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The five-layer TCP/IP model

5. Application layer

DHCP · DNS · FTP · Gopher · HTTP · IMAP4 · IRC · NNTP · XMPP · POP3 · SIP · SMTP · SNMP · SSH · TELNET · RPC · RTCP · RTSP · TLS · SDP · SOAP · GTP · STUN · NTP · (more)

4. Transport layer

TCP · UDP · DCCP · SCTP · RTP · RSVP · IGMP · (more)

3. Network/Internet layer

IP (IPv4 · IPv6) · OSPF · IS-IS · BGP · IPsec · ARP · RARP · RIP · ICMP · ICMPv6 · (more)

2. Data link layer

802.11 (WLAN) · 802.16 · Wi-Fi · WiMAX · ATM · DTM · Token ring · Ethernet · FDDI · Frame Relay · GPRS · EVDO · HSPA · HDLC · PPP · PPTP · L2TP · ISDN · ARCnet · (more)

1. Physical layer

Ethernet physical layer · Modems · PLC · SONET/SDH · G.709 · Optical fiber · Coaxial cable · Twisted pair · (more)

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ITU-T Recommendation G.709 "Interfaces for the optical transport network (OTN))" is a standardized method for transparent transport of services over optical wavelengths in DWDM systems. It also known as Optical Transport Hierarchy (OTH) standard.

G.709 is positioned as a server layer for various client signals, e.g. SDH/SONET, ATM, IP, Ethernet, and OTN ODUk (where k=1, 2 or 3)

The frame structure defined in G.709 is constructed of 4 areas:

1. OPUk is the area in which payload is mapped.
2. ODUk contains the OPUk with additional overhead bytes (e.g. TTI, BIP8, GCC1/2, TCM etc).
3. OTUk is the section and includes framing , TTI, BIP8 and GCC0 bytes.
4. FEC - The standard FEC code (described in G.975) is a Reed-Solomon coding calculated across the payload (OPU)columns. This allows detection and correction of bit errors due to impurities in optical equipment, and problems with marginal equipment and fibers. The FEC code also extends the distance the optical signal can travel before requiring regeneration.

Note: k represents the line rate as follows: 1 means ~2.6Gbit/s 2 means ~10.7Gbit/s 3 means ~43Gbit/s

G.709 offers rich OAM&P capabilities such as TCM (Tandem Connection Monitoring), End to End performance monitoring, misconnection detection and General Communication Channel (GCC).

[edit] SDH/SONET and G.709 rates

SDH Line Signal (G.707)	SONET Line Signal (T1.105)	OTH Line Signal (G.709)	G.709 Line Rate (kbit/s)	OPUk Payload Rate (kbit/s)	OTUk frame period (µs)
STM-16	OC-48 (STS-48 SPE)	OTU1	2,666,057	2,488,320	48.971
STM-64	OC-192 (STS-192 SPE)	OTU2	10,709,225	9,953,280	12.191
STM-256	OC-768 (STS-768 SPE)	OTU3	42,836,900	39,813,120	3.035

Unlike SDH/SONET, the line rate is increased by maintaining the G.709 frame structure (4 rows x 4080 columns) and decreasing the frame period (in SDH/SONET the frame structure is increased and the frame period of 125 µs is maintained).