LAN and MAN Data Link Protocols Local Area Networks Limited geographical area, usually within a building, often on a single floor Typical speeds = 10-16 Mbit/s Backbone LAN Usually within a building, connecting lower speed LANs, often between floors Typical speeds = 100 Mbit/s – 10 Gbit/s Metropolitan Area Networks Within a city, or a campus 24/11/09 15-Datalink

LAN and MAN Data Link Protocols LANs Ethernet Token Bus Token Ring Backbone LANs Fast Ethernet Gigabit Ethernet (also used in MANs and WANs) Fibre Distributed Data Interface (FDDI) MANs Distributed Queue Dual Bus (DQDB) Cable TV (CATV) Networks 24/11/09 15-Datalink

LAN and MAN Standards LANs are standardised by the IEEE 802 Committee The main standards are: IEEE 802.1 Internetworking issues in LANs and MANs IEEE 802.2 Logical Link Control (LLC) IEEE 802.3 Ethernet (CSMA/CD) IEEE 802.4 Token Bus IEEE 802.5 Token Ring IEEE 802.6 Metropolitan Area Networks (DQDB) IEEE 802.11 Wireless Local Area Networks (WLANs) 24/11/09 15-Datalink

LAN Sublayers IEEE LANs operate two sublayers Logical Link Control Common to all LANs Based on HDLC Includes addressing, control information and data Media Access Control Specific to the media type Resolves contention Responsible for synchronisation, flow control and error control 24/11/09 15-Datalink

IEEE 802.2 (LLC) Frame Format The LLC Headers are based on HDLC and are encapsulated in MAC frames They therefore follow immediately after the MAC header DSAP SSAP Control Information 1 byte 1 or 2 bytes Up to 1496 or 1497 bytes 24/11/09 15-Datalink

IEEE 802.2 (LLC) Frame Format Destination Service Access Point (DSAP) used to identify the network layer protocol at the receiver with one bit to signify whether it is a group or individual address Source Service Access Point (SSAP) used to identify the network layer protocol at the transmitter with one bit to signify whether it is a command or a response Control Field used to identify the type of LLC frames (similar to its use in HDLC) 24/11/09 15-Datalink

IEEE 802.3 (Ethernet) The Ethernet was developed in 1976 at Xerox’s Palo Alto Research Center (PARC) It was marketed by a consortium of Digital, Intel and Xerox (known as the DIX Consortium) and soon became the de facto standard for LANs There are two variations of Ethernet in use Ethernet II (the original DIX protocol. It has a type field to identify the network layer protocol) IEEE 802.3 (replaces the type field with a length field and cannot identify network layer protocol so can only be used with one network layer or requires an additional LLC sublayer) 24/11/09 15-Datalink

IEEE 802.3 (MAC) Frame Format © Tanenbaum, Prentice Hall International 24/11/09 15-Datalink

IEEE 802.3 (MAC) Header Fields Preamble 7 bytes of 10101010 to synchronise receivers Start of Frame Delimiter (10101011) Destination and Source Addresses 6 bytes each for MAC addresses Length of Data Field (in bytes) Data 0 to 1500 bytes Padding Up to 46 bytes to make sure that frame is at least 64 bytes long to ensure that collisions can always be detected by the transmitter Checksum (CRC-32) 24/11/09 15-Datalink

IEEE 802.3 Specification Access Method Backoff Algorithm CSMA/CD Binary exponential back-off after n collisions a random number between 0 and 2n-1 is chosen and transmitter backs off for this number of timeslots (timeslot = worst case RTT = 51.2 microseconds at 10 Mbit/s) After 10 collisions maximum backoff time frozen After 16 collisions algorithm gives up Hence delay is variable and there is no guarantee that the frame will ever be transmitted 24/11/09 15-Datalink

IEEE 802.3 Specification Electrical Data Rate Addressing Baseband signaling with Manchester encoding Data Rate 10 Mbit/s to 10 Gbit/s Addressing MAC addresses burnt into Network Interface Cards 24/11/09 15-Datalink

Ethernet Implementations 10Base5 (Thick Ethernet or Thicknet) Leads to stations are tapped into up to 500 m of thick coaxial cable 10Base2 (Thin Ethernet or Thinnet, Cheapnet or Cheapernet) Leads to stations are connected to a thin coaxial cable bus of up to 200m via a three-way connector 10Base-T (Twisted Pair Ethernet) Each station is wired to a hub via twisted pair cable. The hub broadcasts frames to all stations apart from the one from which it received the frame Topology is physically a star, but logically a bus 24/11/09 15-Datalink

Ethernet Implementations 10Base5 10Base2 10Base-T © Tanenbaum, Prentice Hall International Note: the 1st part of the name indicates the speed in Mbit/s, the 2nd part whether it is baseband or broadband and the final part the maximum distance the signals will carry in 100s of meters or the type of media 24/11/09 15-Datalink

Switched Ethernets In order to reduce the number of collisions, modern Ethernets are often connected to a switch rather than a hub Whereas the hub will forward frames to all stations except the one from which the frame was received, a switch will examine the destination MAC address and forward the frame only to the one station it was intended for 24/11/09 15-Datalink

IEEE 802.4 (Token Bus) Developed by General Motors and used in factory automation, but is not very common Topology is physically a bus or tree, but logically a ring Overcomes problem with 802.3 regarding the arbitrarily long time a station may have to wait to transmit Overcomes the problem with ring networks being completely brought down by a cable break Station can only transmit if it has the token which is passed around from station to station in turn 24/11/09 15-Datalink

