1. Local Area Networks Includes some material from Forouzan ‘Data Communications’

2. 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

3. LAN Protocol Architecture
LLC
MAC
Physical
TCP/IP protocol family IPX/SPX protocol family
LLC = Logical Link Control
MAC = Media Access control
OSI Layers
3 (Network) and 4 (Transport)
2 (Data Link)
1 (Physical)

4. Physical Layer Functions
Encoding/decoding of signals
Preamble generation/removal for synchronisation
Bit transmission, reception
Transmission medium specification
(baud rate, distance limitations)

5. MAC functions Medium Access Control Frame assembly, disassembly
Frame transmission and reception
Checksum (re)computation
Address recognition

6. 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

7. MAC Protocols Contention – Ethernet (CSMA/CD)
Round Robin – Token passing
Polling – VGAnyLAN (no longer used)
Reservation – WLAN & satellite

8. IEEE and CSMA/CD / Ethernet
Overview of operation

9. 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
Wireless LAN (WLAN)
Wireless Personal Area Network (WPAN)
Broadband Wireless Access (BBWA)
Resilient Packet Ring (RPR)
Radio Regulatory Technical Advisory Group
Coexistence Technical Advisory Group
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)
Standard for Interoperable LAN Security (SILS)
Demand Priority

10. 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).
Data-Link
Layer
Physical
LLC
MAC
802.2
802.3
CSMA/CD
802.4
Token Bus
802.5
Token Ring
802.6
MAN
LLC frame:
DSAP
Address
(1 byte)
SSAP
Control
(1 or 2 bytes)
Information
(0 or more bytes)

11. Logical Link Control (LLC)
… Bit stream …
MAC
header
LLC
Payload
Packet
Layer 3 PDU
LLC PDU
MAC PDU (Frame)
Layer 3
Layer 2
Layer 1

12. 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.

13. 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).
Switch
Hub
Router

14. 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

15. 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 / Ethernet Frame:
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.
Length
/ Type
2 bytes
Data
0 – 1500
bytes
Pad
0 – 46
Source
MAC
Address
6 byte
Destination
Start-of-frame
Delimiter
1 byte
Preamble
7 bytes of
Frame Check
Sequence
(FCS)
4 bytes
(Preamble fields shown yellow, header fields shown blue, payload fields shown red)

16. 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).

17. 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
Send

18. 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.

19. 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)
K = re-transmission attempt (1,2,3…)
R = random(0, 2^k)
Backoff time = R * slot time (51.2 microsec)

20. Collision Detection
Station ‘A’ must detect the collision before it has finished transmitting its frame.

21. Ethernet Topologies - Physical
Physical - Bus
Node
Hub
Switch
Physical - Star

22. Ethernet Topologies - Physical (continued)
Physical - Extended star (wired as a star)
Node
Hub
Switch

23. Ethernet Topologies - Logical
Hubs are transparent to layer 2.
Thus, Logically, the topology is a bus.
Ethernet Topologies - Logical
Logical - Bus
Hub
Node

24. 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.

25. Ethernet
Detailed operation

26. Provides ‘Type’ information
802.3 MAC frame
Preamble: 7 bytes of alternating 0, 1 to wake up the receivers; part of the physical layer, to alert and time synchronise
SFD: : signals beginning of frame also part of the physical layer. The MAC frame starts with the DA.
DA/SA are 6-byte MAC addresses
Length/Type – depending on whether LLC is present or we are running raw Ethernet under TCP/IP say. The length is the no. of bytes of data carried. Type shows the upper layer protocol (e.g. IP, ARP)
CRC (or FCS): checksum for error detection
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.

27. MAC Addresses Associated with NIC, burnt in address
6 bytes e.g 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

28. Uni, multi and broadcast See e. g. http://www. iana
Look at the first byte: 07 =
The last 1 bit signifies it is a multicast address :
08 = , unicast
This applies to the destination address only. Why?

29. Frame Format
Raw Ethernet does not use LLC, and is used with TCP/IP. It has a type field to specify the higher level protocol.
The IEEE standard frame is meant to be used with LLC and so has a length field defining the number of bytes of data (payload). The maximum is The payload consists of an LLC PDU which contains a type field.
The type fields are always > 1500, so that both types of frame can co-exist, and can be told apart.

30. Ethernet_II Type Codes http://www. cisco
Decimal
>1536
0x = hexadecimal
> 0x 0600
Data type
2048
0x 0800
IPv4
2053
0x 0805
X25 lvl 3
2054
0x 0806
ARP
1536
0x 0600
XNS
33079
0x 8137
IPX
also contains a list (Oct 2007)
Think: How can you tell whether the Type or Length field is in use (from the value)?

