Chapter 12 Local Area Networks Project 802 Ethernet Token Ring FDDI

Project 802 In 1985, IEEE developed project 802. It subdivided the data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). LLC sublayer is non-architecture specific (same for all LANs). MAC sublayer is architecture specific. It also contains a section for internetworking to assure compatibility of different LANs.

OSI Model and Project 802

Project 802

IEEE 802.1 It is devoted to internetworking issues in LANs and MANs. It assures compatibility between networks without requiring modification in addressing, access, … Discussed in Chapter 20.

LLC and MAC IEEE 802 uses the structure of an HDLC frame and divides it into two sets of functions: The first represents the end-user portions (logical address, control, and data) and is handled by the LLC sublayer. The second resolved contention for the shared media and is handled by the MAC sublayer. The MAC contains the synchronization, flag, flow, and error control specification, as well as the physical address of the next station.

Protocol Data Unit (PDU) The data unit in the LLC level is called PDU. It contains four fields: Destination Service Access Point (DSAP), a Source Service Access Point (SSAP), a control field, and an information field. DSAP and SSAP are used by LLC to identify the protocol stacks on the receiving and sending machines. The first bit of the DSAP indicates whether the frame is intended for an individual or a group. While in SSAP, it indicated whether the packet is command or response. The PDU has no flag, no CRC, and no station address.

PDU Format

PDU Control Field

Ethernet (IEEE 802.3) It defines two categories: Naming style: Baseband (Digital Signaling – Manchester) Broadband (Analog Signaling – DPSK). Naming style: - Data Rate in Mbps Type of signaling - Maximum cable length or type of cable. Example : 10Base5.

802.3 Broadband Baseband Analog Digital 10Base5 10Base2 10BaseT 10 Braod 36

Access Method: CSMA/CD The way in which network devices access the network medium Carrier Sense Multiple Access with Collision Detection (CSMA/CD): media-access mechanism wherein devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. If two devices transmit at once, a collision occurs and is detected by all colliding devices. Collision subsequently delays retransmissions from those devices for a random amount of time.

Evolution of CSMA/CD MA Multiple access CSMA Carrier sense Collision detction

Rules for CSMA/CD If the medium is idle transmit, otherwise, go to 2. If the medium is busy, continue to listen until the channel is idle, then transmit immediately. If a collision is detected during transmission (high voltage), transmit a brief jamming signal to assure that all stations know that there has been a collision and then cease transmission. After transmitting the jamming signal, wait a random amount of time, then go to 1.

Addressing: Each station on an Ethernet network has its own Network Interface Card (NIC). The NIC contains the physical address of the station (MAC address) (6-byte). IEEE 802.3 specifies one type of frame containing 7 fields. Ethernet does not provide any mechanism for acknowledging received fames (upper layer). - Preamble: 7 bytes of alternating 1s and 0s. - Start Frame Delimiter (SFD) 10101011: signals the beginning of the frame. - Destination Address, Source address 6-bytes each - Length/type of PDU: number of bytes in PDU or type if the length is fixed. - PDU can be anywhere from 46 to 1500 bytes. - CRC-32.

MAC Frame

10BASE5 – Thick Ethernet Ethernet Segments - 200 stations /segment, RG-8 cable

10BASE5

Transceiver: Each station is attached by an AUI cable to an intermediary device called Medium Attachment Unit (MAU) or transceiver. It performs the CSMA/CD function and functions as a connecter. AUI Cables: Each station is connected to the corresponding transceiver by an Attachment Unit Interface (AUI). It is a 15-wire cable with plugs. Each end terminates in a DB-15 connector. Transceiver Tap: a connecting mechanism to allow the transceiver to tap into the line at any point. It is sometimes called vampire.

Transceiver

Thin Ethernet (10BASE2) Called Thin-Net, Cheap-Net, & 10BASE2 Pros: Reduced cost and ease of installation Cons: Shorter range (185m) and smaller capacity (30 nodes /segment) Thin coaxial cable 75 RG-58. BNC-T connector. The NIC provides the functions of the transceiver.

10BASE2

Twisted-Pair Ethernet 10BaseT Star Topology (Logical bus) Maximum length 100m (Hub-to-Station). 4-pair UTP cable. RJ-45 connector. Each station is connected to the Hub through a port. The Hub fans out any transmitted frame to all of its connected stations.

10BASET

StarLAN: 1Base5 Very slow – infrequently used today Its range can be increased using daisy chaining. 10 stations can be linked each to the next in a chain.

1BASE5

Fast Ethernet 100 Base -T4 100 Base-x 4 pairs of UTP 100 Base -TX 100Base-Fx 2 pairs of UTP or STP 2 optical fibers

Fast Ethernet An extension to IEEE 802.3 (802.3u) approved in 1995. IEEE 802.3 uses Manchester encoding running at 20 MHz. (two pulses /digit  10 Mbps). 100BASE-T4 uses 4-pair UTP: one is to the hub, one from the hub, and the other two are switchable to the current transmission direction. Manchester in not used, 25 MHz ternary signals are used with 4-bit/signal  100 Mbps