IEEE 802.5 (Token Ring) Developed by IBM for office automation Overcomes problem with 802.3 regarding the arbitrarily long time a station may have to wait to transmit Token is circulated around the ring When a station wants to transmit, it seizes the token and transmits its data The receiver marks the frame as having been read When the frame returns to the sender, it checks that it has been successfully read an passes the token on to the next station 24/11/09 15-Datalink

Token Ring Protocol Access Method Electrical Data Rates Addressing Token Passing Electrical Baseband with Differential Manchester Encoding Data Rates 1, 4 or 16 Mbit/s Addressing MAC addresses burnt into Network Interface Cards 24/11/09 15-Datalink

Star-shaped Token Rings Token Rings are often collapsed into MultiStation Access Units (MAUs) which solve the problems associated with cable breaks © Tanenbaum, Prentice Hall International 24/11/09 15-Datalink

IEEE 802.11 (Wireless LANs) Also known as WiFi Uses CSMA/CA access method (RTS, CTS, ACK) Uses LLC and alternative MAC layers IEEE 802.11a Uses Orthogonal Frequency Division Multiplexing (OFDM) with 52 frequencies in the 5 GHz band and supports 6-54 Mbit/s IEEE 802.11b Uses Direct Sequence Spread Spectrum (DSSS) in the 2.4 GHz band and supports 1, 2, 5.5 and 11 Mbit/s IEEE 802.11g Uses OFDM in the 2.4 GHz band to support 54 Mbit/s 24/11/09 15-Datalink

IEEE 802.11 Physical Layers The IEEE 802.11 Standard supports the following physical layers 802.11 (3 variants) Infrared Frequency Hopping Spread Spectrum Direct Sequence Spread Spectrum 802.11a Orthogonal Frequency Division Multiplexing 801.11b High Rate DSSS 802.11g Orthogonal Frequency Division Multiplexing 24/11/09 15-Datalink

IEEE 802.11 Services Distribution Services provided by the Base station Association - Connecting to Base Station Disassociation – Disconnecting from Base Station Reassociation – Changing Base Station Distribution – routing frames to other stations or over wired network Integration – Translating from 802.11 frame format to other formats for non 802.11 networks 24/11/09 15-Datalink

IEEE 802.11 Services Intracell Services used within a cell after association has taken place Authentication – challenge to check if station has correct secret key Deauthentication – used when a station wants to leave the network Privacy – encryption using RC4 algorithm Data delivery – connectionless (unreliable) delivery of data 24/11/09 15-Datalink

Backbone LAN Standards IEEE 802.3u (Fast Ethernet – 100 Mbit/s) IEEE 802.3z (Gigabit Ethernet –1000 Mbit/s) IEE802.3ae (10G Ethernet – 10 Gbit/s) ANSI X3TP.5 (Fibre Distributed Data Interface - FDDI) 24/11/09 15-Datalink

IEEE 802.3u (Fast Ethernet) Implementations 100Base-TX Uses 2 cat 5 UTP or 2 STP cables up to 100m to hub or switch with 4B/5B encoding and NRZ-I signalling 100Base-FX Uses 2 optical fibres up to 2000m to hub or switch with 4B/5B encoding and NRZ-I signalling 100Base-T4 Uses 4 cat 3 UTP cables up to 100m to hub or switch by splitting the 100 Mbit/s capacity into 3x33.66 Mbit/s channels 24/11/09 15-Datalink

IEEE 802.3z (Gigabit Ethernet) Implementations 1000Base-SX Multi-mode optical fibre with short wave lasers up to 550m 1000Base-LX Multi-mode or Single-mode optical fibre with long wave laser up to 550m (multi-mode) or 5000m (single-mode) 1000Base-CX Shielded Twisted Pair up to 25m 1000Base-T Unshielded Twisted Pair up to 25m 24/11/09 15-Datalink

Fibre Distributed Data Interface (FDDI) FDDI is a high performance LAN protocol for use over optical fibres standardised by ANSI and ITU-T The topology is base on two rings with data flowing in opposite directions If there is a break in the cable, the stations either side of the break detect it and simply loop the cables to form one ring As with Token Rings, the FDDI Ring can be collapsed into a wire centre to further protect against cable failures bringing the network down 24/11/09 15-Datalink

FDDI Dual Ring Back-up Cable Break 24/11/09 15-Datalink

FDDI Protocol Access Method Electrical Data Rates Addressing Token Passing Electrical Baseband with 4B/5B (4 bits are represented by 5 bits containing no more than two consecutive zeros) encoding with NRZ-I signaling Data Rates 100 Mbit/s Addressing MAC addresses burnt into Network Interface Cards 24/11/09 15-Datalink

MAN Implementions Distributed Queue Dual Bus (DQDB) Used as Layer 2 protocol in Switched Multimegabit Data Service (SMDS) Cable Television Networks Community Access TeleVision or CATV 24/11/09 15-Datalink

IEEE 802.6 (Distributed Queue Dual Bus) Topology is 2 parallel unidirectional buses Each bus has a head end which generates a steady stream of 53 byte cells Each cell carries a 44 byte payload and has two protocol bits (busy to indicate that a cell is occupied and request to indicate that a station wants to make a request) Each station can see requests for one bus being made on the other bus Stations can implemented a distributed First In First Out (FIFO) queue and know when they can use a cell 24/11/09 15-Datalink

Cable TV Networks Cable TV networks are broadband networks that broadcast TV channels from the head end of the cable network to houses, but they can also support interactive data services using cable modems A standard has been developed for cable modems called DOCSIS (Data Over Cable Service Interface Specification) or EuroDOCSIS Users contend to request short slots on the upstream channel while the downstream channel is managed by the head end Data is transferred downstream in 204 byte packets The user interface is normally 10 Mbit/s Ethernet or USB 24/11/09 15-Datalink