31. Logical Link Control (LLC) – Ethernet Protocols and Headers
Bit stream
MAC Header
(‘Ethernet’ head)
LLC Header
(SAP / SNAP)
Payload
(e.g. IP) Packet
Layer 3 PDU
LLC PDU
MAC PDU (Frame)
Layer 3
Layer 2
Layer 1
Provides logical link control / data link control information
In addition to that in the Ethernet (MAC) header

32. SNAP = Subnetwork Architecture Protocol
SNAP = Subnetwork Architecture Protocol. Allows Ethernet II frame to be used in frame. DSAP / SSAP set to AA , command set to 3.
SNAP allows you to use Ethernet codes with LLC. Used on Novell networks to allow all 3 types of frames to coexist
‘SAP’ fields indicate
this is ‘SNAP’

33. SAP Codes for LLC http://www. geocities
04 - IBM SNA
BC - Banyan
06 - IP
E0 - Novell
80 - 3Com
F4 - Lan Manager
AA - SNAP
FE -CLNS
But this would mean NW hardware has to cope with two sets of type codes.
MAC header
DSAP (1)
SSAP (1)
Control (1)
Data
( )
MAC FCS

34. IP in Ethernet frame AA 00 04 00 32 04 00 00 B0 60 E4 80
BD FD 01 3F D2 C E B AD A1 1B 00 0B A 0B 0C 0D 0E 0F
A 1B 1C 1D 1E 1F A 2B 2C 2D 2E 2F
preamble, and start delimiter (SFD) omitted

35. Ethernet Frame - ARP
FF FF FF FF FF FF D6 7B D6 7B C1 3C 4D C1 3C 4D 0B

36. STP bridge
01 80 C A B A B A B F
Note the multicast address – all bridges
Type/length = 26 (hex), not a valid type, and is < so must be length

37. OSPF over IP
E F4 D C CF 0E AC E C 0A 0A 0A 0A FF FF FF A AC
Destination MAC address is multicast

38. 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.

39. 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)
Four segments, each with 10 Mbps shared by 3 workstations, rather than 10 Mbps between 12 work stations.
Each segment is independent as far as BW is concerned. The switch does address filtering and does not forward collisions. So the LAN segments are in separate collision domains.

40. 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

41. Full-duplex switched Ethernet
This configuration allows 10 Mbps for each station each way. No collisions can occur, but basic format preserved for compatibility

42. Runs over 2 wire-pairs inside a category 5 or above cable
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.

43. Fast Ethernet – IEEE 802.3u 100BaseT4 100BaseTX 100BaseFX medium
UTP3 at least
UTP5, STP
Multi Mode Fibre
Mode
HDX 4-wire
FDX 2-wire
Range
100m (seg)
200m (net)
2 km FDX
412m HDX
Coding
8B/6T NRZ
4B/5B MLT-3
4B/5B NRZI
On-off
Manchester requires 200 Mbaud for 100 Mbps, not very efficient. But it provides a transition in the middle of the bit, hence – self clocking.
4B/5B allows each 4-bit sequence (16 possible patterns) to be coded as a 5 bit symbol (32 possible), avoiding the occurrence of long sequences of 0’s or 1’s. Clocking is thus provided. But requires a baud rate of 125 Mbps for a bitrate of 100 Mbps.
MLT-3 (multiline transmission, 3-level) provides good bandwidth performance, signal rate is 0.25 of the bitrate.
8B/6T: codes each 8-bit sequence (256 possible) as a pattern of 6 ternary signal elements (478 usable patterns). The patterns are chosen to provide DC balance and clocking.
100BaseT4 uses 4 pairs of wires, three of which transmit in one direction at a time at 25Maud each. The 8B/6T encoding enable this 75 Mbaud to deliver a rate of 100 Mbps. 8 bits/sec = 6 signals/sec.
Fibre has high enough bandwidth to use a simple coding scheme - NRZI

44. 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

45. 10G- Standards LANs WANs (over Sonet OC-192 links)
Short Range: 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
Tomso, Tittel & Johnson (Guide to Networking Essentials) p261
SAN = Storage Area Networks
SM = single Mode, MM = multimode
S=short , L = long, E = extended range

46. 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 minimum-sized 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.

47. 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.

48. 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.

49. 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
NRM = Normal Response Mode, one primary station (to issue commands) & several secondary stations (which can only respond). Point-to point & multipoint configuration.
ABM = Asynchronous Balanced Mode – point to point only, both sides are equal, either can initiate communication.