame.
3 The header is 18 bytes long.
4 The minimum frame size is 64 bytes.
4a Thus the Data field + Pad MUST always total at least 46 bytes.
5 The maximum packet size is 1518 bytes.
5a Thus the maximum size of the Data field is 1500 bytes.
Layer 2 Framing (continued)
Fames are susceptible to transmission errors.
The Frame Check Sequence (FCS) field contains a number calculated by the source
node based on the data in the frame.
This FCS is added to the end of the frame before it is sent.
When the destination node receives the frame it recalculates the FCS number.
If the two numbers are different:
an error is assumed,
the frame is discarded,
a NACK is sent to the source (implying that the frame must be retransmitted).
Media Access Control
MAC protocols determine which computer on a shared-medium environment,
or collision domain, is allowed to transmit the data.
MAC is a sublayer of Layer 2.
Deterministic Media Access Control
(taking turns) e.g. Using a Token, as in Token Ring
Non-deterministic Media Access Control
(first come, first served) e.g. CSMA/CD as in Ethernet.
Carrier Sense, Multiple Access, with Collision Detect (CSMA/CD)
The Network Interface card (NIC) listens for an absence of a signal on the shared media
and can transmit if the carrier is clear.
If two nodes transmit at the same time (nearly the same time) a collision occurs
and temporarily none of the nodes are able to transmit.
Node
Listen
Node
Send
Listen
Node
Listen
Send
CSMA/CD in operation
A node that wants to send data works in a listen-before-transmit mode,
→ is the networking media is busy ? (Carrier Sensing).
This is to ensure no other stations are transmitting at the same time (Multiple Access).
If the node determines the network is busy, the node waits a random amount of time
before retrying.
After completing data transmission the device will return to listening mode.
Networking devices detect a collision has occurred when the amplitude of the signal on
the networking media increases (Collision Detect).
→ Each node that is transmitting will continue to transmit for a short time to
ensure that all devices see the collision (Jam Signal).
→ A backoff algorithm is invoked and transmission is stopped.
→ Nodes wait for a random period of time.
When the delay period expires, each device can attempt to gain access to the
networking media.
If the MAC layer is unable to send the frame after sixteen attempts, it gives up and
generates an error to the network layer.
Access Protocol CSMA-CD
• Carrier Sense – Listen Before Transmit (wait till line free)
then:
• Transmit at once (i.e. 1-persistent; always transmits as soon as
line is detected free)
• Collision Detect – (Listen while talk), if collision:
– Stop Tx, send Jam Signal (32 bits)
– If #colls > 16, give up
– All stations wait 96 bit-times (Interframe gap 96 bit-times,
9.6 μsec before sending a frame
– Those involved in collision wait a random interval (binary
exponential backoff algorithm); multiples of slot time (512
bits)
Collision Detection
Station ‘A’ must detect the collision before it
has finished transmitting its frame.
Ethernet Topologies - Physical
Physical - Bus
Node
Node
Node
Physical - Star
Node
Node
Node
Node
Node
Hub
Node
Switch
Node
Node
Node
Node
Node
Node
Ethernet Topologies - Physical (continued)
Physical - Extended star (wired as a star)
Node
Node
Node
Node
Node
Hub
Node
Hub
Node
Node
Node
Node
Switch
Node
Node
Node
Node
Hub
Node
Hub
Node
Node
Node
Node
Node
Ethernet Topologies - Logical
Logical - Bus
Node
Node
Hub
Node
Node
Hub
Hubs are transparent to layer 2.
Thus, Logically, the topology is a bus.
Full Duplex vs Half Duplex operation
Half Duplex → Only one node can transmit at a time (i.e. shared medium).
Logically a bus topology.
Full Duplex → Both nodes (at each end of a private link) can transmit
simultaneously.
Logically the link is seen as a point-to-point link.
Achieving Full Duplex with Ethernet
In order for both nodes to be able to transmit, there must be two carriers.
A UTP category 5 cable has 4 twisted pairs or wire.
In Half-Duplex mode, only one pair is used to transmit.
In Full-Duplex mode, a second pair of wires is employed.
→ One wire pair is used in each direction.
→ Each wire-pair is dedicated, so no collisions can occur.
→ Full link bandwidth is available in EACH direction.
→ The links must terminate at layer 2 or higher devices.
→ Therefore must use Bridges, Switches or Routers.
→ Cannot use Repeaters or Hubs.
→ Allows for larger network architecture designs because the timing
restriction for collision detection is removed.
Ethernet
Detailed operation
Provides ‘Type’ information
802.3 MAC frame
Why limit the maximum frame size? To ensure fair
access to all and to reduce effects of errors
Why minimum size limit? To ensure collisions are
detected before the frame is fully transmitted.
