Computer Networks: IEEE 802.3 Ethernet

• Lecture 03 focuses on Computer Networks and IEEE 802.3 Ethernet at the Datalink Layer.

Network Coverage Area

• Networks can range from personal area networks (1m, square meter), to local area networks (100m-1km, building to campus), to larger networks.
• Metropolitan area networks (10km, city), wide area networks (100km-1000km, country to continent), and the Internet (10,000 km, planet).
• Personal Area Networks (PAN) cover up to 10s of meters and include Bluetooth (IEEE 802.15).
• Local Area Networks (LAN) cover up to 1 kilometer and include Ethernet (802.3), Token Ring (802.5), and WLAN (802.11).
• Metropolitan Area Networks (MAN) cover up to 10s of kilometers and include WiMAX (802.16).
• Wide Area Networks (WAN) cover hundreds/thousands of Km and can be private.
• The Internet is a global-scale public network.

LAN Topologies

• Network topology refers to the physical interconnection of computer systems.
• Wired topologies include bus (horizontal), ring, star, tree, and mesh.
• Wireless topologies include centralized (infrastructure mode) and distributed (ad-hoc mode).
• The star topology, often used in Ethernet networks has nodes interconnected via a central node which is either a Hub or Switch.
• Hubs broadcast to all attached network interfaces, while switches transmit only on the specified network interface.
• In a bus topology, all nodes are connected via a shared medium.
• A signal transmitted by one node is received by all other nodes on the LAN.
• In a ring topology nodes are interconnected with each other in a ring shape.
• Data flow is typically unidirectional, and variants include FDDI, CDDI, and TPDDI.
• A Hybrid Topology might use a Star backbone with 3 Bus Networks

Wireless LANs

• Wireless LANs adhere to the IEEE 802.11 WLAN standard or ETSI HiperLAN standard.
• There are two configurations: centralized (access point serves as infrastructure) and distributed (no infrastructure required, ad hoc mode).

Metropolitan Area Networks (MANs)

• Metropolitan Area Networks cover a 30-50 km area including a whole city and remote areas
• Wired MANs did not gain popularity, but wireless MANs are becoming more common
• IEEE 802.16 Standard (WiMAX) is deployed in Pakistan, and IEEE 802.20 Standard supports high-speed mobility.

Cellular Networks

• 2G uses Global System for Mobile Communications (GSM).
• 2.5G uses General Packet Radio Service (GPRS).
• 2.75G uses Enhanced Data rates for GSM Evolution (EDGE), including Enhanced GPRS (EGPRS).
• 3G/4G uses (CDMA2000, LTE).
• GSM is the ETSI Standard, it describes protocols for 2G digital cellular networks, functioning as a digital circuit switched network for full duplex voice telephony.
• SIM stands for Subscriber Identity Module
• BTS stands for Base Transceiver Station
• BSC stands forBase Station Controller
• PCU stands for Packet Control Unit
• MSC stands for Mobile Switching Center
• VLR / HLR
• AuC stands for Authentication Center
• EIR stands for Equipment Identity Register
• SGSN stands for Serving GPRS Support Node
• GGSN stands for Gateway GPRS Support Node

Network Adapters

• Network adapters can be seen as an Ethernet Card, Network Interface Card (NIC), or LAN Card
• Use the ipconfig command to find your IPv4 address
• Use the arp command to view and modify the IP-to-physical address translation tables used by address resolution protocol (ARP).

LAN Technologies and Standards

• IETF (Internet Engineering Task Force)
• IEEE (Institute of Electrical & Electronic Engineers).
• The IEEE initiated project 802 in the 1980s to develop standards for emerging LAN technologies
• These standards ensure equipment from different vendors can work together
• IEEE 802.3 (Ethernet), IEEE 802.5 (Token Ring), IEEE 802.11 (Wireless LAN), IEEE 802.15 (Bluetooth), and IEEE 802.16 (WiMAX).
• The most popular standard is 802.3.
• CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is a key access method in LANs.