MAC Addresses
•
•
•
•
•
•
•
Associated with NIC, burnt in address
6 bytes e.g. 00-07-E9-41-D3-90 (hex)
Bytes 1-3 assigned to manufacturer
Bytes 4-6 identify the NIC
LSB of first byte = 1 for multicast (& broadcast)
The next bit defines scope (global/local)
FFFFFFFFFFFF = broadcast
Uni, multi and broadcast
See e.g. http://www.iana.org/assignments/ethernet-numbers
07-01-02-03-04-05
Look at the first byte: 07 = 0000 0111
The last 1 bit signifies it is a multicast address
08-07-06-05-44-33:
08 = 0000 1000, unicast
This applies to the destination address only. Why?
Frame Format
Ethernet_II Type Codes
http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hbr_r/br_aph.htm
Decimal 0x = hexadecimal Data type
>1536
> 0x 0600
2048
2053
0x 0800
0x 0805
IPv4
X25 lvl 3
2054
1536
33079
0x 0806
0x 0600
0x 8137
ARP
XNS
IPX
Think: How can you tell whether the Type or Length field
is in use (from the value)?
Logical Link Control (LLC) – Ethernet Protocols and Headers
Layer 3
Layer 2
MAC Header
(‘Ethernet’ head)
Layer 1
LLC Header
(SAP / SNAP)
(e.g. IP) Packet
Layer 3 PDU
Payload
LLC PDU
Payload
Bit stream
Provides logical link control / data link control information
In addition to that in the Ethernet (MAC) header
MAC PDU (Frame)
SNAP = Subnetwork Architecture
Protocol. Allows Ethernet II frame to
be used in 802.3 frame. DSAP /
SSAP set to AA , command set to 3.
‘SAP’ fields indicate
this is ‘SNAP’
SAP Codes for LLC
http://www.geocities.com/SiliconValley/Haven/4824/ethernet.html
04 - IBM SNA
BC - Banyan
06 - IP
E0 - Novell
80 - 3Com
F4 - Lan Manager
AA - SNAP
FE -CLNS
MAC DSAP SSAP Control Data
MAC
(1)
header (1)
(1)
(46-1500) FCS
IP in Ethernet frame
AA 00 04 00 32 04 00 00 B0 60 E4 80
08 00 45 00 00 54 02 BD 00 00 FD 01 3F
D2 C0 72 16 64 84 42 20 01 08 00 84 7E 43
75 00 00 2B AD A1 1B 00 0B 78 35 08 09
0A 0B 0C 0D 0E 0F 10 11 12 13 14 15
16 17 18 19 1A 1B 1C 1D 1E 1F 20 21 22
23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F
30 31 32 33 34 35
preamble, and start delimiter (SFD) omitted
Ethernet Frame - ARP
FF FF FF FF FF FF 00 01 80 05 D6 7B
08 06 00 01 08 00 06 04 00 01 00 01 80
05 D6 7B C1 3C 4D 66 00 00 00 00 00
00 C1 3C 4D 0B 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00
00 00
STP bridge
01 80 C2 00 00 00 00 0A 04 42 5B 01 00 26
42 42 03 00 00 00 00 00 80 00 00 0A 04 42
5B 00 00 00 00 00 80 00 00 0A 04 42 5B 00
80 01 00 00 14 00 02 00 0F 00 00 00
• Note the multicast address – all bridges
• Type/length = 26 (hex), not a valid type, and is
< 1536 so must be length
OSPF over IP
01 00 5E 00 00 05 00 01 30 F4 D0 00 08 00
45 C0 00 40 CF 0E 00 00 01 59 46 80 AC 10
17 01 E0 00 00 05 02 01 00 2C 0A 0A 0A 0A
00 00 00 05 25 74 00 00 00 00 00 00 00 00
00 00 FF FF FF 00 00 0A 02 01 00 00 00 28
AC 10 17 01 00 00 00 00 00 00 00 00
• Destination MAC address is multicast
Switched Ethernet
Bus Ethernet configurations are half duplex, TX & RX
not simultaneous
• Only one station can transmit at a time
• Band width (10 Mbps) is shared by all stations
• A station cannot send and receive at the same time;
collision detect must be used.
• Switches & bridges do not propagate collisions
Switched Ethernet is full duplex, and the CD function
is not needed.
Bridges reduce collisions and provide more
bandwidth
In example below: 10 Mbps required for every per 3 stations (because
of collisions) instead of per 12 (when bridge used)
Switched Ethernet – extension of bridged
Ethernet – one station per segment
Each PC-switch link is a segment, with 10 Mbps
bandwidth, 5 Mbps each way – still HDX
Full-duplex switched Ethernet
This configuration allows 10 Mbps for each station each way. No
collisions can occur, but basic format preserved for compatibility
Fast Ethernet 100 Mbps 802.3u
Runs over 2 wire-pairs
inside a category 5 or
above cable
Uses two strands of optical
fibre, one for receive (RX) and
one for transmit (TX).