Data Link Layer

• The data link layer is divided into two functionality-oriented sublayers: data link control and multiple-access resolution.
• Media Access Control (MAC) involves protocols that control the right to transmit over the physical medium.
• Primary/Secondary Protocols, the primary station controls access on the media in Contention-Free protocols, GSM, WiMAX, and WiFi Infrastructure Mode.
• Peer-to-Peer Protocols gives all stations equal rights to send data but may result in collision, Ethernet, WiFi Ad Hoc Mode.

Multiple Access Protocols

• Random access protocols: ALOHA, CSMA, CSMA/CD, CSMA/CA.
• Controlled-access protocols: Reservation, Polling, Token passing.
• Channelization protocols: FDMA, TDMA, CDMA.

Bus/Star Configurations

• Shared medium bus has a capacity of 10Mbps.
• A shared medium hub has a capacity of up to 10 Mbps, 10 Mbps goes to each node.
• Layer 2 switches can have Total capacity N x 10 Mbps

Ethernet Transmission

• In ethernet transmission data copies are going by.
• Destination and source addresses are included in frame addressing.

Hubs

• Hubs broadcast on all attached network interfaces but hubs create a collision domain, which can occur when two nodes try to send data simultaneously.
• In Ethernet, when a computer sends a message to a hub it is then transmitted to all the other computers in the network.

Switches

• Transmits only to specified network interface.

MAC Addresses

• MAC Address is the physical Address of Network Interface Card (NIC).
• MAC Addresses consist of Lowest level addressing with the hardware address
• MAC Address is a 48-bit address written in hexadecimal notation.
• The first 24 bits of the MAC Address are the organizational identifier and the next 24 bits are the network interface identifier.

Ethernet Frame (Layer-2 Header)

• Preamble: 56 Bits
• SFD to determine the start of the frame
• Destination Address: 48 bits
• Source Address: 48 bits
• Length or Type: 16 bits
• Data/LLC
• Header
• Trailer
• Ethernet type II frame ( 64 to 1518 bytes)

IEEE 802.3 frame Fields

• Preamble + SFD:
• Used for synchronization + Start of Frame Delimiter
• Destination & Source Addresses:
• MAC addresses of receiver & sender
• I/G = Individual/Group Address, LSB of first Octet of address U/L Universal/Local Address, Second to LSB of first Octet
• Length/Type
• Length of Frame in bytes
• Type of next protocol header; e.g. ARP, IP etc
• Frame Check
• 32-bit CRC Value, used for error detection

Error Detection with CRC

• Detects only the environmental/ accidental (random) errors
• May not detect errors intentionally induced by some attacker
• When a node detects the collision, it immediatley stops transmitting data and a jamming signal if sent so all other nodes are aware of a collission has accured
• CSMA Algorithm has many varients
• Collision Detection (CD), used in Ethernet, 802.3
• Collision Avoidance (CA) used in WIFI, 802.11

CSMA/CD

• CSMA/CD is a peer-to-peer protocol
• Shares a common transmission medium
• Only one station can emit at a time But which station
• When a node detects the collision, it immediately stops transmitting and sends a jamming signal
• It is the message that allows the nodes to be aware of the emergency or collision happening on the network
• Contention solution

###Delay

• We need a collision detection mechanism
• Transmission delay = time to transmit data
• The propagation delay between them
• Can still occur in following conditions
• Nodes should try to emit
• The medium needs valid voltage

Binary Exponential Backoff(BEB)

• The algorithm was developed to prevent collisions in the Ethernet network
• Base of BEB is the contention window
• Initially contending nodes randomly choose from 2-contention slots
• Nodes wait for the time equal to the randomly chosen period of contention slots
• After this random time wait, nodes again come and sense the mediums, and if found they are able to emit
• If only one node that is shortest, then contention is resolved

###Evolution of Ethernet Ethernet

• Pioneering work done by Dr Robert Metcalf in 1972, DEC and intel in 1979
• standard is implement in bus/star topology Fast Ethernet (100Mbps) approved by IEEE in 1995 and is star topology Gigabit Ethernet is (Gbps) 1Gbps stoodized through IEEE in 1998

###Device Differences

• Layer 3 Router
• Layer 2 Switch
• Layer 2 Bridge
• Layer 1 Hub