Old, required 4 twisted
copper pairs, within a
category 3 or above cable
Same frame format, addresses, min/max frame sizes.
Compatible with 10 Mbps standards.
Auto negotiation between nodes (speed, HDX/FDX).
Star topology retained.
Fast Ethernet – IEEE 802.3u
100BaseT4
100BaseTX
100BaseFX
medium
UTP3 at least
UTP5, STP
Multi Mode Fibre
Mode
HDX 4-wire
FDX 2-wire
FDX 2-wire
Range
100m (seg)
200m (net)
100m (seg)
200m (net)
Coding
8B/6T NRZ
4B/5B MLT-3
2 km FDX
412m HDX
4B/5B NRZI
On-off
10 Gbps Ethernet IEEE 802.3ae
adopted 2002
• Fibre, single or multi-mode, FDX only
• Up to 40 km, useful for backbones, WANs and
MANs; POPs and Local Loops
• LANs (R- standards), MANs & WANs
(10GBase-W)
• Frame format and addressing the same, but
CSMA/CD abandoned.
• Compatibility with Frame Relay and ATM
10G- Standards
• LANs
– Short Range: 26-82 m, Multi Mode fibre, connections to
high speed servers, SAN
– Long Range: 10 km, Single Mode fibre, campus
backbones, MANs
– Extended Range: 40 Km, Single Mode fibre; MANs
• WANs (over Sonet OC-192 links)
– Short Wan: Multi Mode fibre, 300m
– Long Wan: Single Mode fibre 10 km
– Extended Wan: Single Mode fibre 40 km
Timing Considerations
Bit Time, and Propagation
On 10Mbps Ethernet one bit requires 100 nanoseconds (ns) to transmit.
At 100Mbps that same bit requires 10 ns to transmit and at 1000Mbps only
takes 1 ns.
Propagation speed of light in a vacuum is 3 * 108 Meters per Second.
Electrical signal in a cable (travels) at 2/3 the speed of light i.e. 2*108 M/S.
→ For 100 meters of UTP, it takes just under 5 bit-times for a 10BASE-T signal
to travel the length the cable.
With CSMA/CD, the sending station must become aware of a collision before it
has completed transmission of a minimum-sized frame.
At 100Mbps the system timing is barely able to accommodate 100Meter
cables.
At 1000Mbps special adjustments are needed as nearly an entire minimumsized frame would be transmitted before the first bit had travelled 100 meters
of UTP cable.
→ Half duplex is not used in 10Gigabit Ethernet – no collisions in full-duplex
mode.
Timing Considerations (continued 2)
Slot Time
To guarantee that collisions will ALWAYS be detected:
Slot time is just longer than time required to travel diameter of
the collision domain,
collide with another transmission at the last possible instant,
and have the collision fragments return to the sending station
and be detected.
Slot time for 10 and 100-Mbps Ethernet is 512 bit-times, or 64
octets.
Slot time for 1000-Mbps Ethernet is 4096 bit-times, or 512 octets.
Slot time is not relevant to 10 Gigabit Ethernet.
Timing Considerations (continued 3)
Extension field
For the system to work the first station must learn about the collision before it finishes
sending the smallest legal frame size.
To allow 1000-Mbps Ethernet to operate in half-duplex the extension field was added
when sending small frames purely to keep the transmitter busy long enough for a
collision fragment to return.
This field is present only on 1000-Mbps, half-duplex links and allows minimum-sized
frames to be long enough to meet slot time
requirements. Extension bits are discarded by the receiving station.
Interframe Spacing
The minimum spacing between two non-colliding frames.
After a frame has been sent, nodes on a 10-Mbps Ethernet must wait a minimum
of 96 bit-times (9.6 µS) before any station may legally transmit the next frame.
On faster versions of Ethernet the spacing remains the same, 96 bit-times,
but the time required for that interval grows correspondingly shorter.
The interframe gap is intended to allow slow stations time to process the previous
frame and prepare for the next frame.
Other Data Link Protocols
• SDLC (Synchronous D/L control) – first DL protocol,
proposed by IBM
• HDLC (High-level D/L control) – ISO version; NRM &
ABM operation
• LAP/LAPB – CCITT for X.25
• PPP – on the Internet (user access from home)
– Over a single link (connection), no addressing
– Byte oriented protocol unlike HDLC
– LCP & NCP (link/network control protocol) for link and
network parameter negotiation
– Link level security – PAP (password authentication protocol)
and CHAP (challenge handshake authentication protocol)
– Compression, line quality monitoring functions available